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Juglans - Growing Guide
In all there are around 15 species of walnuts originating from China, Japan, Europe and North America. At Caerhays and Burncoose we grow four of them as largeish freestanding specimen trees in the woodland gardens. These trees have furrowed bark and pinnate leaves with toothed leaflets. In late spring and early summer male and female flowers appear on the same tree. The males are in conspicuous hanging catkins and the females inconspicuous. The fruits or walnuts are rounded and green, becoming brown when ripe. With the outer pith removed the centres contain edible nuts.
Common walnut trees growing here have taken decades to produce walnuts. This is perhaps because of our high rainfall and the consequent quick growth of those trees who seem to do best being baked in full sun in drier conditions in the east of the country. Certainly walnut timber has little value in Cornwall for this reason whereas, elsewhere, it is prized for making gunstocks and ornate furniture.
These are perfectly hardy plants but early new foliage can sometimes catch a frost which stunts leaf development for a bit. The nuts can readily be sown in a seedbed when ripe.
J. ailantifolia is a little known but spectacular species originating from Japan. Stocks remain limited as a result. It is a quick growing, spreading tree with gigantic pinnate leaves (24in) with 11 to 17 leaflets. In a parkland or garden context it is a tree of great ornamental value and we were excited to see it flowering vigorously as a young tree here recently as you can see in the pictures below. Sadly we have yet to see any walnuts develop. The tree will grow to 50ft.
J. mandshurica is, again, only available in small numbers. This is a large suckering tree eventually achieving a height of 70ft or so again with very large pinnate leaves.
J. nigra, the Black Walnut, is far more widely grown in the UK although it is of American origin. This tree grows to around 100ft in some cases with a similarly huge spread. The pinnate leaves are aromatic, about 26in long with 11 to 23 leaflets. The walnut fruits are around 2in long.
J. regia, the Common Walnut, grows to a similar size and is also a spreading tree. Its leaves are about 12in long with five to nine glossy leaflets. The new growth is bronzy and the fruits are also about 2in in length.