Pterocarya are not often planted in the UK but that is a real shame because they are a veritable gem for the larger garden, parks and estates. They are fast growing, becoming a large, spreading tree within 25 years and they produce very attractive, long catkins whose flowers turn into the winged nuts which give them their common name – these are papery and brown and persist on the tree into winter, making them a lovely feature.
The genus resides in the walnut family, Juglandaceae. Like the walnuts Juglans, their shoots have chambered pith. They have similar pinnate leaves to walnut, though the leaflets are alternate unlike the opposite leaflets of the walnut. They turn a nice golden yellow in the autumn and they are monoecious trees so produce both male and female catkins.
There are 10 species of Pterocarya, all hailing from the Caucasus to East and South East Asia. They are hardy but cannot cope with prolonged winter temperatures below -12C.
The most commonly planted in the UK is P. fraxinifolia, which has the longest catkins (up to 45cm) and foliage resembling that of the ash tree. It is said to be capable of putting on growth of around 1m a year. It can develop more than one trunk and is suited to being planted near water as it thrives in moist soil.
The most vigorous and hardy is the hybrid P. x rehderiana, which can reach 30m. One disadvantage is that it produces lots of suckers.
P. stenoptera is another well-liked species – it is even planted as a street tree in China – and is a good choice for compacted soil, which it copes with brilliantly.
In their native habitat Pterocarya are found alongside rivers so they prefer moist soil. They can even grow in positions where they become temporarily submerged underwater. However, the roots can establish well in drier soil and once taken they thrive. One thing they cannot abide however is salt-laden sea winds.
Several of the species produce suckers, including P. fraxinifolia and P. x rehderiana. P. tenoptera only does so when the tree is in a bad condition so it is a highly-visible sign that something is wrong. Trees tend to have several main branches, which start low down on the trunk, meaning that as the tree ages they tend to be broader than they are tall. The trunk also becomes deeply grooved with age.
What the Specialists Say
Mike Glover, MD, Barcham Trees, Cambridgeshire
A lovely tree but not for the faint-hearted. They are quick to grow and a tree of large ultimate size. It is best planted in fields large enough to accommodate it, and it tends to grow best where it’s root system can access a good water supply.
P. fraxinifolia is always the best bet. This tree is quick to dehydrate if supplied bare rooted or root balled so beware of poor transplantation results when using trees of this type. Containerised trees are the most successful way of planting this genus. It is rarely grown in the UK and is imported for sale from Europe, which is not in accordance with Arboricultural Association biosecurity guidelines.
James Harris, co-owner, Mallet Court Nursery, Somerset
Pterocarya are a fantastic group of trees and there are several species. P. fraxinifolia from the Caucasus grows to a majestic tree but it does send out a lot of suckers. There is the Chinese species, P. stenoptera – a large tree with female catkins up to 30cm long. The variety ‘Fernleaf’ is a most impressive foliage tree. P. macroptera var. insignis is the most impressive form of the species.
From Japan comes P. rhoifolia a large fast-growing tree with leaves 30 cm long. P x rhederiana is a hybrid between P. fraxinifolia x P. stenoptera. There is a fine specimen in Segrez Arboretum south of Paris.
I sell quite a lot of Pterocarya and they are quite popular - especially P. fraxinifolia and P. stenoptera.
I have found that they are compatible with most soils. Young plants of P. stenoptera can get damaged by late spring frosts. I prefer to grow plants from seed but I have not tried growing from root cuttings but I see no reason why this should not succeed.
Kevin Croucher, Managing Director, Thornhayes Nursery, Devon
Pterocarya are handsome trees in the landscape, for use in large gardens and parks. The large pinnate foliage can turn a good yellow in autumn. In summer the large catkins can be very impressive, hanging down beneath the foliage. Often dividing low down on the stem, the trees can become very large multi-stemmed architectural specimens
All the species and varieties are good, but P. fraxinifolia has the longest catkins. Ecologically they are denizens of lowland riverine forests, so do best in rich moist soils on sheltered sites. Not a tree for a ‘blasted’ hilltop.
Species and varieties
- P. fraxinifolia or the Caucasian wingnut is the most commonly grown Pterocarya in the UK. It is a vigorous, broadly spreading, deciduous tree with large pinnate leaves and small green flowers. These are held on long catkins and become highly decorative winged fruits in the late autumn. Height: 20-25m.
- P. fraxinifolia ‘Heerenplein’ is a Dutch selection which resembles the species, though the crown is regular. Very similar leaves to the species. Produces lots of root suckers. Height: 15-20m.
- P. fraxinifolia var. dumosa is a variety which is considerably smaller than the species. It will eventually become a small tree with a flattened, spherical to rounded crown. It produces branches low down on the tree and several heavy main branches. Differs from the species by having smaller pinnate leaves, with fewer leaflets, grouped more closely together. Height: 7-10m.
- P. hupehensis is a Chinese species which becomes a large tree. It has shorter leaves than P. fraxinifolia with a small number of leaflets. Fruits have semiorbicular wings. Height: 20m.
- P. macroptera or the large-winged Wingnut is a Chinese species which becomes a large tree and has more narrow leaflets than other species, with densely serrate margins. They are smooth above and downy below, with a rust red midrib. Height: 20m.
- P. macroptera var. insignis is a large, highly vigorous, hardy, deciduous tree from China. It has long pinnate foliage and very long racemes of flowers which becomes lovely wingnuts. Best in a site that doesn’t dry out excessively. This species doesn’t appear to sucker.
- P. rhoifolia or Japanese wingnut is hardy in the UK but is rarely seen, sadly, as considered by some to be the most attractive species. It flowers in June, with the seeds ripening between October and December. Likes a position in full sun, but in moist soil. Height: 25-30m.
- P. stenoptera or the Chinese wingnut becomes a large tree with a broad, rounded crown. It produces a dense canopy of large pinnate leaves, which are shorter and contain less leaflets than those of P. fraxinifolia. This species produces wings on the stems and narrow wings on the nutlets. The twigs are also well marked with lenticels. Height: 15-25m.
- P. stenoptera ‘Fern Leaf’ has narrow, deeply divided, fern-like leaves. Its deeply cut leaves give the tree a more delicate, open appearance than the other wingnuts. Height: 20m.
- P. x rehderiana is a vigorous hybrid of P. fraxinifolia and P. stenoptera. As well as being very vigorous and spreading by suckers, it is also the most frost resistant. Leaves about half the size of P. fraxiniflia but a similar number of leaflets. Height: up to 30m.