One of the most easily overlooked trees, small and rather plain, and unlikely to interest ornament-minded gardeners, yet it boosts biodiversity in country hedgerows, often hidden among other species, and sustains the larvae of one of north-west Europe's most distinctive butterflies, the sulphur-yellow Brimstone (Gonepteryx rhamni).

Native across the whole of Europe (even inside the Arctic Circle), to Algeria and Tunisia, and east through Iran, Iraq, Central Asia, China and Russia to central Siberia. It grows only on alkaline (calcareous) soils, commonest in central and southern England but easily overlooked in hedgerows among Blackthorn and Common Hawthorn. Where introduced in parts of North America it is invasive and hard to control through suckering.

Dioecious, inconspicuous yellow-green flowers on trees of both sexes. Insect-pollinated. Distributed by birds and small mammals.

Small (10m), with an often very short trunk and untidy, twiggy branching. Bark dark brown, small-scaled, becoming gnarly. Twigs slender, grey-brown, the lateral shoots usually spine-tipped; buds dark brown, scaled, usually opposite. Foliage bright green, turning golden in autumn. Leaves small (6cm), usually opposite, oval, finely round-toothed, with 3-4 vein pairs curving forward to the apex. Flowers small, yellow-green, four-parted, clustered in leaf axils. Fruit a small black globular drupe (10mm) with 2-4 seeds. Alder Buckthorn differs: acid soils, five-petalled flowers, spineless twigs, outward leaf veins.

Deeply shade-tolerant and hardy. Its tendency to sucker can be a problem in hedgerows, spreading into neighbouring fields. Its roots produce an allelopathic compound (emodin) that reduces competition from other plants. It deserves wider inclusion in hedgerows and woodland edges to promote biodiversity.

A larval food plant for the Brimstone butterfly. Fruit remains on tree long into the winter, sustaining many birds, especially visiting winter thrushes.

Generally healthy. Quite capable of regrowing vigorously when cut harshly, as they often are when flailed in field edges and hedgerows.

Both bark and fruit can irritate the skin and were once an important traditional medicine, including as a strong purgative (hence the common name); just a few berries have a significant laxative effect, so children should avoid them. The timber is attractively yellow-orange and fine-grained, used in turnery and carving.