Alnus cordata Seeds | Italian Alder

Italian Alder (Alnus cordata) seed — Italian Alder (*Alnus cordata*): Betulaceae (birch family), subfamily Betuloideae, native to Southern Italy — Campania, Basilicata and Calabria in the southern Apennines — and the mountains of Corsica, on deep moist soil in ravines and along streams.

LATIN NAME: Alnus cordata

COMMON NAME: Italian Alder

Plant Profile

Most alders carry leaves that are round or bluntly notched at the base, and the Italian species breaks the pattern with a clean, symmetrical heart. The blades are glossy, dark green, finely toothed and held long into the autumn, and they keep their shine through the summer in a way that the common alder of wet British woods never does. The heart-shaped base, together with a native range that stops at the Alps, is the quickest way to name the tree.

Alder roots bear clusters of knobbly nodules in which a filamentous actinobacterium lives and fixes atmospheric nitrogen for the tree. The nodules are perennial, they branch like coral as they age, and they die back if the soil dries out or floods for too long. This partnership explains the alder’s independence from soil fertility and its value as a nurse crop on poor ground, and it is why alder litter is rich in nitrogen compared with the leaf fall of birch or oak.

Italian alder grows fast, roots deeply and holds a narrow, conical crown with a strong central stem, and these three qualities together make it wind-firm in a way that many quick-growing trees are not. On exposed ground it is used for shelter belts and for screening, and it also tolerates air pollution and urban soil well enough that British local authorities planted it by the thousand along streets and on housing estates from the later twentieth century onwards.

The species has a very small natural range, restricted to the mountains of the Italian south and to Corsica, but cultivation has carried it across western Europe and North America. Its tolerance of both wet and reasonably dry ground, its appetite for poor soils and its fast growth have made it a favourite for land reclamation, coastal shelter and amenity planting. Common alder, grey alder and green alder occupy the colder and wetter end of the genus, while this species fills a warmer niche.

Science & Biotechnology

Alder timber is pale, soft, fine-grained and easily worked, and it resists decay when it is kept wet, so it went into piles, sluice gates, water pipes and the soles of clogs. In the ground, exposed to air, it rots quickly instead, which restricts it to wet uses and to short-lived articles. The bark of the common alder has long been used in tanning and dyeing.

Uses & Benefits

The male catkins form in autumn and hang on the naked branches all winter as long, slim, purplish-grey cylinders, and they open in late winter into loose yellow tassels that shake pollen onto the wind. The female catkins are much smaller, upright and green, held further along the twig, and after pollination they ripen into small woody cones.

Growing from Seed

The same kind of symbiosis made the alder a pioneer of wet ground across Europe, and this species uses it in the same way on the moist soils of mountain ravines, so a mixed planting of alder with willow or poplar starts on raw ground more successfully than either group would alone.

Each cone is a strobile of woody scales, and unlike the cones of the conifers it is not much bigger than a thumbnail. The scales open to release small flat nutlets, each with a pair of air-filled wings, and the empty cones stay on the tree into the following season. Sowing is a matter of collecting the ripening cones before they open and drying them in a paper bag indoors, so that the light seed falls out clean.

The seed is small, flat and winged, and it carries no hard dormancy, so it can be sown as soon as the cones have released it. Fill a deep tray with a moistened mix of two parts multipurpose compost to one part horticultural grit, scatter the seed thinly on the surface and press it in without covering it deeply, because light and moisture on the surface suit a seed of this size better than burial. Keep the tray damp, shaded and out of the wind, prick the seedlings out into deep tubes when they have made their first true leaves, and plant them out in the dormant season while the root is still upright and unbranched.

A young alder makes a long, soft shoot in its first summer and can be rocked by wind before the root has gripped. Drive a short stake beside the root ball, tie with a broad flexible tie well above the ground, and check the tie each autumn so that it does not strangle the swelling trunk.

Choose the wettest part of the garden, or the windiest; the species will take either, but it will not enjoy a hot, dry, shallow soil over chalk. Plant in late autumn or early winter with the root collar level with the surrounding soil, water thoroughly in the first season, and keep grass and weeds half a metre away from the stem while the tree establishes. Alders are not much troubled by pests in cultivation, though aphids collect on the young shoots in early summer and the leaves are stripped by alder beetles in some seasons; neither does lasting harm to a healthy tree, and no spray is needed. Do not plant an alder where the drainage is genuinely stagnant, because the roots need moving water rather than a sump.

Our Take

If you want a quick screen on wet ground, this is one of the few trees that fixes its own nitrogen while it does the job, and it will not need feeding at any point in its life. For other alders with different tolerances, see our pages on grey alder, the choice for cold northern sites, and green alder, the low mountain scrub species. Black alder is the timber tree of the floodplain, and it wants even wetter ground than this one.

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