THE walnut tree is the plant version of the domestic dog or the horse. It has been a part of human life for millennia, and is one of the most useful living things around. You鈥檒l often find them near homes, churches and pubs 鈥� a sure sign of a close relationship. People plant them partly because of the shade their large bowl-like canopies offer, partly because of the delicious fruit they bear and partly because they seem to ward off flies, probably due to a toxic vapour emitted by the leaves.
A natural insecticide? Yes, but it鈥檚 much more than that. The chemical responsible, juglone (5-hydroxy-1,4-naphthoquinone), is also toxic to a number of plant species and to aquatic life. The tree embarks on a kind of chemical warfare to keep intruders off its patch. Most potent of all is the black walnut, found in eastern North America. Juglone is produced mainly by the roots, from where it leaches into the soil and interferes with the respiration of nearby plants, which turn yellow, wilt and eventually die. The chemical is used as a natural herbicide.
鈥淣ASA has even used pulverised walnut shells as insulation in its rocket nose-cones鈥�
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Is juglone harmful to humans? Although there have been reported cases of toxicity in horses, there have been none in people. It seems juglone may even have medicinal properties: Chinese herbalists have long used it to get rid of intestinal parasites such as tapeworms. The edible fruit of the walnut is certainly beneficial to health, rich in antioxidants, vitamins and omega-3 fatty acids. Last year the US Food and Drug Administration announced that eating a handful of walnuts a day could reduce the risk of coronary heart disease. And a study published last month shows that they can be especially beneficial to people with type 2 diabetes (Journal of the American Dietetic Association, vol 105, p 1087).
What else can you do with walnut trees? A great deal. The outer husks of the nuts can be processed to make dyes 鈥� furniture stain, hair dye and ink. The wood is used for furniture, gun stocks and the dashboards of Jaguar cars. The dried whole husks of the black walnut were once used as baseballs. Crushed walnut shells, which are exceptionally hard, are used in industrial abrasives to clean runway lights at airports; they also work well in cosmetic face masks. NASA has even used pulverised walnut shells as thermal insulation in its rocket nose-cones. Then there鈥檚 walnut oil, pressed from the kernels, in which the ancient Egyptians embalmed their mummies.
But the most unusual use of walnuts has to be the ancient Roman practice of throwing them at the bride and groom at weddings 鈥� the walnut represented fertility. We urge readers not to try to reinstate this ritual, however fun it sounds.
