How to Plant, Grow, and Care for American Mountain Ash Trees (2024)

If you love birdwatching and your yard needs some extra shade and color, an American mountain ash tree is the perfect addition to your landscape. Sorbus americana is a compact ornamental tree native to North America. It grows wild throughout the Great Lakes, New England, and the Appalachian Mountains and performs excellently in urban and suburban yards.

The thick clusters of showy white flowers are followed by vibrant red berries, adding aesthetic charm and wildlife value through summer. At the end of the season, its golden-orange autumn leaves captivate a landscape in the fall and the berries remain into winter. Mountain ash averages 15-35 feet at maturity and grows with a nice, rounded, open-top crown. Sometimes, it grows from a single trunk, but it commonly has a shrubby multi-stemmed base.

For gardeners in cool eastern regions from hardiness zones 2-6, this native tree will grow quickly without much fuss. Let’s dig into everything you need to know about growing a mountain ash tree!

Overview

Plant Type Native deciduous small tree or shrub Plant Family Rosaceae Plant Genus Sorbus Plant Species americana Hardiness Zone 2-6 Planting Season Spring	Plant Maintenance Low Plant Height 15-35 feet Fertility Needs Low Temperature Cold climates Companion Plants Balsam fir, American yew, honeysuckle, woodfern Soil Type Acidic, well-drained loamy or sandy	Plant Spacing 10-20 feet Watering Needs Moderate to high Sun Exposure Full sun to partial shade Lifespan 10-50 years Pests Borers, aphids, scale, deer Diseases Fire Blight, cankers

History and Cultivation

American mountain ash has a rich history and intriguing folklore. The plant is one of 7 mountain ash species native to the U.S., and the genus Sorbus includes over 100 or 250 species, depending on your definition of the genus. American mountain ash (Sorbus americana) is smaller and smooth-leaved compared to the larger, hairy-leaved European mountain ash (S. aucuparia). Here are more details of this gorgeous red-berried tree and its origins.

What is American Mountain Ash?

The native Sorbus americana tree is nicknamed American mountain ash, rowan tree, dogberry, roundwood, and sometimes just mountain ash. The deciduous perennial shrub or small tree is a member of the rose (Rosaceae) family.

It is called a mountain ash because its pinnately compound leaves of lance-shaped serrated leaflets resemble ash trees. However, it is not actually related to true ash trees of the genus Fraxinus, which are members of the olive (Oleaceae) family.

Where Does It Originate?

Sorbus americana grows wild in the Eastern U.S. from the Appalachian mountains of Georgia up to Maine and into Nova Scotia and Newfoundland. Its native range also extends through Minnesota, Illinois, Michigan, and Pennsylvania. It is a cool climate tree that prefers cold winters and moist soils in zones 2-6.

It thrives in moist, cool, acidic soils of swamps, bogs, and rocky outcrops. We don’t recommend it for hot, humid climates or southern gardens warmer than zone 6.

This wild native has been cultivated as an ornamental since 1811 and is commonly used in home landscapes and parks.

Folklore

Mountain ash trees are sometimes nicknamed “witchwood.” In 18th-century Europe, many believed European mountain ash trees could cast away witches and guard your home from evil. Some people would burn the twigs outside their houses or make necklaces of the ash wood to protect themselves. When colonizers came to America and discovered this tree covering the northern landscape, they believed they were heavily protected. The five-pointed star on the berry stalks represented good luck.

Old folklore aside, modern plantings of mountain ash are mostly good luck for native bees and birds. The showy white spring blossoms are magnets for bees, and the vibrant red summer berries draw in birds near and far. This wildlife-friendly tree is beautiful, functional, and important for local ecosystems.

Propagation

Mountain ash trees can be propagated by seeds (gathered from the berries), cutting, or bare root and container trees from a nursery.

Seed

Growing American mountain ash from seed is a fun and rewarding endeavor. It involves harvesting the ripe fruits (red berries) in early fall and extracting the seeds immediately. If you can’t separate the seeds right away, it’s best to store the fruits in the refrigerator to ensure good germination.

Be sure to get to the red berries before the hungry birds do! Use clean pruners to remove the clusters and collect them in a basket or harvest container.

Fall is the best time to plant Sorbus americana because this is when their seeds would naturally germinate in the wild. If you want to wait until spring, you’ll have to cold-stratify the seeds in a refrigerator for 1-2 months to mimic the outdoor winter conditions. Be sure to keep them in a bag or sealed container with damp sand or vermiculite blend so they don’t dry out.

When you’re ready to plant, soak the seeds for 24 hours. Sow the seeds 1/16 inch deep and cover very lightly. You can plant them directly in the ground or in a container with a blend of sand and peat moss.

Keep the seeds consistently moist, and be very patient. Avoid heavy watering, as this can wash away the seeds. Seeds germinate erratically and may take up to a year to emerge, so plant lots of them!

Cutting

If you have access to a wild mountain ash or a friend’s garden, these attractive native trees are easy to propagate by semi-hardwood cutting. Berry collection and seed germination can be time-consuming and erratic, so propagating by cutting is a quicker way to replicate Sorbus americana. Although you won’t get as much genetic diversity as seed propagation, you’ll have baby mountain ash trees in two to three months.

Begin with a young semi-mature mountain ash with lots of healthy new growth. Older trees have a harder time regenerating and producing new branches. The best time to take cuttings is in spring and summer when the current year’s growth has turned slightly hard.

When you look at your tree or shrub, you should be able to clearly differentiate between old growth and new growth. Semi-hardwood cuttings are like an intermediary between green (softwood) cuttings and brown (hardwood) cuttings. They are typically semi-flexible, partially green twigs that have started to semi-harden. Avoid brown old growth.

Give your cuttings a slight tug to check if they’ve formed roots. If there is a slight give, they are probably good to go. If they pull out completely, check that the bottom has not rotted. Cut off the base and repeat steps 6-11 if you find rotted areas.

Saplings grow relatively quickly and are ready to transplant in the ground as soon as they reach one to three feet tall. American mountain ash is a wild native tree that readily propagates under moist conditions in its native range.

Bare-root or Container Trees

For those seeking young trees that can quickly take off growing in their yard, your local nursery is the best option. Native plant nurseries in the Great Lakes, Northeast, and Appalachian regions are highly likely to carry American mountain ash.

The tree is usually available in three forms:

When picking out a nursery tree, be sure to consider:

• Size of the root ball: Can you handle planting it on your own or with help?
• Trunk health: Fire blight is a major issue on mountain ash trees, causing visible cankers that look like they’ve been burned. Avoid these plants at all costs.
• Shoots: Do the fresh green shoots grow straight upward? If they are wilted or bending downward in a hook-shape, the plant may have fire blight and should not be purchased.
• Foliage health: Are the leaves vibrant, green, and turgid? Avoid specimens with lots of yellow, wilted, or dead foliage.
• Pests and diseases: Check under the leaves for bugs and smell the root zone to ensure there are no signs of rot.

If you specifically want to plant the native variety, check that the species is Sorbus americana, not Sorbus aucuparia, which is the European variety.

Planting

American mountain ash is the perfect plant for a cool-climate front yard, a native plant garden, or an exposed slope. Here’s how to plant the trees and water them properly for a strong growth start.

How to Transplant

The best time to plant or relocate mountain ash trees is in early spring while they are still dormant. Before planting your seedlings or saplings, be sure that the location has full to partial sun and moist, acidic soil.

Young trees need a fair amount of moisture, but it’s important that the soil never gets soggy. The key is to water deeply (i.e., enough to fully saturate the newly planted rootball) but less frequently. Use your hand to check that the first round of irrigation has penetrated the soil at least 6-12 inches deep.

Spacing

Sorbus americana grows as a large multi-stemmed shrub or a small single-trunk tree. It requires at least 10 feet of space from other trees and shrubs. You can space them up to 30 feet if you’d like the tree to grow to its maximum height and spread.

How to Grow

This northeastern-native plant is easy to grow in cold climates from USDA 2 through 6. It takes off readily in the northernmost parts of its native range but may require a little more tending in urban environments. Overall, the tree is easy to grow and rewards you with one to three feet of annual growth. You can expect pretty umbel flowers and vibrant red berries after a few years of maturity.

Light

American mountain ash prefers full sun, but it will tolerate partial shade. The tree naturally grows on open slopes as well as open forest understories. It needs at least 6 hours of sunlight and enjoys more in its northern range. Maximum sunlight ensures faster growth and an abundance of flowers and berry clusters. In zones 6 and 7, it will appreciate dappled sunlight in the afternoon.

Water

Moisture is key for this native species. It loves the wet forests of New England and the Great Lakes. The trees struggle in dry climates and appreciate extra irrigation during times of drought. However, they do not tolerate standing water or soggy clay soils.

It’s best to water your tree deeply but less frequently. In well-drained soil, generously soak around the root zone and check that water penetrates 6-12” into the soil.

As the trees get established, they can often subsist on the rainfall off their native cool mountain climates. If growing in a suburban or dryer environment, supplemental irrigation is essential.

Soil

Mountain ash grows best in acidic, humus-rich, well-drained soil. Amending with sand, peat moss, and compost can improve conditions before planting. Waterlogged clay soil or dry, cracked soil will result in poorly performing, ugly trees that are more susceptible to pests and diseases.

Climate and Temperature

American mountain ash is hardy in USDA zones 2-6 and thrives in cool mountainous climates. It enjoys the damp chill of Northeastern forests, where it grows alongside cold-loving balsam fir, American yew, and woodfern.

This shrubby tree will not grow well in regions with hot, humid summers or dry soils. Avoid planting it in southern zones and areas with high urban pollution.

Fertilizing

Mountain ashes are not heavy feeders. In humus-rich soil, little to no fertilizer is needed for this native species. Excessive nutrients can cause problems with insect outbreaks and diseases, so it is especially important to avoid concentrated quick-release fertilizers.

Maintenance

Little to no maintenance is needed to keep these trees happy and beautiful. After the trees drop their leaves in fall, you can leave them in place to nurture the soil or rake them up and add them to your compost pile. If lower stems become scraggly or diseased, prune them away in late fall to early spring while the tree is dormant.

Varieties

The Sorbus genus contains over 100 or 250 species of trees and shrubs around the world, with a large concentration in Europe and North Africa. Many different species of mountain ash go by the nickname “Rowan tree.” Interestingly, American mountain ash trees are known to naturally hybridize with chokecherry shrubs (Pyrus melanocarpa and P. floribunda) in the wild. But in the world of ornamental landscaping, these three species are the most commonly available:

American Mountain Ash (S. americana)

This is the native species found growing wild throughout the Northeastern United States and up into Canada. It has an attractive form and is prized for its autumn and winter color in the garden. Do not plant it anywhere with insufficient moisture, or the trees will suffer.

Showy Mountain Ash (S. decora)

This close cousin of Sorbus americana is native to the Great Lakes states and looks very similar to S. americana. However, the leaves, flowers, and fruits are about three times smaller.

European Mountain Ash (S. aucuparia)

The European mountain ash grows rapidly to a 30-foot-tall tree with a dense, oval crown that is prized in landscaping. It looks extremely similar to its cousins and is easy to find in nurseries. Commonly nicknamed the Rowan tree, this species is sometimes invasive and can displace the native American mountain ash in the Northeast.

Companion Plants

For a truly wildlife-friendly forest garden, plant these comrades near your American mountain ash. Moose and white-tailed deer are particularly fond of mountain ash and use them as important browsing during the winter.

Balsam Fir (Abies balsamea)

This dense evergreen tree is a common canopy plant around wild mountain ash shrubs, particularly on the Isle Royale in Michigan. Both species enjoy damp, acidic soil, which is perpetuated by the fall of the fir needles. Ensure at least 30 feet of spacing between fir trees and mountain ash so they don’t become shaded or competitive.

American Yew (Taxus canadensis)

This key companion for mountain ash also occurs regularly in its native range. The attractive frilly needles of this evergreen create a verdant green backdrop to mountain ash berries and foliage.

Honeysuckle (Lonicera canadensis)

This New England native plant enjoys the same well-drained, moist soils as mountain ash. Its tubular yellow flowers and red, fleshy fruits attract hummingbirds.

Woodfern (Dryopteris disjuncta)

The perfect plant for growing around the base of a mountain ash tree, woodferns enjoy plenty of shade and moisture. Though it’s sometimes called an oak fern, it is not usually found with oaks. This species thrives in forest gardens with coniferous trees and native shrubs like mountain ash.

Pests and Diseases

American mountain ash trees are sometimes short-lived in urban and suburban environments due to stressors like pests and diseases. Some of them are preventable, while others can only be slowed.

Fire Blight

The most common issue with mountain ash trees is fire blight. Caused by the bacteria Erwinia amylovora, this disease also attacks pear, apple, and crabapple trees. It is a major issue in the Great Lakes states. The bacteria enters through blossoms, shoot tips, and wounds in the bark. The tree branches will suddenly turn brown, and new shoots will turn black as if they were burned in a fire. Purplish, reddish, or brown cankers form on the bark and wood, sometimes oozing sticky droplets.

It is essential to prune away and dispose of infected branches ASAP. Cut at least six inches below infected parts and burn or destroy infected parts. Sterilize all of your tools regularly. In severe cases, copper fungicides can be used to halt the disease. To prevent infection, avoid wounding the plants because bacteria easily enter through gashes in the wood.

Apple Tree Borer and Round-Headed Borer

Borers are the major pest nuisance on American mountain ash trees. The beetles emerge in the spring. The yellow worm-looking larvae have black heads that burrow into the trunk. You will notice round holes about the diameter of a pencil and noticeable frass (sawdust-looking larval poop) nearby.

Prune and burn any stems that are dying around the borer holes. In the summer, you can use a sharp knife to cut out the borer holes and crush any visible eggs. Nicotine sulfate is a common pesticide treatment, but Bt is a safer organic option.

Emerald Ash Borer

Fortunately, this shiny, problematic bug only attacks true ash trees and other tree species. Emerald ash borer does not pose a risk for mountain ashes.

Other Pests

Aphids and scale may also feed on the plant juices of your mountain ash. You can blast aphids off the tree with a strong stream of water from a hose. Scale need to be removed manually with a cotton swab or q-tip dipped in alcohol. Simply pop them off the branches they’re feeding on. Deer may also take a nibble here and there. To deter them, plant a perimeter of plants they don’t like around your mountain ash.

Plant Uses

This species is primarily grown as a lawn specimen, small shade tree, or native garden ornamental in cold northern regions. It does not do well in heat or humidity.

The vibrant berries are important food sources for birds, and the shrubs are often browsed by moose and deer. The berries are edible for humans but too acidic to eat raw. They are sometimes made into jelly or meat marinades. Native Americans used the fruit and bark for medicinal purposes.