Sorbus aria Seeds | Whitebeam
LATIN NAME: Sorbus aria
COMMON NAME: Whitebeam
Plant Profile
Every whitebeam leaf is a two-sided trick. The upper surface is a glossy dark green, almost varnished; the underside is covered in a dense white felt so complete that you can write on it with a fingernail. When a breeze turns the crown, the whole tree flashes from green to silver and back again, and a whitebeam in a gust looks like a different tree from the one standing still. The leaves are simple and broadly oval, not divided into leaflets the way a rowan’s are.
In Britain the whitebeam has gone in for mathematics. The genus throws fertile hybrids between Sorbus aria and the rowan, and those crosses then set seed without fertilisation, a form of agamospermy in which the embryo develops from an unfertilised cell of the ovule. Because no genetic reshuffling takes place, each resulting line breeds true from seed and behaves like a species. British botanists recognise a crowd of them, among them the Bristol, Devon, Wiltshire, Lancaster and Arran whitebeams, several of them triploid and several confined to a single gorge.
Wild whitebeam is a tree of thin, dry, lime-rich ground: chalk downland edges, limestone scars, scree, the ledges of old quarries and the skirts of beech hangers. It tolerates drought, wind and shallow soil better than most broadleaves. In cultivation the same tolerance makes it a useful street and car-park tree, and the cultivar ‘Lutescens’ is planted for its silver foliage and upright habit while ‘Majestica’ offers a broader, larger-leaved crown.
The whitebeam’s fruit is a pome, a miniature apple rather than a berry, speckled with pale lenticels and ripening from green through orange to a deep red. It hangs in heavy clusters through the autumn, and the winter thrushes work through it fast: redwings and fieldfares strip a tree in a few visits and drop the seed onto turf and scree, where the seedlings survive because grazing animals find the tannin-rich leaves unpalatable.
Science & Biotechnology
Biotechnology has been surprisingly useful here, because the microspecies are hard to tell apart by eye. Flow cytometry measures the DNA content of a leaf sample and sorts diploid from triploid and tetraploid plants in an afternoon, while DNA fingerprinting checks whether a rare population is a distinct apomictic line or a local form of a common one. The rarest Scottish whitebeams have also been brought into culture, with shoot multiplication from axillary buds providing planting material when a single surviving tree is the whole population.
Condensed tannins of the procyanidin type accumulate in whitebeam bark and leaf, and the genus lends its name to sorbitol, the sugar alcohol first identified in the fruits of these trees. The combination of tannin and sorbitol gives the fruit its drying, mealy quality, and it also makes the foliage unpalatable, so a seedling established in a grazed downland sward has a real advantage over the palatable broadleaves beside it.
Uses & Benefits
Whitebeam is not trouble-free. Leaf blotch mottles the foliage in wet summers, silver leaf can follow hard pruning, and fireblight, caused by the bacterium Erwinia amylovora, has killed whitebeams in southern England and along the coast, where the disease spreads with wind-driven rain and insects among hawthorns and ornamentals of the apple tribe. Any pruning tool used on a blighted branch should be cleaned before it touches another tree.
Growing from Seed
The felt is built from dendritic, non-glandular trichomes: hairs that branch as they grow, tangling into a mat of air-filled pockets rather than lying flat like the simple hairs on an apple leaf. That structure reflects much of the light reaching the underside, cuts transpiration from the shaded surface, and gives the young shoots and flower stalks the same frosted finish. Veins run in ten to fourteen pairs, sweeping from the midrib toward the toothed margin, and they stay visible through the felt all summer.
Collect the haws when they turn orange-red, then macerate the fruit in water, pour off the floating pulp and keep the sinkers; because so many populations are apomictic, the seedlings usually come true to the parent tree. Sow at once in a gritty loam-based mix, ten millimetres deep, and leave the pan outdoors in a cold frame, since the seed must not dry out. Prick the seedlings out at the second-true-leaf stage, pot them on when the stems reach fifteen centimetres, and plant out at forty to sixty centimetres tall, spacing hedgerow plants at forty-five centimetres and specimen trees at four metres. Prune in late summer rather than spring to reduce the risk of silver leaf.
Our Take
Whitebeam is at its best with other trees of the same dry, limey world. My favourite partner is the Harrow whitebeam, a Chinese species with long leaves and grey-white undersides that sets off the native tree’s felt beautifully, and Sorbus harrowiana will take the same free-draining site. The Swedish whitebeam makes a denser, more wind-resistant crown for exposed places, and Sorbus intermedia is the one I would plant on a coastal bank. For the hedgerow below, add hawthorn and let the two knit together, because Crataegus monogyna is unbothered by chalk and flowers just before the whitebeam.
Related Reading
Continue exploring with these related plants: