Alnus

( AL - nus )

At-A-Glance Information

Commonly called Alder, Alnus is a genus of mostly deciduous trees and shrubs that can take sun or shade and regular to ample water. These often multi-trunked trees will attract birds to the garden. They love moist soil, and can do well in flood situations, making them fantastic for planting near creeks or lakes. Very fast-growing, males produce tassels of greenish-yellow catkins, and female flowers produce woody cones. These trees are useful as ornamentals in the landscape, providing lush foliage throughout the spring and summer months. Alders are also fantastic land reclamation plants, with many species serving as great nitrogen-fixing additions to the landscape.

alnus Species and Varietals

The genus Alnus includes around 35 species and is a member of the plant family Betulaceae, making it a relative of Birch. There are three sub-genera within the genus: Alnus, Clethropsis, and Alnobetula. Growing zones for Alnus depend on the species; for more information, see the descriptions below. To find your zone, click here: Zone Map.

Subgenus Alnus
These trees (and some shrubs) produce male and female catkins in fall which stay closed over the winter months, opening up in springtime for wind pollination. This subgenera includes between 13-15 species.

Alnus acuminata (Andean Alder, Aliso Andino, Aile)
This species is native to Mexico through Argentina in montane forest areas, and reaches up to 80 feet tall at maturity. Used as a timber tree in its native range. Foliage is green, and catkins are yellow to tan in color. This species does well in USDA hardiness zones 8-11.

Alnus cordata (Italian Alder, Neapolitan Alder)
This species grows best in USDA hardiness zones 5-9 or Sunset zones 2b-9 and 14-24. They are natives to Italy and Corsica, and have an upright growth habit when young, eventually transforming into 40-foot tall, 25-foot wide trees. Leaves are heart-shaped and rich, glossy green above with pale undersides. Briefly deciduous. Great for the Southwest United States except in high-desert climates.

Alnus glutinosa (Black Alder, Common Alder, European Alder)
These trees grow best in USDA hardiness zones 3-7 or Sunset zones 1-10 and 14-24. Native to Europe, North Africa, and Asia, it eventually reaches 70 feet tall and 30 feet wide. Coarsely toothed leaves are shiny dark green. Great as a screening plant. Variety 'Imperialis' has lighter green leaves than the species, and the foliage is deeply cut and fern-like.

Alnus incana (Grey Alder, Tag Alder, Speckled Alder, Hoary Alder, Hazel Alder, European Gray Alder, Thinleaf Alder, White Alder)
This species forms a multi-stemmed shrubby tree and has a wide native range in the northern hemisphere. These plants slowly spread by runners, and are common in swamps and wetland areas. Leaves are broad and green, and bark is smooth, grey and speckled. They can reach between 50 and 65 feet tall and can live for between 60 and 100 years. Provide full sun for this fast-grower that does well in poor soils. USDA hardiness zones 2-6.

Alnus japonica (Japanese Alder)
This deciduous species is native to Japan, Korea, Taiwan, eastern China, and Russia. These trees are tough and adapt well to a wide variety of growing conditions. They reach between 12 and 25 feet tall and can take frequent standing water around their roots. Full sun to partial shade is best. This species does well in USDA hardiness zones 4-8.

Alnus jorullensis (Mexican Alder, Evergreen Alder, Aile)
This evergreen to semi-evergreen species of Alder is native to eastern and southern Mexico, Honduras and Guatemala. These trees can reach between 65 and 85 feet tall, and also make great hedge plants, taking well to pruning and trimming. These plants prefer very moist soils, and can do well as ornamental plants in warm climates such as Southern California. Spreads to form colonies or stands. Mexican Alder does well in USDA hardiness zones 7-12.

Alnus oblongifolia (Arizona Alder, Mexican Alder, Aile)
This species reaches up to 75 feet tall at maturity, and is native to the southwestern United States into Mexico, where it grows along mountain ranges. Foliage is green and bark is light brown to whitish-grey. Trees have an upright habit and are common on rocky stream banks and canyons in their native range.

Alnus rhombifolia (White Alder, California Alder)
Grows best in USDA hardiness zones 8-11 or Sunset growing zones 1b-10 and 14-21, and is native to California's inland foothills as well as the mountains of Oregon, Washington, British Columbia, and Idaho. Fast grower to 50-90 feet tall and 40-60 feet wide, with branches that spread and droop at the tips. Coarsely toothed green leaves are pale green beneath. This species of Alder can tolerate heat and wind, but is susceptible to tent caterpillars, borers, and mistletoe in its native habitat.

Alnus rubra (Alnus oregona) (Red Alder)
Grows best in USDA hardiness zones 5-10 or Sunset zones 3-7 and 14-17, native to stream banks and marshes from Alaska down to Northern California. This is the most common Alder species in the Pacific Northwest, and can get to 90 feet tall and 20-30 feet wide, though it usually tops off at 45-50 feet. Bark is light grey and smooth, and dark green leaves are rust-colored and hairy underneath. Useful where underground water is somewhat saline, and is a favorite of tent caterpillars.

Alnus serrulata (Hazel Alder, Smooth Alder)
This thicket-forming species of Alder is native to North America's eastern and coastal regions. These are small trees or large shrubs, reaching 8-13 feet tall at maturity. Leaves are dark green, and flowers are reddish-green. Bark is grey to brown, smooth, and has a bitter taste to it. Hazel Alder does well in USDA hardiness zones 4-9, and is well-suited for rain gardens and erosion control.

Subgenus Clethropsis
This group of Alder are trees or shrubs that produce their male and female catkins in fall, expanding and pollinating in the same season. There are three species. including:

Alnus formosana (Formosan Alder, Taiwan Alder)
This species of Alder is endemic to Taiwan where it is found growing along riverbanks, and forms a medium-sized tree that reaches 66 feet tall with greyish-brown bark. Leaves are glossy and green and flowers appear from May through June. These trees do well in USDA hardiness zones 8a-11b.

Alnus maritima (Seaside Alder, Brook Alder, Beach Alder)
This species of shrub or small tree is native to the United States with distinct populations in Oklahoma, Delaware, Georgia, and Maryland. It is considered to be endangered in its native range due to habitat destruction. Leaves are green to dark green and catkins are cream-white to yellow-tan in color. These trees grow well in USDA hardiness zones 4-7.

Subgenus Alnobetula
These trees and shrubs produce male and female catkins in late springtime after leafout, expanding and pollinating in the same season. There are between one and four species in this group, including:

Alnus alnobetula (Green Alder, American Green Alder, Mountain Alder)
This large shrub or small tree is native to Europe, Asia, and North America and has shiny green leaves. These plants can be used for revegetation or soil revitalization purposes. These plants produce shallow root systems that sucker and spread and is fast-growing. Green Alder is best in USDA hardiness zones 2-8.

Alnus maximowiczii (Montane Alder, Maximowicz's Alder)
This tree Alder is native to Japan, Korea, and the Russian Far East, and prefers growing in alpine or mountainous regions. This species grows best in USDA hardiness zones 4-8 or Sunset growing zones A1 and 1-9. They reach up to between 10 and 30 feet tall when mature.

Alnus pendula (Drooping Alder, Rhombie Leaf Alder, Japan Alder)
This exceptionally attractive ornamental tree is native to Honshu and Hokkaido Islands of Japan (and possibly some areas of Korea, though it may be an introduced species there), and has dark grey-green teardrop-shaped foliage. It forms a large shrub or small tree that reaches between 20 and 26 feet tall at maturity. Best in USDA hardiness zones 5a-7b.

Caring for Alnus

These trees prefer full sun, but some can grow in partial shade (especially the shrubbier species), and love well-draining soil that is moist and fertile. Choose a space in the landscape that receives between 6 and 8 hours of sun each day, and provide shade for trees in the late afternoon hours when temperatures are at their peak, especially when trees are young. Ensure that soil is well-draining, rich in nutrients, and has a pH of between 5.5 and 7.2 (acidic to neutral). 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. Apply a layer of mulch around the planting area to suppress weed growth, maintain consistent soil temperatures, and to increase soil moisture retention. Ensure that mulch stays at least 4-5 inches away from the trunk. After planting, if the Alder tree is quite young, it may be a bit floppy; young Alders commonly benefit from staking or tying to support them until they are established enough to stand on their own. Water regularly, especially while young trees are establishing themselves. These trees do best when soil is kept moist; naturally, they are happy growing near bodies of water.

Fertilize yearly in early springtime with an application of 2 inches of compost around the base of plants. Pull mulch back before applying, work the compost gently into the soil a bit, water it in, and then replace mulch. These plants are not heavy feeders, and this should be sufficient to provide the necessary nutrients for the active growing and flowering seasons. Pruning Alder trees is simple, and is done mainly to remove suckers, crossing branches, and dead or dying wood. They are best pruned in August to avoid attracting borer insects. Although these are generally hardy trees, younger plantings may benefit from some winter protection in harsh climates. Wrapping trunks in burlap can ensure that young trees or newly pruned trees don't suffer from sunburn during the warmer, sunnier months of the year. If properly cared for, Alnus trees can live for between 40 and over 100 years in the landscape before needing to be replaced.

Propagation

Alnus trees and shrubs 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 Alder, follow the instructions below:

Propagating Alnus 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. If you don't have any potting medium on hand, you can make some. Just mix together equal parts (by volume) potting soil, vermiculite and perlite.
In spring or early summertime, choose the plant that you'd like to propagate. You'll be taking a softwood (greenwood) cutting from the tree or shrub; hardwood cuttings of Alder are quite hard to root. 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 3-4 inch portion(s) of the end of a non-blooming, greenwood 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 28 days have passed, your cutting should be rooted. When you notice new growth on your propagated Alder tree or shrub, 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 shrub.

Propagating Alnus 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 sealable plastic bag, horticultural sand, a pot or tray filled with well-draining compost or other organic material, a spray bottle and a fork. If you don't have any potting medium on hand, you can make some. Just mix together equal parts (by volume) potting soil, vermiculite and perlite. You will also, after a few weeks, need individual pots for each seedling that grows.
Harvest your seeds. The seed cones on Alder plants are pretty easy to spot, they're large and dangle from the branches, left behind after flowers have faded. Once they have dried out and begun to split open, you can harvest the seeds inside. 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, dark, cool area. They will begin to split open and release the seeds contained inside. Once they have been released, take them out of the bag and put them in a sealable container.
Alder seeds can be stored in sealed containers in a cool, dry place until about four weeks before they will be sown. April is a great month to sow these seeds. Four weeks before sowing, take the seeds out of storage and lay them out. Fill a bowl with clean, cold water and leave them for between 24 and 48 hours, then lay them out to dry. Next, put some moistened horticultural sand in a plastic bag and add the seeds, mixing them in to the sand before placing the sealed bag in the refrigerator. Leave for four weeks.
After four weeks has passed, prepare a seed tray or pot filled with equal parts potting soil, vermiculite, and perlite. Water your prepared seed tray or pot to ensure that good drainage is occurring. Take the seeds out of the bag, remove them from their sand bed, and plant them about a quarter inch to a half-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 a few weeks from sowing. Never allow the soil to dry out, keep it consistently moist. Once you begin to see seedlings sprout and they are able to be handled, transplant them to their own individual 3-inch pots, filled with the same potting medium as described in Step 1. Loosen the seedlings from the potting medium by digging down with the tines of your fork about an inch and a half, gently working the seedling up until it is out. Then, replant the seedling in its individual pot.
Keep the seedling's potting medium moist. 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 until the next spring (after the danger of frost has passed), when you can transplant it to its new home.

Alder cones will begin to turn brown, dry out, and open, spilling the seeds held inside. It is best to harvest them as they start to turn brown, but before most of them have dried out and opened.