Alnus glutinosa aka Alder

Alnus glutinosa is known as black, common, or European alder. This deciduous tree belongs to the birch family. It comes from Europe, northern Africa, and western Asia. It is native to Britain and Ireland too. Garden trees often reach 40–60 feet [12.2–18.3 metres] tall. Their spread is about 20–40 feet [6.1–12.2 metres]. Some sources report heights of 82 feet [25 metres]. Wild trees can exceed 100 feet [30.5 metres]. It grows fast but has a short life span.

A single trunk supports a narrow, upright crown. Trees with several trunks tend to stay smaller and spread. Young bark looks smooth and grey-green. With age, it turns grey-brown and breaks into small patches. Young twigs and leaves feel sticky. Purple-brown buds sit on short stalks along the twigs. The leaves grow in an alternate pattern. Each leaf is simple, round to oval, and glossy green. Its edge has teeth, and its tip may have a notch. Leaves fall green or brown, without a bright autumn show.

Male and female flowers grow on the same tree. The male catkins hang down and look reddish-brown. Female catkins are small and may look purple. Flowers appear in spring, with March listed by one source. Wind carries the pollen between flowers. The female catkins form small, woody, cone-like fruits. These turn from green to brown as they ripen. They hold many small, winged nutlets and last through winter. The tree gives birds nesting cover and food. Deer and rabbits also feed on it.

Grow this alder in sun or some shade. It does best in moist to wet soil. It also copes with dry, poor soil and brief floods. Root bacteria fix nitrogen, helping it grow on poor ground. It accepts clay and a wide range of soil pH. Cool climates suit it best, within USDA zones 3–7. The RHS rates its cold hardiness as H7. Uses include wet low spots, pond edges, and rain gardens. People also grow it for hedges, screens, and land repair. Its wood has served for timber and carving.

Root suckers can build dense stands, and seeds spread it. It has become invasive in parts of North America and elsewhere. Canker can cause serious harm; powdery mildew also occurs. Other risks include Phytophthora and honey fungus. Aphids, lace bugs, flea beetles, and leaf miners may attack. Sawflies and tent caterpillars can also damage it. No edible uses are known. The bark has a history of medicinal use. Fresh bark can cause vomiting, despite no general hazards being listed.