Walnuts are large, slow-growing forest trees. The Black Walnut, Juglans nigra, is grown for its beautifully figured timber. The Persian or English Walnut (J. regia) is grown for its nuts. Limits on productivity include late frost, insufficient winter chilling and insufficient water as the nuts ripen. They have similar climatic requirements to the apple and should do well in traditional apple growing areas. Because of the long tap root, they need deep, fertile soils to perform well. The soil must be well drained to avoid the root diseases the walnut is heir to.
The nutritional requirements of walnuts necessitates heavy feeding with compost. As they also suffer badly from boron deficiency, the addition of seaweed meal is advisable.
Walnuts are very difficult to graft successfully; the union is prone to failure. Sometimes the weakness of the union is not discovered until the tree is a couple of decades old, when the tree snaps off at the graft. Chip budding onto black walnut (J. nigra) stock is the usual method. Black walnut is less susceptible to root disease and produces a tree of more moderate height than grafting onto J. regia. The latter root stock produces a better union, but is susceptible to root rots. The hybrid root stock Paradox (J. hindsii x regia) is resistant to fungal disease and is tolerant of poorer soils.
The walnut carries separate male and female flowers. The shortage of female flowers on many varieties means that as many as possible need to be pollinated to ensure good yields. Cross pollination is essential for this to happen. Pollination occurs by wind.
Traditionally, walnuts have been planted 12-15 m apart. This produces large trees that take a long time to bear. Earlier and higher yields per unit area can be achieved by planting closer, 8-9 m apart. They are trained to have a short (1 m) central stem. Pruning is largely confined to making sure the selected leaders do not form acute angles and that the tree remains open.
Walnuts mature in autumn and the green husk splits, allowing the nuts to fall to the ground. The nuts are then picked up by hand and the husks removed. Commercially, tractor-attached tree shakers are used. The nuts are dried in the sun. Conventional growers often bleach the walnuts to make the shells more attractive.
Yields are very variable, but expect 10-20 kg per tree after 10 years. In the USA and France, maximum yields are said to be about 10 tonnes per hectare.
Pests include parrots, possums and people. See the remarks under chestnuts. Codling moth and bacterial blight are also potential problems. Preventive treatment of the latter is by spraying a 0.5% Bordeaux mixture. Root rots can be conveyed by planting into land recently under native bush.
| Name | Blossom | Potential yield | Remarks |
| Ashley | early | high | Medium nut size, good quality, small to medium tree. |
| Esterhazy | late | high | Large nut, very good quality, suits dry climate. |
| Eureka | mid | low-medium | Medium size nut, very good quality, unsuitable warm climate, susceptible to bacterial spot. |
| Franquette | v. late | low | Large nut, very good quality, most common commercial variety. |
| Freshford Gem | mid | medium | Small to medium size nut, very goodquality, thin shell, early bearing, small tree. |
| Geisenheim 26 | early | high | Medium size nut, very good quality, small tree. |
| Geisenheim 129 | late | medium | Small to medium size nut, fair quality, cross pollination not necessary. |
| Mayette | late | low | Large nut, very good quality, thin shell, used to pollinate Franquette. |
| Wilson's Wonder | early | medium | Large nut, small kernel, good flavour, thin shell, susceptible to bacterial spot. Precocious bearer. |
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Copyright Jonathan Sturm 2003