Weeds
"A weed is a plant that has mastered every survival skill except for learning how to grow in rows."
- Doug Larson
- Doug Larson
Weeds are nobody's favorite subject, but it is an important one nonetheless when it comes to tending to outdoor areas. Ask any gardener what their least favorite part of caring for their yard is and you'll most likely hear something about weeds, the unwanted, awkward cousins of our more aesthetically pleasing, intentionally chosen plants. Weeds are commonly found in gardens and other areas, making do with little to no water, soil, or nutrients. They are born survivors, many able to re-sprout from damaged foliage quickly and with ease, and are nuisances to those who value order in their garden. Plants are called weeds when they interfere with the intended use of a piece of land, causing health problems for surrounding plants, being hosts for disease and pests, and reducing the aesthetic value of a site. Any plant technically can be considered a weed if it is growing in a location where it is unwanted. For example, a tomato plant in a vegetable garden is not considered a weed, but a tomato plant growing amongst a front yard flower bed may be a weed. A mustard plant growing in a grass lawn is a weed, but on a hillside it is quite the opposite; it provides structure for the soil and prevents erosion damage.
Of the 250,000 plant species identified today, only about 250 (0.1%) are considered weeds. Although they are small in species number, they can cause large amounts of damage to agricultural crop lands in California, and are the number one pest problem in the garden landscape. In this state, more money is spent on weed control than on any other plant pest, and some are so difficult to control that laws are now in place providing for their intensive control or eradication. The list below is just a sampling of the many weeds that you may come across in your gardening journey. You may recognize some names as those of intentionally grown plants featured in the Plant Identification modules of this course; these are plants that have an aesthetically pleasing appearance, but can become a bit unmanageable if not kept in check.
Weed (n.): A valueless plant growing wild, especially one that grows on cultivated ground to the exclusion or injury of the desired crop.
weed Classification & Identification
The most important single piece of information needed when determining effective eradication strategies is a weed's identification. The identity of a weed will, by association, give valuable information about its life cycle, growth and development, and invasive potential. There are several references available to gardeners for the purpose of identifying weeds that provide photographs, identification keys, and physical descriptions to help with identification of weed species:
- Weeds of California and Other Western States (DiTomaso and Healy, 2007)
- Weed Pest Identification and Monitoring Cards (DiTomaso, 2013)
- Weeds of the West (Burrill et al., 2006)
The majority of weeds belong to just a few plant families, so identification is somewhat simple. General categories that are used to classify weeds make identification even easier. The first system used to classify weeds is the binomial system. This system uses the scientific names of weeds to categorize them into families, genera, and species, just like all other plants. This system allows for a common language to be used when referring to weeds, and also includes the common names of weeds. For example, Euphorbia maculata, a commonly found weed in California, also goes by the common names of Spotted Spurge, Prostrate Spurge, Spotted Pulsey, and Sandmat. Scientific identification of weeds is possibly the most important, as knowing the true name of a weed will provide gardeners with a plethora of information that can then be used to treat weed problems.
A second way that weeds are classified is through their life cycle or seasonal information. Weeds are classified as annual, biennial, or perennial, just like ornamental and crop plants. Annuals are plants that complete an entire life cycle in just one year. This group of weeds is further divided into summer and winter annuals. Winter annual weeds will germinate in autumn, live through the winter months, and produce seeds in late winter and springtime. Summer annuals, on the other hand, will germinate around early March and continue growing through the summer months, releasing seeds in late summer or fall. Knowing when the seeds of certain species will germinate is an important piece of information, because it informs how and when treatment is applied. Biennial weeds will complete their life cycle in two years' time, producing vegetative growth in the first year and reproductive growth in the second year. These weeds are not usually considered to be a serious problem in gardens, as they are usually pulled up in their first year before they can set and release their seeds. Perennial weeds live for over two years, and are the most difficult to eradicate. They usually have more extensive root systems than annual or biennial weeds and most produce seeds each year, meaning that they can reproduce very well. In this perennial group, there are also many weeds that will spread via underground rhizomes tubers, stolons, or bulbs.
Third, and finally, weeds can be classified as either broadleaf (dicots) or grassy (monocots). Professional weed specialists will commonly classify weeds as annual or perennial broadleaves or annual or perennial grasses. This classification seriously narrows down the field when searching for a weed's identity. Two less commonly used classification systems that concern weeds are their regulatory status (how they are treated under the law) and their use (some weeds are edible or poisonous to humans or livestock).
A second way that weeds are classified is through their life cycle or seasonal information. Weeds are classified as annual, biennial, or perennial, just like ornamental and crop plants. Annuals are plants that complete an entire life cycle in just one year. This group of weeds is further divided into summer and winter annuals. Winter annual weeds will germinate in autumn, live through the winter months, and produce seeds in late winter and springtime. Summer annuals, on the other hand, will germinate around early March and continue growing through the summer months, releasing seeds in late summer or fall. Knowing when the seeds of certain species will germinate is an important piece of information, because it informs how and when treatment is applied. Biennial weeds will complete their life cycle in two years' time, producing vegetative growth in the first year and reproductive growth in the second year. These weeds are not usually considered to be a serious problem in gardens, as they are usually pulled up in their first year before they can set and release their seeds. Perennial weeds live for over two years, and are the most difficult to eradicate. They usually have more extensive root systems than annual or biennial weeds and most produce seeds each year, meaning that they can reproduce very well. In this perennial group, there are also many weeds that will spread via underground rhizomes tubers, stolons, or bulbs.
Third, and finally, weeds can be classified as either broadleaf (dicots) or grassy (monocots). Professional weed specialists will commonly classify weeds as annual or perennial broadleaves or annual or perennial grasses. This classification seriously narrows down the field when searching for a weed's identity. Two less commonly used classification systems that concern weeds are their regulatory status (how they are treated under the law) and their use (some weeds are edible or poisonous to humans or livestock).
weeds: the pros and cons
Believe it or not, there are both pros and cons to weeds growing in the garden. The major downside of weeds is that they compete with intentionally grown crops and other ornamentals for water, mineral nutrients, sunlight, carbon dioxide and oxygen. Weeds can outcompete other plants due to their resiliencies. They seed prolifically, and seeds can be dormant for long periods of time before the correct environmental conditions are met for germination. They can also produce underground structures like rhizomes and bulbs that can withstand environmental stress and survive for years underground before re-sprouting. Weeds are more tolerant of heat, drought, and floods than other plants as well. Another large issue with weeds in the garden is that they can serve as host plants for many insect pests and pathogens, allowing them to survive in the area while their preferred host crop is dormant. Weeds also provide coverage and food for vertebrate pests, like rodents, that are unwanted in the garden. Finally, weeds can cause allergies in many people. Weed pollen contributes to Hay Fever, and some weed foliage (as in that of Poison Oak plants) causes skin dermatitis.
Although weeds do have many downsides, they can also be beneficial to garden spaces in a few ways. Weeds can reduce soil erosion and dust in areas of the garden that are uncultivated, providing free coverage for these bare areas. Weeds also provide coverage and a food source for many beneficial garden insects and animals. Furthermore, weeds can provide a nutrient source, adding additional organic matter to soil when they perish. Finally, some weeds can be eaten as a food source by humans. This group of edible weeds includes purselanes, lambsquarters, dandelions, and nettles. Overall, weeds usually do more harm than good, and good horticultural practices should be employed to promote favorable conditions for desired plants and discourage weed growth.
A Bit ABout Invasive SPecies
Invasive plants represent a distinct group of weeds that occurs in natural habitats where they are not native. They are also called 'noxious' plants. These weeds infest natural landscapes, degrading their ecosystems. The main difference between these plants and regular weeds is that they can disperse, establish, and spread in uncultivated areas, where they are much more problematic than they are in gardens. Many invasives were intentionally brought into areas via the horticultural trade, due to a lack of understanding of how they could potentially affect (and in most cases, severely damage) local ecosystems. Invasive plants displace native species, altering habitats for birds, insects, and other wildlife. These plants can also affect livestock forage quality, jeopardize animal and human health, and increase the threat of fire or flooding. Aquatic invasive weeds, for example duckweed, can impact the movement and navigation of water vessels, clog up irrigation and other plumbing systems, and impede livestock's access to clean, drinkable water.
In California, we boast the greatest amount of natural biodiversity of any other state in the country. We have around 5,000 native plant species, and unfortunately we also have somewhere around 1,300 non-native species that have taken up residence and enjoy our mild Mediterranean climate. Of the native species, 200-300 are weeds of agricultural crops, turf, or home gardens. The remaining thousand have naturalized themselves in our wildlands, and some of these are important threats to our native ecology. The nonprofit organization The California Invasive Plant Council (CAL-IPC) maintains and publishes online a useful inventory of invasive species in the state, using a process based on 13 criteria. Their website lists about 200 species as threats to our wildlands, and continues to monitor new invasives as they take root in our state. Their website can be visited by clicking the link below:
Cal-IPC promotes horticultural alternatives to planting invasive species that have the same beneficial qualities as invasives without the danger of invasiveness. Another organization, Plant Right, also has valuable lists and publications on their website towards the same end. To visit their site, click the link below:
It is important to recognize the illegality of bringing in plants or seeds from foreign countries without proper authorization and inspection. It is a federal offense, but it is also an easy mistake to make, as seeds of invasive plants are available through mail-order or over the internet from out-of-state sources. Because of this, it is important to research in-depth any plant parts or seeds obtained from out-of-state sellers before purchasing and importing them.
Dealing with weeds in the home garden
Weed seeds are usually already present in soil before any cultivation is begun in a garden space. There can be as many as 140 viable weed seeds in a given pound of surface soil (about 200 million seeds per acre of land). This content of weed seeds in garden soil is called the soil seed bank. Weed seeds can also make it into prepared packages of crop seeds or other seed mixes inadvertently, adding to their potential spread. Reseeding is one of the top skills of weedy plants, and they have several interesting and effective ways of dispersing their seeds. As with many plants, animals are effective spreaders of seed via their excreta. Many weeds are also well-adapted to disperse their seeds using the wind or latching on to passing humans' or animals' clothing and fur. An important weed seed spreader to acknowledge is the home gardener themselves. Many of our tools and wearable garden gear becomes contaminated with weed seeds as we work within our yards. Cleaning tools, shoes, used containers, gloves, and tires can do a lot of good in the fight against weed seed spread. There are many ways to control the spread of weeds, from chemical and physical treatments of existing weeds to preventive measures and correct gardening cultural practices.
Principles of Weed Management
The main principle of weed management is very straightforward: remove unwanted plants and prevent new ones from growing. In practice, however, weeds are very difficult to eradicate because they are so resilient and well-adapted to garden environments. Soil seed banks can remain dormant for long periods of time, making their control a continuous part of gardening. The objective of controlling annual weeds is to prevent seed production and deplete the soil seed bank on the property. For perennial weeds, the objective is to eradicate underground vegetative reproductive organs and prevent seed production. The goal in any created weed management program is to select a practice that is least harmful to the biodiversity of the garden. This will protect the health of both intentionally grown plants as well as the critically important insects and animals that call your garden home. It is also to apply your chosen method at the correct time when it will be most effective.
Methods of Weed Control in the Home Garden
The home gardener has a few tried-and-true methods for weed control to choose from when creating a holistic weed program for their landscape. These methods are categorized as cultural, mechanical, physical, and chemical. The best overall strategy to use depends on many factors unique to each garden space, and is called Integrated Pest Management, or IPM. This method is woven in to the many tasks and practices employed by gardeners, and allows for the use of multiple strategies for long-term management of weeds (and other pests, as well). A central goal of any IPM system is to minimize the impact treatment has on human health, the environment, and nontarget organisms (the good insects and other organisms living in the garden). An integrated pest management system brings together the following methods for weed eradication.
Principles of Weed Management
The main principle of weed management is very straightforward: remove unwanted plants and prevent new ones from growing. In practice, however, weeds are very difficult to eradicate because they are so resilient and well-adapted to garden environments. Soil seed banks can remain dormant for long periods of time, making their control a continuous part of gardening. The objective of controlling annual weeds is to prevent seed production and deplete the soil seed bank on the property. For perennial weeds, the objective is to eradicate underground vegetative reproductive organs and prevent seed production. The goal in any created weed management program is to select a practice that is least harmful to the biodiversity of the garden. This will protect the health of both intentionally grown plants as well as the critically important insects and animals that call your garden home. It is also to apply your chosen method at the correct time when it will be most effective.
Methods of Weed Control in the Home Garden
The home gardener has a few tried-and-true methods for weed control to choose from when creating a holistic weed program for their landscape. These methods are categorized as cultural, mechanical, physical, and chemical. The best overall strategy to use depends on many factors unique to each garden space, and is called Integrated Pest Management, or IPM. This method is woven in to the many tasks and practices employed by gardeners, and allows for the use of multiple strategies for long-term management of weeds (and other pests, as well). A central goal of any IPM system is to minimize the impact treatment has on human health, the environment, and nontarget organisms (the good insects and other organisms living in the garden). An integrated pest management system brings together the following methods for weed eradication.
Cultural Methods of Weed Control
These techniques modify the garden environment, giving intentionally planted flora an upper hand and decreasing the competitive edge that weeds bring to the table. The methods included in this category are foundational practices that can be incorporated into the care of any garden, and include proper soil preparation, soil testing, irrigation management, correct plant selection, crop rotation, proper lawn mowing height, thatch control, and reduction of soil compaction activities. These controls can eliminate about 70% of weed problems in the landscape, but are often overlooked due to their somewhat time-intensive qualities. |
Physical and Mechanical Methods of Weed Control
These are some of the most commonly used methods of weed control used in gardens today, and include hoeing/cultivation, hand-pulling, rototilling or disking, and mowing or chopping. Physical barriers are included in this category, as well. These controls work well with annual weeds, as long as the weeds are cut at or below the soil line before they set seed. Perennial weeds will require more work, as they are more difficult to physically control due to their underground storage organs. Working a physical weeding time into an ongoing garden task list is essential for controlling the population of weeds in a garden space, and only requires a small time investment each week. A great project to engage in while prepping an area for cultivation or installation is to get the existing weed seeds in the soil to germinate and sprout, and then physically pull them out of the ground while they are still young. This method is called the 'water, wait, cultivate' method, and is very effective.
Another method of weed control that is widely used to both increase the aesthetic beauty of a garden space as well as suppress weed growth is the use of mulches. Generally speaking, a mulch is a layer of opaque material applied over the soil surface. These materials block light from reaching weed seeds, and therefore discourage germination. These materials can be either synthetic (for example, plastic sheeting or rock applied over plastic sheeting) or organic (for example, bark, compost, hay, straw, or rice hulls). When a material is mixed into the soil, it is no longer considered a mulch, it is then called an amendment. The coarser the material used, the more effective it will be at combatting weeds. Organic mulches will also have the added benefit of cooling the soil, but should not be placed too close to the trunks and low branches of plants, as this can create a very favorable environment for disease to take hold. As a general rule, mulch should be about 1-3 inches deep for finer materials (such as sawdust or grass clippings) and 3 to 6 inches deep for coarser materials (such as bark, straw, or shredded plant matter). Inorganic mulches include weed blocks, weed barriers, polyethylene plastic and landscaping fabric. They can be expensive, but can provide many years of protection in a garden bed.
These are some of the most commonly used methods of weed control used in gardens today, and include hoeing/cultivation, hand-pulling, rototilling or disking, and mowing or chopping. Physical barriers are included in this category, as well. These controls work well with annual weeds, as long as the weeds are cut at or below the soil line before they set seed. Perennial weeds will require more work, as they are more difficult to physically control due to their underground storage organs. Working a physical weeding time into an ongoing garden task list is essential for controlling the population of weeds in a garden space, and only requires a small time investment each week. A great project to engage in while prepping an area for cultivation or installation is to get the existing weed seeds in the soil to germinate and sprout, and then physically pull them out of the ground while they are still young. This method is called the 'water, wait, cultivate' method, and is very effective.
Another method of weed control that is widely used to both increase the aesthetic beauty of a garden space as well as suppress weed growth is the use of mulches. Generally speaking, a mulch is a layer of opaque material applied over the soil surface. These materials block light from reaching weed seeds, and therefore discourage germination. These materials can be either synthetic (for example, plastic sheeting or rock applied over plastic sheeting) or organic (for example, bark, compost, hay, straw, or rice hulls). When a material is mixed into the soil, it is no longer considered a mulch, it is then called an amendment. The coarser the material used, the more effective it will be at combatting weeds. Organic mulches will also have the added benefit of cooling the soil, but should not be placed too close to the trunks and low branches of plants, as this can create a very favorable environment for disease to take hold. As a general rule, mulch should be about 1-3 inches deep for finer materials (such as sawdust or grass clippings) and 3 to 6 inches deep for coarser materials (such as bark, straw, or shredded plant matter). Inorganic mulches include weed blocks, weed barriers, polyethylene plastic and landscaping fabric. They can be expensive, but can provide many years of protection in a garden bed.
Mulches are effective for two reasons: they block the sunlight required for the germination of weed seeds, and they also prevent already germinated seeds from reaching an environment where weed seedlings can get adequate light for growth. Black plastic is best for weed control, not clear. Plastic mulches require some additional work irrigation-wise; they do not allow water to infiltrate the soil from above, and so drip irrigation systems need to be installed underneath. These coverings can also interfere with the gas exchange below the soil, encouraging root diseases. The choice between using organic and inorganic mulches is a personal one, and in some cases a combination of mulches are used in the garden. Adding mulch to a garden is a critical step in suppressing weeds, and it also has the added benefit of reducing surface evaporation of soil water, and can raise or lower soil temperature. Organic mulches lower soil temperature, which slows plant growth a bit in the springtime. Another point to consider when choosing a mulch is that when mulches are used in a garden, irrigation needs to be tweaked a bit. Because mulches increase soil moisture retention, irrigation should be lessened somewhat to avoid overwatering and inviting the development of a root rot fungal issue.
Solarization of the soil is an alternative to using mulch, and uses a clear polyethylene plastic sheet (1-4 mil thickness) weighted down on all sides. The purchased plastic sheeting should be treated with a UV light inhibitor; this quality increases the longevity of the clear plastic. The plastic sheeting must be left in place for 4-6 weeks in the summertime (June, July, or August). The more hours the clear plastic is left covering the soil where temperatures are at or above 95 degrees Fahrenheit Before laying down the plastic, ensure that soil is loosened, moist, and graded. This technique controls many weeds, and also can effectively stave off certain soil-borne diseases like Verticillium wilt and nematodes. Weed species that are most effectively controlled using soil solarization include annual bluegrass, annual sowthistle, barnyard grass, Bermuda buttercup, cheeseweed, chickweed, henbit, lambsquarters, prickly lettuce, pigweed, and shepherd's purse. It is also effective against bermudagrass, creeping woodsorrel, bindweed, crabgrass, purselane, and wild oats. It is not effective with sweetclover, burclover, and filaree.
Biological Methods of Weed Control
These methods discourage weed growth through the use of natural predators (like insects and diseases) that feed on or can infect weed plants and their seeds, thereby reducing the population enough to be manageable. This control strategy is appealing due to its use of already existing, natural processes, but it is primarily only used in naturalized, wildland areas rather than home gardens and landscaped areas. PUNCTUREVINE/YELLOW STARTHISTLE.
These methods discourage weed growth through the use of natural predators (like insects and diseases) that feed on or can infect weed plants and their seeds, thereby reducing the population enough to be manageable. This control strategy is appealing due to its use of already existing, natural processes, but it is primarily only used in naturalized, wildland areas rather than home gardens and landscaped areas. PUNCTUREVINE/YELLOW STARTHISTLE.
Chemical Methods of Weed Control
Herbicides are considered only when all other methods have failed, and are organic or inorganic substances used to kill plants. These chemicals are applied as foliar sprays or as dry granules. When used properly, these methods are very effective, though not all weed species can be controlled using herbicides. A successful and safe weed control program that uses chemicals as a component of control (and not the only control) are most effective. When incorporating chemical controls, make sure to read any and all instructions thoroughly, as they will provide critical information about application, effectiveness, and toxicities. Labels will also warn users about environmental conditions that should be avoided to minimize herbicide contamination of surface water and groundwater, herbicide drift, and poor efficacy. Knowing the terminology used on herbicide labels can give a gardener a head-start on choosing a product or method that is best for their garden.
Herbicides are considered only when all other methods have failed, and are organic or inorganic substances used to kill plants. These chemicals are applied as foliar sprays or as dry granules. When used properly, these methods are very effective, though not all weed species can be controlled using herbicides. A successful and safe weed control program that uses chemicals as a component of control (and not the only control) are most effective. When incorporating chemical controls, make sure to read any and all instructions thoroughly, as they will provide critical information about application, effectiveness, and toxicities. Labels will also warn users about environmental conditions that should be avoided to minimize herbicide contamination of surface water and groundwater, herbicide drift, and poor efficacy. Knowing the terminology used on herbicide labels can give a gardener a head-start on choosing a product or method that is best for their garden.
Preemergent vs. Postemergent
Herbicides are preemergent if the application is made before the weed seeds germinate. They are referred to as soil-residual herbicides because they prevent germination of seeds and inhibit young seedling growth for a period of time. They must be 'worked in', meaning they must be moved into the germination zone in soil by mechanical incorporation, irrigation or rainfall. Control provided can last anywhere from a few weeks to a few years, depending on the particular chemical, the rate at which it is used, and soil characteristics. Certain types of preemergent herbicides need to be activated at certain times after application (for example, 24 hours, 7 days, or three weeks). Postemergent herbicides are applied to weed plants' foliage and require a period of no overhead water (usually between 1 and 24 hours). Most of these herbicides do not have any associated soil activity. Examples of postemergent herbicides are 2, 4-D, diquat dibromide, fluazifop, glyphosate, MCPP, fatty acid soaps, and triclopyr.
Herbicides are preemergent if the application is made before the weed seeds germinate. They are referred to as soil-residual herbicides because they prevent germination of seeds and inhibit young seedling growth for a period of time. They must be 'worked in', meaning they must be moved into the germination zone in soil by mechanical incorporation, irrigation or rainfall. Control provided can last anywhere from a few weeks to a few years, depending on the particular chemical, the rate at which it is used, and soil characteristics. Certain types of preemergent herbicides need to be activated at certain times after application (for example, 24 hours, 7 days, or three weeks). Postemergent herbicides are applied to weed plants' foliage and require a period of no overhead water (usually between 1 and 24 hours). Most of these herbicides do not have any associated soil activity. Examples of postemergent herbicides are 2, 4-D, diquat dibromide, fluazifop, glyphosate, MCPP, fatty acid soaps, and triclopyr.
Contact vs. Systemic
Foliar applied herbicides can be either contact or systemic. Chemicals that only kill the parts of the plant that they touch are called contact herbicides. If enough of the plant is killed, the plant will die. They are most effective on younger plants that do not have the ability to bounce back from damage as effectively. Examples include diquat dibromide, plant oils (clove oil or vinegar), and petroleum distillates. Systemic herbicides, on the other hand, are absorbed into the plant and moves through its conductive tissues. Foliar-applied herbicides of this type will move from leaves to other plant parts, but have the strongest effect on the areas on which they are applied. Control of established perennial weeds can be achieved using systemic herbicide with repeated treatments. Examples of systemic herbicides are glyphosphate, fluazipfop, and 2,4-D.
Foliar applied herbicides can be either contact or systemic. Chemicals that only kill the parts of the plant that they touch are called contact herbicides. If enough of the plant is killed, the plant will die. They are most effective on younger plants that do not have the ability to bounce back from damage as effectively. Examples include diquat dibromide, plant oils (clove oil or vinegar), and petroleum distillates. Systemic herbicides, on the other hand, are absorbed into the plant and moves through its conductive tissues. Foliar-applied herbicides of this type will move from leaves to other plant parts, but have the strongest effect on the areas on which they are applied. Control of established perennial weeds can be achieved using systemic herbicide with repeated treatments. Examples of systemic herbicides are glyphosphate, fluazipfop, and 2,4-D.
Selective vs. Nonselective
These descriptions refer to the type of plant life that are affected by a particular herbicide. Nonselective herbicides should only be used when all vegetation in an area is to be eradicated. Selective herbicides will kill certain types of plants, and are meant to be used in areas where weeds are competing with other plant life. Uninjured crop plants are said to be 'tolerant' of the selective herbicide, and the weed species targeted by it is said to be susceptible. Generally, contact herbicides are nonselective and systemic herbicides are either selective or nonselective.
These descriptions refer to the type of plant life that are affected by a particular herbicide. Nonselective herbicides should only be used when all vegetation in an area is to be eradicated. Selective herbicides will kill certain types of plants, and are meant to be used in areas where weeds are competing with other plant life. Uninjured crop plants are said to be 'tolerant' of the selective herbicide, and the weed species targeted by it is said to be susceptible. Generally, contact herbicides are nonselective and systemic herbicides are either selective or nonselective.
Selecting the right herbicide for a weed issue is a critical step in controlling weeds in the landscape. Selection of an herbicide requires knowledge of the characteristics of herbicides, the environment (including soil characteristics and climate), and the weed-crop complex, which refers to the plants to be controlled in an existing landscape area or crop field. Several questions need to be answered to determine which herbicide will be appropriate for the specific situation.
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- What weed species are to be controlled? What are their life cycles and growing seasons?
- Are the weeds present in a non-crop area, turf, or ornamental site?
- Is the herbicide in question registered for the intended application (referring to the site it is to be used on as well as the plants that are there?
- What are the soil characteristics in the area to be treated?
- What are the environmental conditions in the area to be treated? What is the temperature range, rainfall amount, soil moisture present, and sun exposure?
- What type of herbicide application equipment is available for use?
- What is the desired duration of weed control?
- If a preemergent herbicide has been chosen, how and when will it be irrigated or incorporated into the soil?
- Would it be easier and simpler to just remove the weeds by hand?
Herbicide Application Methods
Herbicides, when used, need to be applied uniformly over the treatment area at the correct rate of application. They can be applied using granule spreaders (handheld or push), hand-operated compressed-air pumps (backpack sprayers), or power sprayers. Herbicide labels will have a recommended method of application on the container; it is of utmost importance that these instructions be read and fully understood before use of the product inside. An even travel speed, regardless of the method of application, is key, as well as using the proper nozzles and the correct spraying height. Backpack sprayers are the most effective in small areas and for scattered patches of weeds. Regardless of the method used, spray equipment must be calibrated accurately if liquid herbicides are to be used safely and effectively.
Herbicides, when used, need to be applied uniformly over the treatment area at the correct rate of application. They can be applied using granule spreaders (handheld or push), hand-operated compressed-air pumps (backpack sprayers), or power sprayers. Herbicide labels will have a recommended method of application on the container; it is of utmost importance that these instructions be read and fully understood before use of the product inside. An even travel speed, regardless of the method of application, is key, as well as using the proper nozzles and the correct spraying height. Backpack sprayers are the most effective in small areas and for scattered patches of weeds. Regardless of the method used, spray equipment must be calibrated accurately if liquid herbicides are to be used safely and effectively.
Rates for herbicide use are generally given in ounces or fluid ounces per 1,000 square feet. Water is often the liquid vehicle that helps to spread the herbicide evenly. Premixed granular and liquid ready-to-use formulas are popular amongst home gardeners, because they eliminate calculations, use less-complex application methods, and eliminate the mixing and disposal steps required by other types of herbicides. Granular herbicides are often sold in pre-mixed bags alongside fertilizers, and are created for use on turf areas.
Weed Susceptibility to Chemical and Nonchemical Methods
Although they are commonly marketed as such, herbicides are not the cure-all for every weed problem. Both selective and non-selective herbicides will not work on all weed species. It is critical to the process to understand which weeds are needing to be eradicated.
Although they are commonly marketed as such, herbicides are not the cure-all for every weed problem. Both selective and non-selective herbicides will not work on all weed species. It is critical to the process to understand which weeds are needing to be eradicated.
Weed Management checklists
Turf Weed Management
Keeping a healthy lawn will discourage weeds from emerging simply due to the fact it encourages a highly competitive environment for weeds to attempt growing in. The following guidelines can ensure that turf areas of the garden are kept weed-free:
Keeping a healthy lawn will discourage weeds from emerging simply due to the fact it encourages a highly competitive environment for weeds to attempt growing in. The following guidelines can ensure that turf areas of the garden are kept weed-free:
- Properly manage irrigation. Ensure that the amount, frequency, uniformity, and time of day of water application are appropriate for the area. Overwatering and frequent watering can both encourage seedlings of crabgrass and other annual weeds to sprout and thrive. Underwatering also carries problems; it stresses the turf grass and welcomes drought-tolerant broadleaf weeds to pop up.
- Evaluate and correct problems associated with soil acidity, salt level, and water infiltration.
- Properly apply selected fertilizers. Fertilizing at times that are not conducive to turf growth can encourage weeds to take hold. Turf will need adequate fertilizing to be able to successfully compete with weeds.
- Do not apply thatch more than 1/2 inch thick; this can interfere with water infiltration into the soil and can increase stress on the turf grass.
- Mow at a height that is correct for the season and turf species. Summer weeds will gain a competitive edge if cool-season turfgrasses are mowed too short (below 1.5-2 inches).
- Select the correct turfgrass for the garden. Turf-type tall fescues have fewer weed issues than bluegrass-ryegrass mixes and Bermuda lawns because they can be mowed higher and the plant swill shade the soil more, excluding light to weeds and seeds under the soil surface. It is essential to choose a turf species that grows vigorously in your location and under the local conditions present (including shade, soil type, drainage and climate).
- Prevent weeds from taking hold by mowing flowers down before they mature and set seed.
Ornamentals and Weed Management
For managing weeds around ornamental plantings, there are many techniques that can be considered, including:
For managing weeds around ornamental plantings, there are many techniques that can be considered, including:
- Use mulch properly around the bases of plants. The mulch will suppress weed growth, whether it be organic mulch, landscape fabric, or stone (or a combination). The weeds that do pop through are easily hand-pulled.
- Herbicides can be used in ornamental beds, though they can 'drift' through the air or water to areas not meaning to be treated. Always read and follow all label directions.
- Properly manage irrigation. Ensure that the amount, frequency, uniformity, and time of day of water application are appropriate for the area. Overwatering and frequent watering can both encourage seedlings of crabgrass and other annual weeds to sprout and thrive. Underwatering also carries problems; it stresses the ornamental plants and welcomes drought-tolerant broadleaf weeds to pop up.
- Groundcover plants can be used to shade the soil, discouraging weed seeds to germinate and not allowing weed seedlings to get the proper amount of sunlight needed for regular growth.
Vegetable Garden Weed Management
Herbicides are not recommended for use in vegetable gardens, so the methods used in these areas are mainly physical - mechanical and cultural. This prevents any unwanted, dangerous chemicals from touching vegetables grown for consumption. Some accepted and widely used methods for controlling weeds in vegetable gardens include:
Herbicides are not recommended for use in vegetable gardens, so the methods used in these areas are mainly physical - mechanical and cultural. This prevents any unwanted, dangerous chemicals from touching vegetables grown for consumption. Some accepted and widely used methods for controlling weeds in vegetable gardens include:
- Use mulch properly around the bases of plants. The mulch will suppress weed growth, whether it be organic mulch, landscape fabric, or stone (or a combination). The weeds that do pop through are easily hand-pulled.
- Solarize areas to be used as vegetable plantings before installing plants. This will discourage both weed seed germination and other soil issues that can cause problems for crops.
- Properly manage irrigation. Ensure that the amount, frequency, uniformity, and time of day of water application are appropriate for the area. Overwatering and frequent watering can both encourage seedlings of crabgrass and other annual weeds to sprout and thrive. Underwatering also carries problems; it stresses the ornamental plants and welcomes drought-tolerant broadleaf weeds to pop up.
Common garden weeds in california and recommended controls
The following list includes photographs, descriptions, and treatments for some of the more commonly-encountered weeds in California gardens.
Bellis perennis (English Daisy)
This perennial is low-growing and has oval basal leaves and produces white or pink flowers. For control, apply a postemergent herbicide or physically dig out plants. |
Convolvulus arvensis (Field Bindweed)
This perennial is very deep rooted (6-10 feet down into the soil) and has twining stems and white or pink flowers. Flowers resemble those of Morning Glory. To control, cultivate repeatedly to starve roots over 1-2 years; apply a postemergent herbicide. |
Cynodon dactylon (Bermudagrass)
This perennial is prostarte with a spreading habit (rhizomes and stolons). Leaves have a tuft at their bases. Used as turfgrass. To control, raise mowing height to more than 1.5 inches in cool-season turfs. |
Cyperus esculentus (Yellow Nutsedge)
This perennial goes dormant in wintertime, and spreads by seeds and tubers. Grassy, stiff and upright foliage. One plant can create 400 new tubers in just one year. To control, remove plants frequently to starve nutlets, and apply a postemergent herbicide. |
Oxalis corniculata (Creeping Woodsorrel)
Perennial with runners that prefers shady conditions, Leaves are similar to clover and have a sour taste. Seeds are ejected 10-13 feet out from the parent plant. To control, encourage a healthy turf and water properly. A preemergent or postemergent herbicide can be applied, as well. |
Paspalum dilatatum (Dallisgrass)
This perennial weed has a spreading crown, growing via rhizomes. Goes dormant in wintertime. Control includes removing established clumps with a shovel or treating with postemergent herbicide. In February or March, a preemergent herbicide can also be used. |
Pinnisetum clandestinum (Kikuyugrass)
This prostrate perennial has thick rhizomes and stolons, and is difficult to control, even when using both preemergent and postemergent herbicides. |
Rumex crispus (Curly Dock)
Grown from a large, thick taproot with wavy leaves growing from a basal rosette. Control includes removing the taproot and applying postemergent herbicides. |
Digitaria spp. (Crabgrass)
This summer annual spreads and roots at nodes. Appears commonly in overwatered turfgrass. Control includes watering deeply and infrequently, and applying a preemergent herbicide in February or March. |
Euphorbia maculata (Spotted Spurge)
This summertime annual has milky sap and leaves with red spots in the center. Control includes using mulch, and mowing grass at recommended height. Pre and post emergent herbicides can also be used. |
Lolium multiflorum (Italian Ryegrass)
This wintertime annual has a glossy appearance and long seed stalks with spikelets. Control includes hoeing and mulching and application of preemergent herbicide in September. |
Malva parviflora (Little Mallow)
Winter annual or bi-annual plant with large, rounded leaves with red spots at blade bases. Common in turf. Control includes encouraging healthy turf and applying preemergent or postemergent herbicides. |
Poa annua (Annual Bluegrass)
This winter annual is a bunchgrass that is tufted and light green. Seeds will continue to form even if mowed as low as a quarter of an inch. Control includes reducing soil compaction, improving water drainage, and watering less frequently. Preemergent herbicide can also be applied in September. |
Polygonum arenastrum (Knotweed)
This summer annual forms a circular mat of foliage and can grow well in compacted, drought-affected soil. Controls include encouraging a healthy turf area and fertilizing and watering properly. |
Setaria glauca (Yellow Foxtail)
This summer annual is a bunchgrass with flat leaves and spikelets with bristles. Controls for this weed include mulching and hoeing the area, and applying preemergent herbicide after February. |
Picris echoides (Bristly Oxtongue)
This is a biennial plant that is prostrate for the most part with rough, hairy foliage. Flower heads are yellow. Controls include mulching and hoeing, though it is difficult to control without preemergent or postemergent herbicides. |
Plantago spp. (Plantain)
There are two weed species in this genus, both are perennial. Seed stalks are tall and skinny. Controls include hoeing or physically pulling up plants as well as application of postemergent, selective herbicides in turf areas. |
Portulaca oleracea (Purselane)
This summer annual has a prostrate habit and very fleshy stems and leaves. Produces pale yellow flowers and is edible. Controls include hoeing or pulling up plants. |
Stellaria media (Chickweed)
This winter annual has many-branched stems and opposite, pointed leaves. Controls for this weed include encouraging healthy turf growth, mowing at the proper height and hand pulling or hoeing plants. |
Veronica persica (Birdseye)
This weed is a winter annual with stems reaching 4-16 inches long. Leaves are rounded and flowers are tiny and blue with white centers. Controls include mulching and hoeing as well as applying a preemergent herbicide. |
resources and references
Learn about commonly encountered California garden weeds.
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How to identify weeds in a lawn area.
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How to physically remove weeds.
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How preemergent herbicides work.
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