Cosmos

( KOZ - mohs )

At-A-Glance Information

This genus of flowering plants includes both perennials and annuals and needs full sun and moderate water. The flowers will attract birds and butterflies to your garden, and are native to the tropical Americas (mainly Mexico). Blooming in summer and fall in a variety of colors and forms, they are great for mass color in border gardens, in the back of other shorter plants, or as filler amongst shrubs. They make beautiful cut flowers in arrangements when cut just after they bloom and placed in cool water. Cosmos are popular in cottage, meadow, and pollinator gardens.

Cosmos Species and Varietals

The genus Cosmos is a member of the plant family Asteraceae and includes just under 40 distinct species. Cosmos grow best in USDA hardiness zones 2-11 or Sunset growing zones 1-24, though this varies somewhat by species. See the descriptions below for more information. To find your zone, click here: Zone Map.

Cosmos atrosanguineus (Chocolate Cosmos, Black Cosmos)
This perennial grows from tuberous roots and grows best in USDA hardiness zones 9-11 or Sunset growing zones 4-9 and 14-24. Gets to 2.5 feet tall and 1.5 feet wide at maturity when in bloom. In late summer and fall, brownish-red flowers pop and have a strong chocolate or vanilla fragrance. They need well-draining soil and to be mulched in wintertime. These plants are native to Mexico, and are extinct in the wild due to over-harvesting for the flower trade.

Cosmos bipinnatus (Garden Cosmos, Mexican Aster, Cosmea)
This half-hardy annual can reach up to 8 feet tall and 2.5 feet wide. Flowers have tufted centers of yellow, and outer petals in white, pink, rose, lavender, purple, and crimson. The flowers will attract bees and other beneficial insects to your yard. Garden Cosmos grow well in USDA hardiness zones 2-11. Varieties are outlined below:

'Candystripe'
Reaches 3-4 feet tall and has white and rose colored flowers.

'Daydream'
With pale pink flowers that have yellow centers. To 3-5 feet tall.

'Dazzling'
Part of the Sensation Series; 3-6 feet tall with crimson blooms.

Doubleclick Series
4 feet tall with frilly double flowers in a range of colors.

'Gazebo'
Compact form that is good for containers with a mix of flower colors.

'Picotee'
Grows to 2.5 feet tall and has white flowers edged with red. Good for containers.

'Radiance'
Part of the Sensation Series, Flowers are rose with a red center.

'Sea Shells'
Grows to 3 feet tall and is grown for its unique, quilled flowers that mimic cones.

'Sonata'
Dwarf strain that gets to 2 feet tall

'Sweet Dreams'
Gets to 3 feet tall with pale pink to white blossoms that have rose centers.

Versailles Series
Cut flowers in many colors; 3.5 feet tall and holds blossoms on strong stems.

'Xsenia'
Large flowers that blend peach, rose and terra cotta hues. Great cut flower.

Cosmos caudatus (King's Salad, Estrella del Mar, Cambray, Flor de Muerto, Ulum Raja, Wild Cosmos, Kenikir)
This annual plant can produce flowers in white, pink, or purple and is native to Latin America. It grows well in USDA hardiness zones 9-11. These plants are considered edible and even palatable, commonly added to salads in their local ranges. Plants reach almost seven feet tall at maturity. This species is considered invasive in Southeast Asia, Africa, Australia, and the Pacific Islands.

Cosmos ochroleucoflorus
This species, native to Durango in Mexico, reaches about 32 inches tall at maturity. Plants will spread over time to form colonies via underground creeping rhizomes. Green foliage is undivided and narrow, and flowers are pink to lavender in color. These flowers thrive in USDA hardiness zones 2-11.

Cosmos pacificus
Native to southwestern and central-western Mexico, these flowering herbaceous annual species form branching plants up to 80 inches tall. Fern-like leaves are highly lobed and dissected. Flowers are pink to lavender. This species grows well in USDA hardiness zones 2-11.

Cosmos parviflorus (Southwestern Cosmos)
These plants are commonly considered to be weeds, and are native to the southwestern United States. They do well in USDA hardiness zones 2-11. Southwestern Cosmos reaches up to 40 inches tall when in bloom, and produces red, pink, or white flowers (flower color will sometimes differ within a given population). Foliage is deeply divided. These plants are potentially invasive in the Northeastern United States.

Cosmos sherffii (Sherf's Cosmos)
This species of Cosmos is native to Oaxaca, Mexico and reaches 32 inches tall when in bloom. It does well in USDA hardiness zones 2-11. Underground roots of this species are thick and tuberous, making these plants relatively drought-tolerant choices for water-wise gardeners. Blooms are produced singly on thin stems, and foliage is heavily dissected. Flowers are pink with yellow centers.

Cosmos sulphureus (Yellow Cosmos, Sulfur Cosmos, Orange Cosmos, Klondike Cosmos)
This half-hardy annual reaches to 7 feet tall and 2.5 feet wide when in bloom, and has yellow-centered yellow or orange blooms. These plants are native to Mexico, Central America, and northern areas in South America. Can get weedy-looking at the end of the season, and are considered invasive in some regions due to this quality. Consult your local authority before purchasing or installing these flowers. This species is considered invasive in the Southeastern United States, Africa, Asia, and various Caribbean Islands. Varieties are outlined below.

Bright Lights Series
3-4 feet tall, semidouble flowering with yellow, gold, orange, or orange-red blooms.

Klondike Series
3-4 feet tall, with semidouble flowers in orange and yellow hues.

'Dwarf Klondike'
1.5 feet tall.

'Sunny'
1.5 foot tall strain.

'Ladybird Mix'
1-foot tall with semidouble flowers in scarlet, yellow or orange.

'Sunny Red'
Orange-red semidouble blooms on a 1 foot tall plant.

'Yellow Garden'
Gets to 3 feet tall and wide, with light yellow blooms with white in the center and at the edges of petals.

Caring for Cosmos

Cosmos can be grown from seed (either direct-sown into the garden or pre-germinated indoors). They have a tendency to self-sow freely, so make sure they are planted in an area of the landscape where this will not be a problem. Plant Cosmos in an area of the garden that receives full sun; between 6 and 8 hours each day is ideal for the best blooms. In areas that have extremely hot climates, these flowers will need some afternoon shade when temperatures are at their peak. These plants can be installed in the outskirts of the garden as they do not require any formal irrigation to do well. Ensure that soil is well-draining before planting. If local soils are particularly poor in nutrients or do not drain well, make sure to amend the planting area with plenty of organic compost before installing. Cosmos are best in a soil with a pH of between 6.0 and 8.0.

Provide water to these plants for the first year in the ground to assist them in developing a strong taproot system. After they are mature (around a year in the ground), they will not need as much water. After planting Cosmos, mulch around the base of the plant with organic mulching material to help retain soil moisture and suppress weed growth. Keep mulch away from the plant stems, as this can encourage an environment for root rot issues to take hold. Fertilizing these plants is not necessary; they can perform well even in poor soils. No pruning is needed other than to deadhead for a neater appearance. If plants are getting leggy, they can be sheared back by about half to maintain a bushier habit.

Propagation

Cosmos can be propagated either by using cuttings or from seed. To ensure that your propagated plant will have the exact same qualities and characteristics as the parent plant, use the cutting method. If you would like to experiment and see what plant you can grow from seed, just keep in mind that it will not necessarily be the same as the parent plant. To propagate your Cosmos flowers plant, follow the instructions below:

Propagating Cosmos by Cutting

Gather your materials. You'll need a clean, sanitized, sharp pair of garden shears, rooting hormone, a pencil, and a 3-inch pot filled with well-draining compost or other organic material. A good substrate for propagation of cuttings is a light blend of 1/3 pasteurized soil or compost, 1/3 sand, vermiculite or perlite, and 1/3 coconut coir or peat moss.
In spring or early summertime, choose the plant that you'd like to propagate. Ensure that this plant has had a good watering the night before, as this will improve the chances your cutting will take root. Cut one or more 6 inch portion(s) of the end of a non-blooming branch and strip the leaves off of the lower half. Make a hole in your planting medium by pushing your pencil into the soil. Dip the end of the stem in rooting hormone, and plant it in your pot. Fill in soil around the cutting to support it, but do not pack it down too much.
Moisten with water, and ensure that the pot has good drainage. Your pot should be kept in warm, sunny conditions indoors (and not in direct sunlight). Water whenever your soil is dry under the top layer. After about four to six weeks have passed, your cutting should be rooted. When you notice new growth, you'll know you have roots.
Keep your new plant in an area with bright light (though not direct sunlight) and temperatures between 55 and 70 degrees Fahrenheit. Ensure that the soil stays moist, but not wet, until the next spring, when you can transplant it to its new home. Ensure that the last frost of the year has occurred before doing this, as it can seriously damage your new flowers.

Propagating Cosmos by Seed

Keep in mind that the plant you get from seed will not necessarily mimic all of the characteristics of the parent plant. To propagate by seed, you will need a paper bag, a pot or tray filled with well-draining compost or other organic material, a spray bottle and a fork. The best potting medium for seed germination is a light blend of 1/3 pasteurized soil or compost, 1/3 sand, vermiculite or perlite, and 1/3 coconut coir or peat moss.
Harvest your seeds. The seed heads on Cosmos plants are pretty easy to spot, they're left behind after flowers have faded and look a bit like spiky balls. Once they have begun to dry out, you can harvest the seeds. Cut them off of the plant, placing them in a paper bag. Harvest as many as you can, and place the paper bag in a dry, cool, dark area. They will begin to release their seeds. Once they have been released, take them out of the bag and put them in a container so you can work with them.
Water your prepared seed tray or pot filled with soil, and ensure that good drainage is occurring. Place your seeds in your prepared pot or seed tray about a quarter inch under the soil surface. Cover them with soil and water with your spray bottle. Place your tray of seeds in a warm area that receives bright, indirect light. A greenhouse is the most ideal climate, but a planting tray inside a window will do as well. Germination usually takes 5-10 days from sowing.
Keep the seedling's potting medium moist, but not wet, by spraying the soil with your spray bottle. Keep your new plant in an area with bright light (though not direct sunlight) and temperatures between 55 and 70 degrees Fahrenheit. Ensure that the soil stays moist, but not wet, until the next spring (after the danger of frost has passed), when you can transplant it to its new home.

Cosmos seed heads look a bit like spiky balls.

Cosmos seeds are thin and blackish-brown.

Problems and Pests

Certain species of Cosmos are considered invasive in some areas of the world; see species descriptions above for more information. Check with local authorities before planting. Cosmos are trouble-free additions to the garden for the most part,

Aphids
Thrips
Leafhoppers
Plant Bugs
Powdery Mildew
Aster Yellows
Bacterial Wilt
Gray Mold

For more information on these issues, plus other frequently-encountered garden problems and how to solve and prevent them, consult the following links:

Pests & Pest Management

Plant Issues