Black alder (Alnus glutinosa)

Synonyms: Betula alnus, Alnus vulgaris, Alnus rotundifolia

Common Names: European alder, European black alder, common alder, sticky alder

Description:

Black alder (Alnus glutinosa) is a fast-growing wetland tree from Europe, western Asia, and North Africa, brought to North America long ago as an ornamental and for erosion control [1][3]. It has escaped into wet ground, where it forms dense stands that crowd out native plants [1][2]. MISIN tracks it because it spreads readily along water, takes over wetlands and streambanks, enriches the soil with nitrogen in ways that favor other weeds, and cross-breeds with native alders [1][2].

Habit:

Black alder is a medium deciduous tree, often about 30 to 60 feet (9?18 m) tall, with a narrow shape when young that becomes more irregular with age, growing as a single trunk or several stems [1]. The easiest leaf clue is the shape: its leaves are broadly rounded and often have a blunt, notched or flattened tip ? unlike the pointed leaves of our native alders ? and young leaves are sticky to the touch [1]. The bark is greyish, speckled with pale horizontal marks (lenticels) [1].

This tree is easiest to identify in late winter and early spring, when small woody, pinecone-like seed "cones" and long drooping male catkins hang on the bare branches ? and the old cones stay on the tree well into winter, a giveaway when leaves are gone [1]. Look for it in wet places: streambanks, floodplains, marshes, fens, and lakeshores [1].

At-a-glance cues:

1. Rounded leaves with a blunt, notched or flattened tip (native alders have pointed tips); young leaves sticky [1]
2. Small woody, pinecone-like seed "cones" that persist on the branches through winter [1]
3. Long, drooping catkins on bare branches in late winter to spring [1]
4. Greyish bark speckled with pale horizontal marks; grows in wet ground [1]

Leaves:

1. Arrangement: alternate, simple [1]
2. Shape: broadly oval to rounded, with a blunt, notched, or flattened tip (not pointed) ? a key difference from native alders [1]
3. Margin: toothed (doubly serrate) [1]
4. Surface: dark glossy green above, lighter below; young leaves are sticky to the touch [1]
5. Size: about 2?5 inches (5?13 cm) long [1]

Stems:

1. Habit: fast-growing deciduous tree to about 30?60 feet (9?18 m), single- or multi-stemmed [1]
2. Bark: smooth and greyish-green when young, maturing to grayish-brown speckled with pale horizontal marks (lenticels) and shallow fissures [1]
3. Twigs/buds: twigs speckled with short lenticels; buds are stalked [1]
4. Roots: dense and spreading, with nitrogen-fixing root nodules; can resprout and sucker from the stump and roots [1][2]

Flowers:

1. Sex: monoecious ? separate male and female flowers on the same tree [1]
2. Male flowers: long, narrow, drooping catkins, reddish-brown [1]
3. Female flowers: small, in clusters, maturing into woody pinecone-like cones [1]
4. Bloom time: early spring, about March to May, before or as the leaves expand [1]

Fruit and seeds:

1. Fruit: small woody, pinecone-like cones (strobiles) that turn brown to nearly black and stay on the tree into winter [1][2]
2. Seeds: small winged nutlets, produced in huge numbers ? a mature tree can release hundreds of thousands of seeds [1][2]
3. Dispersal: mainly by water ? each seed has a tiny air bladder and can float for over a year, carrying it far downstream [1]
4. Also spreads vegetatively by resprouting and suckering from cut stumps and roots [2]

Habitat:

Black alder is a tree of wet ground. It thrives along rivers and streams and in floodplains, marshes, fens, bogs, wet forests, and lakeshores, and it can also move into grasslands and forest edges [1]. It prefers full sun and wet soil but tolerates shade, drier ground, and a wide range of soil acidity [1]. A key advantage is that its roots host bacteria (Frankia) that pull nitrogen from the air, so it can grow fast even on poor, low-nutrient soils where other trees struggle [2].

The tree is native to Europe, western Asia, and North Africa [2][3]. It was planted across North America since colonial times for landscaping and erosion control and has since naturalized, and it is now considered invasive on wet sites in parts of North America and other regions [1][2]. It is not native anywhere on this continent [3].

Reproduction:

Black alder spreads mainly by floating seed. A mature tree drops huge numbers of tiny winged seeds into the water, and because each seed has a built-in air bladder it can float for more than a year, carrying the tree far downstream and seeding new wet ground along the way [1]. The tree can also resprout and send up suckers from cut stumps and roots, so a damaged tree often grows back [2]. Historically it also arrived simply because people planted it [1].

Trees produce seed in woody, pinecone-like cones that ripen in fall and release seed into winter and spring, and the cones often hang on the bare branches for months [1]. Because seeds travel on water, the most effective control starts upstream ? removing isolated outlier trees and seed-source trees before they can seed the waters below ? and revisiting cut stumps that resprout (see Management) [1][2].

Impact and Damage:

Black alder is not a danger to people or pets ? its concern is what it does to wet natural areas. It grows fast and can form dense, single-species stands in under ten years, shading out and crowding native wetland plants and turning diverse streamsides and marshes into alder thickets [1][2]. Dense root mats and heavy shade can also change how water moves and degrade habitat along watercourses [1].

It also changes the ground itself. Its nitrogen-fixing roots add nitrogen to soils that are naturally low in it, which can shift the plant community and give other weeds a foothold, and it has been linked to increased activity of invasive earthworms [1][2]. On top of that, it readily cross-breeds with native alders, which can blur and threaten the genetic identity of native populations [1][2]. These combined effects are why managers in parts of North America rate it a high priority for control [1].

Similar species:

Black alder is most easily confused with our native alders, which are usually smaller and shrubbier.

Speckled alder (Alnus incana subsp. rugosa) is a common native wetland shrub, but its leaves have a distinct pointed tip and it usually grows as a multi-stemmed shrub rather than the single, taller tree form of black alder.

Smooth (hazel) alder (Alnus serrulata) is another native wetland shrub of streambanks and swamps, but it too has pointed-tipped leaves and a shrubby habit, unlike black alder's rounded, notch-tipped leaves.

Green alder (Alnus viridis) is a native shrub with sticky young leaves like black alder's, but its leaves are pointed and it stays a low, multi-stemmed shrub rather than becoming a tree.

Monitoring and rapid response:

Signs to watch for:

1. A wet-site tree with rounded, notch-tipped leaves and sticky young leaves, often in dense stands along water [1]
2. Small woody, pinecone-like seed cones and drooping catkins on the branches, especially visible on bare branches in late winter and spring [1]
3. Thickets taking over streambanks, floodplains, marshes, or lakeshores where native shrubs and plants used to grow [1][2]

What you can do:

1. Pull or dig out seedlings and small saplings, roots and all ? easiest in spring when the soil is wet and soft [1].
2. Cut larger stems, but expect the stump and roots to resprout, so plan to return and cut the regrowth; removing or bagging the cones before seeds drop helps limit new seedlings [1][2].
3. Because seeds float downstream, tackle isolated outlier trees and the trees farthest upstream first, so they can't keep seeding the water below [1].
4. Natural area crews can clear dense stands over repeat visits, prioritize seed-source trees along waterways, and pair mechanical removal with follow-up monitoring for resprouts and new seedlings [1][2].
5. Report sightings (below) to help track where this tree is spreading along waterways [1].

When to call a professional:

1. Large or mature trees, dense stands, vigorous resprouting after cutting, or work in sensitive wetland and streamside areas ? a land manager or licensed applicator can plan a safe, multi-season approach [1][2].
2. For chemical control options, contact a licensed applicator or your local cooperative extension office.

Report sightings to MISIN. Help track this species' spread by submitting observations through the MISIN web site or the MISIN mobile app (Android, iOS, Windows, and macOS). Photos at multiple angles paired with a precise GPS location are the most valuable data for confirming new occurrences and triggering rapid-response review.

Credits:

1. Wisconsin Department of Natural Resources. Black (European) Alder (Alnus glutinosa). dnr.wisconsin.gov
2. IUCN SSC Invasive Species Specialist Group, Global Invasive Species Database. Alnus glutinosa. iucngisd.org
3. USDA NRCS PLANTS Database. Alnus glutinosa (L.) Gaertn. (black alder). plants.usda.gov
4. Ontario Invasive Plant Council. European Black Alder (Alnus glutinosa) ? Best Management Practices. ontarioinvasiveplants.ca

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