Chestnut hybrids

Family

Fagaceae

Habit

Tree

Height

20 m

Spread

12 m

Growth

medium

Lifespan

150 years

First yield

after 5 years

Hardiness

zone 5–9

Shade tolerance

light shade

Soil

Sand, Silt, Loam

Soil moisture

very dry – very moist

pH

4.5 – 6.5

Times apply to the northern hemisphere in a temperate climate and shift with altitude and cultivar.

Edible: The nuts are as a rule distinctly larger than those of the European chestnut and can usually be peeled more easily — the seed coat penetrates less deeply into the grooves of the kernel, which is decisive when processing larger quantities and was the actual reason for crossing with the East Asian species. In aroma they usually fall short of the European: sweeter, but flatter. Harvest is as windfall from October; the nuts are still moist at that point and must be used quickly or preserved. Traditionally this was done by soaking for several days followed by drying, today mostly by chilling or freezing. They are prepared roasted, boiled, as puree or ground into flour; the flour is naturally gluten-free but binds poorly and is mixed with other flours for bread.

Other uses: Like the parent species, the hybrids serve twice over: as a nut tree and as a timber tree. The wood is ring-porous, medium-heavy and contains up to thirteen per cent tannin; it therefore stays sound in ground contact for decades without any treatment and was traditionally the material for fence posts, vine stakes and trellises — in southern European viticulture to this day. The same tannin sits in the bark, once processed in tanneries and still used for tannin extraction. In the garden a third use is added: flowering in June and July falls in a lean period for forage, and the honey from it is dark and strong. The tannin-rich leaf litter rots slowly, works as mulch and bedding, but noticeably acidifies the soil beneath.

Soil and climate requirements match those of the genus: acid, deep and free-draining soils in a pH range of roughly 4.5 to 6.5, no waterlogging and no lime. Frost hardiness depends heavily on which species have been crossed in and on how well the wood has ripened; crosses involving the Japanese chestnut are more frost-sensitive, those involving the Chinese hardier. Crosses with an Asian component often shed their leaves poorly in autumn and are therefore at risk of snow breakage. Many cultivars come into bearing early, frequently from the second to fifth year after planting. Susceptible to: chestnut blight (Cryphonectria parasitica, only partly fended off depending on parentage), oriental chestnut gall wasp (Dryocosmus kuriphilus, galls on young shoots), chestnut weevil (Curculio elephas, larvae inside the nut), chestnut leaf spot (Mycosphaerella maculiformis, leaf blotches).

Propagation is exclusively vegetative, by grafting, since seedlings from crosses do not come true and their characteristics segregate uncontrollably. Grafting is onto seed-raised rootstocks, and compatibility between stock and scion must be watched: swellings at the graft union or a missing wound callus indicate poor affinity, and such trees break off at the union after a few years. For fruit set at least two, better three different cultivars are needed — and that matters especially here, because it is precisely the large-fruited crosses that mostly produce sterile male flowers and therefore absolutely require an outside pollen donor.

Pollination is both by wind and by insects. The flowers are visited heavily by bees and other insects and count as valuable bee forage — the flowering period in June and July fills a gap when little else is in bloom. The nuts are eaten by squirrels, dormice and corvids, which also disperse them.

Special: A collective entry for the crosses between chestnut species; many large-fruited commercial cultivars belong here but are sold as pure European chestnut — for instance 'Bouche de Bétizac' + 'Marigoule', both European × Japanese; the purpose is almost always resistance to chestnut blight + ink disease, contributed by the East Asian parent; not related to horse chestnut.

Cultivated worldwide, with a concentration in the commercial orchards of south-western Europe. There are no wild populations — every taxon in this group has come out of deliberate breeding. The parent species come from three separate ranges: European chestnut from the Mediterranean and Asia Minor, Japanese chestnut from Japan and Korea, Chinese chestnut from northern and central China. Crossing was done mainly in France, Italy and the USA, from the 1930s onward under pressure from chestnut blight introduced from East Asia. Today the hybrids stand in the plantations of France, Italy, Spain and Portugal, and increasingly in Central European gardens as well.

The hybrids have no natural habitat of their own — they exist only in cultivation. Their requirements follow from those of the parent species, which agree on one point: all chestnuts avoid lime. European chestnut grows in open, summer-warm oak and birch woodland over silicate rock, on acid, deep, free-draining soils well supplied with potassium and phosphorus; in the Alps it reaches about 1000 metres, in Spain 1600. The East Asian parents occupy summer-moist montane forest with distinctly higher summer rainfall. For a hybrid this means: acid, airy, deep, no waterlogging. Heavy, calcareous or waterlogged sites are out, whatever the cultivar.