Alders (genus Alnus) are seriously under-rated trees. They are part of the birch family (Betulaceae), but are often overlooked in favor of their close cousins, the birches (genus Betula). Most people, if they know alders at all, think of them primarily as lumber trees – and for good reason: their wood is strong yet flexible, and remarkably resistant to rot, even when continuously submerged (most of the docks and pilings in the city of Venice are made of alder wood).
One reason alders are such valuable timber trees is because they are both fast-growing and extraordinarily adaptable – a rare combination of traits in a tree. In the wild, types of alder trees are “pioneer species,” often the first woody plants to colonize areas that have suffered from environmental disturbance. They’re drought-resistant, yet also grow well in soils that don’t drain well, and even tolerate flooding.
They also have the ability to convert nitrogen in the air into a usable form, with help from bacteria living in their roots. This process, called nitrogen fixation, is common among plants in the legume family (Fabaceae) but quite rare in non-legume plants. It allows alders to thrive on soils that are too infertile for any other trees to grow.
Alders are also surprisingly strong, despite their rapid growth rate and relatively soft wood. Unlike poplars, sycamores, silver maples, and other fast-growing landscape trees, alders rarely drop limbs, and aren’t vulnerable to pests or diseases.
If you haven’t considered incorporating different alders into your landscaping, take a moment to learn a little bit about these appealing, adaptable, and fascinating trees. Whether you’re trying to landscape a “problem yard” with mucky or infertile soil, planting a hedge or screen, or just looking for something a little different from the usual oaks, maples, elms, etc. – alders may be just what you’re looking for!
Key Takeaways
- Alders are trees in the genus Alnus, and are closely related to birches (Betula). There are about 30 species worldwide, almost all of them native to Europe, Asia, or North America (sometimes all three, as is the case with green alder).
- Most alders are shrub-sized to medium-sized (<10-50 ft tall), but a few species can grow to a hundred feet.
- The most common species of alder are red alder (Alnus rubra), native to North America, and European or black alder (Alnus glutinosa), which is native to Europe.
- Most alders are fast-growing, hardy trees, and can often be found growing near or even in water.
- Alders are able to survive in harsh environments because they “trade” some of the sugar they photosynthesize with bacteria living in their roots in exchange for nitrogen (nitrogen fixation). This nitrogen is returned to the soil as their leaves decay, improving the soil for plants growing around them.
- Most alders are fairly short-lived (100 years or less), but unlike many fast-growing, short-lived trees, alders are resistant to pests and diseases and remain healthy for most of their lives.
- Alders tolerate most environmental conditions well, but are very shade-intolerant and must be planted in full sun.
12 Tough, But Gorgeous Alders for Any Landscape
1. Black Alder (Alnus glutinosa)
Black or European alder is one of the most common species of alder, both in the wild and as a cultivated tree. Black alder is hardy, fast-growing, pest and disease-resistant, and on top of all that it has great aesthetics! Its broad, pyramidal crown reaches up to 60 feet in height and makes an excellent shade tree with no pruning required. Their popularity means that many varieties of black alder are available that maximize aesthetics, including a few that produce fall color.
2. ‘Imperialis’ Alder (Alnus glutinosa var. ‘Imperialis’)
Technically a variety of black alder, the ‘Imperialis’ is so different in appearance and habit from other varieties that it deserves its own place on this list. ‘Imperialis’ alders tend to grow more slowly than other black alders, and generally don’t reach above 30 feet in height. They also have extraordinarily lacy foliage that resembles pine needles more than leaves; combined with their elegant, conical shape, these are trees to be showcased, especially near water features or on wet, mucky soils.
3. Red Alder (Alnus rubra)
Native to western North America, red alder is one of the largest alder species, regularly reaching 80 or 90 feet tall, with a canopy spread about half its height. Cultivated trees will probably not reach these heights, but are still plenty large enough to make them great shade trees. Red alders are also some of the faster-growing alders: with sufficient water, they can reach 20 or 30 feet in height within ten years! Like black alder, red alder has a “cut-leaf” variety (A. rubra f. pinnatisecta), which sports lovely fern-like leaves.
4. White Alder (Alnus rhombifolia)
South of zone 6 in North America, red alder is replaced by white alder, which has slightly curled leaves but is otherwise quite similar in appearance. White alder tolerates heat much better than red alder, growing as far south as Florida, and is a bit more drought-tolerant. White alder is a great choice for thick or mucky soil, but make sure to give it space: its shallow root system can damage sidewalks, driveways, and foundations if planted too close.
5. Siebold Alder, Oobayashabushi (Alnus sieboldiana)
Native to Japan, Alnus sieboldiana is becoming more popular outside its native range as a landscape tree because of its showy catkins and large, handsome foliage. It’s smaller than many of the more common species, and might be better described as a “shrub” than a “tree” – but it’s just as robust and fast-growing as larger species. Its smooth bark and multi-trunked habit make it an attractive specimen planting, and it takes well to pruning and shaping.
6. Gray Alder, Speckled Alder (Alnus incana)
Gray or speckled alder is native to both Eurasia and North America, and in North America is common in the Midwest and western US & Canada. Its small stature and thick growth make it a good choice for hedges or screens, and as an understory plant it’s also more shade-tolerant than most alders (though it still will not tolerate complete shade). Speckled alder is fast-growing and highly tolerant of flooding, but must be protected from drought.
7. Japanese Alder (Alnus japonica)
Japanese alder makes a handsome mid-sized specimen tree, reaching 20 to 30 feet in height and showing two very different “faces”: from late spring through December, its symmetrical crown is thickly grown with elm-like foliage, but in early spring is decorated with yellow (male) and purple (female) catkins that will remind you of autumn leaves. While it grows best in moist-to-wet soils, it’s also quite tolerant of dry soils, which along with its fast growth has made it popular as a street tree.
8. Smooth Alder (Alnus serrulata)
Hazel alder, also called smooth alder, is one of the smaller and more shrub-like alders, generally reaching about ten feet high and often producing multiple trunks. It’s native to forests and marshes throughout eastern North America, and as an understory tree it’s more shade-tolerant than red or white alder, the most common native species. Hazel alder’s stature and dense growth make it a good choice for hedges or screens, particularly because it’s a magnet for wildlife: birds and small mammals eat the flower clusters in spring, and it is a host plant for the rare harvester butterfly, which eats aphids and is the only known carnivorous butterfly.
9. Siberian Alder (Alnus hirsuta)
Siberian alder is closely related to gray alder, and is native to far eastern Russia and northern China, where its extensive root system allows it to cling on rocky, exposed soils. It is the most drought-tolerant species, yet also tolerates occasional flooding, and isn’t vulnerable to any serious diseases or pests. Tough as it is, Siberian alder is still a handsome tree, with a broad crown that actually changes color in the fall, a rarity among alders. For home landscaping, consider the “Prairie Horizon” cultivar, which was selected not only for its symmetrical growth but its tolerance of urban pollution.
10. Green Alder (Alnus viridis)
Green alder is one of the most widespread alder species, native in both Eurasia and North America. In the western US, it is sometimes called “slide alder” because of its ability to colonize areas cleared by avalanches or landslides, and will even grow in mine tailings. It’s one of the hardiest alders and also one of the most frost-tolerant, growing as far north as zone 1 (northern Alaska)! It’s also one of the smallest, rarely rising much above the height of a person, making it a good choice for planting under other trees or in small spaces.
11. Italian Alder (Alnus cordata)
Italian alder, as its name suggests, is native to Italy and southern Europe, but has become a popular ornamental tree, even winning the Royal Horticultural Society’s Award of Garden Merit for its hardiness, upright shape, and large, glossy foliage. It tolerates a wide range of environmental conditions, but is particularly well-suited for highly alkaline soils, which are inhospitable to most landscape plants. Italian alder is a well-shaped, medium-sized tree, generally reaching about 40 to 50 feet. It’s an ideal yard tree for yards with poor soil, or to fill a low spot in the landscape that never seems to dry out.
12. Andean Alder (Alnus acuminata)
Andean alder is the only species in the genus native to the Southern Hemisphere, growing along the west coast of the continent from Chile north to Central America and Mexico.It also tolerates highly acidic soils, and grows better in dry soils than many other alders, although it still needs fairly consistent moisture. One thing Andean alders do not tolerate well is frost, and in more northerly climates they should be planted in sheltered spots to protect them from winter winds.
12 Enchanting and Adaptable Alder Trees With Colorful Canopy
Alders are exceptionally hardy, fast-growing trees, capable of thriving on sites where other trees simply can’t survive. They’re particularly adept at growing on mucky, poorly-drained, and even flood-prone soils, but many species can survive equally well on dry and droughty soils. Their unique relationship with soil bacteria allows them to establish on infertile soil, which they improve as they grow, paving the way for less tolerant plants.
If your landscape has areas with drainage or fertility issues, an alder may be your superhero tree! Yet there’s a lot more to alders than hardiness, and they would be worth planting even if they weren’t such tough trees. Their birch-like foliage, colorful catkins, and unique fruits provide not just aesthetic appeal but food and shelter for wildlife, and the plants that grow around them will get free fertilizer every year in the form of fallen leaves!
This isn’t to say that alders don’t have issues of their own: they’re thirsty trees, and even when cared for don’t live nearly as long as oaks or maples – and speaking of oaks and maples, if you’re looking for fall color, keep looking (although the leaves of a few species will turn a lovely gold in late fall).
Frequently Asked Questions About Alder Trees
How do I identify an alder tree?
Regardless of species, alders generally grow near water, so your first clue in identifying an alder should be its location. Look at the leaves, too: different species have different leaves, but most common alders have similarly-shaped leaves: small, roughly egg-shaped, and with toothed or serrated leaf margins.
Finally, look for the distinctive reproductive structures: male trees have long, dangling catkins resembling those of oaks, while female trees have woody structures called strobiles.
What is special about alder trees?
Alders are known for being tough, adaptable trees: they can grow in almost any soil, and tolerate both drought and flooding. They’re also fast-growing – up to two feet per year – but unlike most fast-growing trees, they don’t have weak wood and aren’t vulnerable to losing limbs.
Alders have one other interesting trait: they are one of the very few genera of non-leguminous plants that can fix nitrogen from the air, via a symbiotic relationship with bacteria in the soil. This allows alders to grow in poor soil and actually enrich it as they grow, adding nitrogen into the soil via their root system and making it more fertile.
What does a European alder look like?
The black alder or European alder (Alnus glutinosa) is a medium-to-large deciduous tree with a roughly conical or pyramidal crown. Its leaves are small (2-4 inches long) and egg-shaped, and stay green much later in the season than the leaves of most other trees.
Since alder trees are monoecious (meaning that they will have all male flowers or all female flowers), the best way to identify them is by their flowers, which are borne on structures called catkins.
Male catkins are about two inches long, green, and hang down from long stalks at the ends of twigs. Female catkins are inconspicuous, but mature in the fall into fruits called strobiles that resemble little pine cones. Since these stay on the trees for a long time, they can often be used to identify alders well after their season has passed.
What type of wood is alder?
Alder wood, like the wood of many fast-growing trees, is quite soft — comparable to pine, but much closer-grained — and thus easy to shape with hand or machine tools. it’s popular with woodworkers for this workability and for its grain, which is quite knotty and similar in appearance to cherry wood. It is also quite resistant to decay when submerged in water, unlike most woods, and was used extensively in the construction of the Venetian canals. One of the most common uses of alder wood today is the production of electric guitars, as alder is said to impart a good “tone” to instruments.
What is the difference between an alder and an elm?
Alders and elms grow in similar places — often near water — and have similar leaves, so they can sometimes be hard to tell apart. The easiest way to distinguish alders and elms is by the bark: alders have smooth bark, which develops shallow fissures as the tree ages, while elms have corky, deeply fissured bark with distinct “channels.”
Elms and alders also have very different fruits: elm fruits are small, winged samaras (like maple and ash fruits), each with a single seed, while alder seeds are contained in pine cone-like structures called strobiles.
Is alder better than oak?
This depends a lot on your location and your needs. Alders are pioneer species, meaning that they tolerate a wide range of soil conditions and are quite fast-growing; oaks tend to be found in mature woodlands, and are pickier about growing conditions. They also generally grow slower than alders, although there are a few species with fairly high growth rates.
If you need a tree for a “problem area” in your yard where other plants won’t grow, an alder may be just right: not only will it handle adverse conditions better than most other trees, it will enrich the soil with nitrogen. On the other hand, if your yard has good soil and you want a long-lived specimen tree, an oak is probably the way to go.
One more important difference to mention is the trees’ foliage: oak foliage is well known for its spectacular fall colors. Alder leaves, on the other hand, often don’t change color at all: they just wither on the tree, falling off in the winter. If you’re hoping for some fall fireworks, oaks have a definite edge on alders!
Is alder better than pine?
Alders and pines are so different in appearance that aesthetics alone may make the difference: pines tend to have a more distinct shape, while alders often have diffuse crowns, and sometimes even multiple trunks. Of course, whether or not you want an evergreen tree will also play an important role in your decision.
As far as cultivation requirements, alders are generally more tolerant of poor soils than pines. Pines also will not tolerate flooding or poorly-drained soils, so in these locations alders are definitely to be preferred.
Ultimately, it’s less a question of one tree being “better” than the other, more a question of the right tree for the right spot.
