Black (European) alder

A fast-growing, multi-stemmed tree in the birch family. Trees' roots have nitrogen-fixing microbial symbionts. New leaf growth is sticky to the touch.

Other names for this plant include:

• Common names: European alder.
• Scientific names: Alnus alnus, Alnus vulgaris, Betula alnus var. glutinosa.

Classification in Wisconsin: Restricted (all cultivars exempt)

Ecological Threat

• Invades all habitat types: floodplain forests, forest edges, prairies, wetlands, grasslands, streamsides, bogs and fens, marshes, and lakeshores.
• Widely planted since colonial times and naturalized.
• Prefers wet soils and full sun. It can be tolerant of dry conditions and shade.
• Trees can produce hundreds of thousands of seeds each year.
• Readily hybridizes with other alder species, threatening native populations.
• Seeds are dispersed mainly by water; seeds can float for over a year.
• Black alder increases invasive earthworm (Lumbricus terrestris) activity.

Identification

Leaves & stems: Young leaf growth is sticky to the touch. The bark of the young trees is greyish-green and smooth. Older trees' bark turn brown-speckled with horizontal striations and shallow fissures. Leaf-out occurs after flowering.

Flowers, fruits & seeds: Dangling male flower clusters, catkins, appear in fall and remain on the trees through winter. Female flowers resemble cones, known as strobiles. Cones are green until seed dispersal, turning brown and woody. Female cones are also persistent on the tree through winter. Seeds are small and winged. Seeds contain a small air bladder that serves as a floatation device, allowing seeds to float on water for over a year.

Roots: The root system develops densely both at the soil sub-surface layer and deeper, taking advantage of available groundwater at multiple levels. Roots are nodular due to the nitrogen-fixing symbionts. Layering or rooting branches allows for trees to spread laterally.

Similar species: Wisconsin contains two similar-looking native alder species: Speckled alder (Alnus incana, Alnus rugosa) and green alder (Alnus viridis, Alnus crispa). These typically are considered shrubs, while black alder is a larger, often multi-stemmed tree. Black alder can be distinguished from these species by its round leaves, as opposed to elliptical. Mature leaves have a distinct notched tip.

Control

Mechanical:

• Cut trees just below the soil surface. Monitor for re-sprouts. Note that trees commonly re-sprout from the trunk after cutting. Follow-up cutting with an herbicide to reduce re-sprouting.
• Cover the infestation with black landscape fabric/plastic in early spring, when leaves emerge. Keep covered for a minimum of one growing season.

Chemical:

• Cut-stump treatment methods with glyphosate are successful. Cutting trees alone without herbicide may result in re-sprouting.
• Foliar spray with triclopyr.

Resources

Sources for content:

• Funk, David T. European Alder. United States Forest Service, Northeastern Forest Experiment Station, Radnor, PA.
• Global Invasive Species Database.
• Invasive Plant Atlas of New England.
• University of Wisconsin - Stevens Point; Robert W. Freckmann Herbarium.

Links for more information:

• Global Invasive Species Database [exit DNR]
• Invasive Plant Atlas of New England [exit DNR]
• Pennsylvania Department of Conservation and Natural Resources [PDF, exit DNR]
• Wisconsin State Herbarium [exit DNR]