A black alder tree in early spring is decorated with showy male catkins and the previous season's empty conelike strobili.
strobili and foliage
Black alder is a cone shaped tree that can get up to 80 ft (24 m) tall with lower branches spreading up to 40 ft (12 m), narrowing towards the top. However, most cultivated specimens are smaller, some even shrubby, 20-50 ft (6-15 m) tall. The deciduous leaves are alternate, 2-4 in (5-10 cm) long, doubly toothed on the margins, and can be nearly round to elliptic, and usually without lobes. They persist long into autumn and rarely change color before falling. Young leaves and twigs are covered with glutinous hairs, making them very sticky to the touch. Separate male and female flowers are carried on the same tree and bloom with or just before the leaves unfurl in early spring. The male catkins are reddish or yellowish brown, around 4 in (10 cm) long, and hang down in clusters of 3-5. The tiny female flowers are borne in egg shaped aggregates called strobili (singular: strobile), about a half inch (1 cm) long. The strobili turn brown as they mature and look for all the world like tiny pine cones. In late summer they open to release tiny winged nutlets. The empty strobili then remain on the tree throughout the winter, and into the next growing season
The nominate (or typical) variety, Alnus glutinosa var. glutinosa, is as described above. Alnus glutinosa var. denticulata, from the Mediterranean region, has very finely toothed leaves that are not lobed; var. barbata, from the Caucasus and northern Iran, has finely toothed leaves that are wooly pubescent beneath. Cultivars of the nominate variety include ‘Aurea’ which is a smaller tree, maxing out at 40 ft (12 m) or so, and has young shoots that are orange, and young leaves that are yellow, aging to yellowish green; ‘Maculata’, with white-dotted leaves; ‘Pyramidalis’, with dark green leaves and upright branches with a rigid erect habit; and the shrublike ‘Imperialis’, with narrowly lobed, deeply cut leaves, giving a finer texture than the species.
Location Alnus glutinosa is native to Europe, including the British Isles, east through the Caucasus to Siberia, and south to northern Africa and Asia Minor. It has escaped cultivation and is now established in much of NE North America from Ontario south to Missouri, Tennessee and Kentucky, where it sometimes forms dense stands along waterways. Black alder also has become naturalized in Australia and New Zealand.
Light: Black alder does best in full sun, but tolerates partial shade. Moisture: Black alder thrives in a moist to wet soil but, once established, it does well in normal, drier soils, too. Black alder can survive extended periods in standing water. Hardiness: USDA Zones 3 - 7. Black alder does best in cool to cold climates. Propagation: : Seed can be sown as soon as ripe, or dried and chilled for three months for sowing in spring. Hardwood cuttings taken in winter can be rooted, as can be soft greenwood cuttings taken in spring. Black alder self-seeds abundantly under ideal conditions, especially at the water’s edge.
These black alder's male catkins are the sturctures that produce pollen.
With the help of symbiotic nitrogen-fixing bacteria on their roots, the alders are able to use atmospheric nitrogen (like legumes) and thus can thrive in nutrient-poor soils. They are well suited to growing near water and can tolerate wet, poorly drained soils better than most trees. Widely used for erosion control and reclamation of disturbed wetlands, black alder is a good choice for naturalizing in wet areas. Under ideal conditions (read: wet) it suckers freely, forming dense thickets unless kept under control.
Black alder is a fast growing (but short lived) deciduous tree, tolerant of urban conditions and air pollution, and makes a fine shade tree. Black alder provides winter interest with cute little “pine cones” that persist long after the seeds have scattered. In fact, last year’s strobili usually are still decorating the tree the following summer even as new ones are developing. In late winter and early spring, the leafless black alder, hung with countless yellowish to reddish 4 in (10 cm) catkins, makes a very attractive specimen.
The wood is used for furniture and general construction. Cut stems bearing the dried conelike strobili look nice in arrangements.
There are about 35 species of alders, all occurring in the Northern Hemisphere except for one species that occurs in Peru.
Black alder is known to be an invasive species in parts of North America, Australia and New Zealand.