Alnus viridis Seeds | Green Alder

Green Alder (Alnus viridis) seed — Green Alder (*Alnus viridis*): Betulaceae, genus Alnus, native to Mountain ranges across the northern hemisphere: the Alps, Carpathians, Pyrenees and Balkans, the Caucasus and Siberia, Kamchatka and the Aleutians, the Rockies and Pacific ranges of North America, and lowland Greenland.

LATIN NAME: Alnus viridis

COMMON NAME: Green Alder

Plant Profile

Alder is the only genus in its family to form a symbiosis with nitrogen-fixing actinobacteria. Frankia enters the root and induces nodules in which atmospheric nitrogen is converted into compounds the shrub can use, and in return the bacterium is housed and fed. The nodules work well enough that green alder grows on raw mineral soil with very little available nitrogen, and when the shrub sheds its nitrogen-rich leaf litter the ground around it becomes fertile for other species. This is the mechanism behind the shrub’s ecological role as a nurse plant.

Green alder is wind-pollinated and monoecious. In late summer the male catkins form along the bare branches and hang through the winter, lengthening and shedding pollen as the weather warms; the female flowers sit behind them as small upright structures that ripen into little woody cones. The cones stay on the bush for a season, and the small winged seeds inside are shed gradually. Leaves are broad, doubly toothed and glossy, and the young shoots are distinctly sticky with resin, which deters some insects and gives the crushed twig a pleasant smell.

Alder pollen is one of the first tree pollens of the year in Europe, and it is released well before birch, at a time when allergy sufferers have usually stopped expecting symptoms. The genus carries a major pollen allergen that is closely related to the birch allergen, so people sensitive to birch often react to alder as well, and the two seasons then run together from late winter into spring. Where green alder forms thickets near towns, the pollen load is high, and a single warm day can push the count up quickly.

Science & Biotechnology

Alder bark is rich in diarylheptanoids, a group of phenolic compounds built on a seven-carbon chain with two aromatic rings. Oregonin is the best known, and it occurs with related molecules in the bark and leaves of the green alders. Extracts have been examined for anti-inflammatory and antioxidant activity, and the reddish dye that leaches from cut alder bark is a mixture of the same type of compound plus tannins. Triterpenes including betulin and betulinic acid occur in the outer bark, and these are the compounds behind the interest in alder as a source of anti-inflammatory material. Tissue culture has been used to study the pathway, and the plants are easy to multiply from softwood cuttings or stool shoots.

Uses & Benefits

The same vigour creates a management problem. On alpine pastures that are no longer grazed, green alder spreads inwards and converts open grassland into shrub, reducing the forage available and changing the vegetation on which rare alpine flowers depend. Swiss and Austrian researchers have documented the process closely, and cutting or grazing are used to hold it back. For a gardener the lesson is simple: this is a plant that rewards being kept on a short rein.

Growing from Seed

The classic account of plant succession on ground exposed by retreating ice comes from Glacier Bay in Alaska, and this alder is the central character. Bare till is colonised by dryas and other creeping pioneers; the green alder, arriving by light seed, then builds dense thickets that add nitrogen and organic matter and shelter spruce seedlings; the spruce overtopping the alder forms the forest. The whole sequence is one of the best-documented examples of how one species can change the ground so much that it engineers its own replacement.

Because it puts roots and stems exactly where soil is trying to move, green alder is planted on avalanche tracks, over steep road cuts and on mine spoil and tailings. It tolerates acid and heavy-metal-rich substrates better than most woody plants, and its nitrogen fixation lets it grow where nutrient levels would otherwise stall a plantation. Seedlings establish quickly and spread by layering, so a small planting thickens into a protective band. Land managers in the Alps use it deliberately as a nurse shrub before planting spruce and larch.

Alder seed is small, winged and produced in quantity. It needs light and a damp surface, so sow on the surface of a fine, moist seed mix and press it in; do not bury it. A cold, moist period before sowing improves uniformity, and outdoor autumn sowing in a shaded frame is the simplest approach. Seedlings must be kept in light shade rather than deep shade, because they lose vigour quickly under dense cover. Once planted out, green alder needs no feeding and little water, and it can be cut hard back in late winter if it outgrows its space.

Our Take

Use it where you need something to fix a bank, shelter young trees or green a patch of poor ground, and be ready to cut it back once the job is done. On any damp or windy site, it earns its keep within a few seasons. Alnus incana, the grey alder, goes further north and tolerates even poorer soil, and Alnus cordata is the southern European alder for a hotter, drier position. If you need a larger tree for the same ground, Alnus nepalensis is the one I would plant on a cool hillside.

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