The Common Walnut - Juglans regia - is a slow growing, medium to large tree native to a wide belt of Asia stretching from the Balkans to China. The largest forests are found in the Jalal-Abad province of Kyrgyzstan at altitudes of 1000-2000 meters. The first historical account of walnut cultivation dates back to Babylon (now Iraq) circa 2000 B.C. However, archaeological excavation of Neolithic sites in southwest France has uncovered roasted walnut shells, indicating walnuts were being eaten in Europe at least 8000 years ago.
The selective breeding of walnuts is thought to have begun with the Ancient Greeks. The walnut tree was then widely cultivated across Europe and parts of North Africa by the Romans. By the Middle Ages, walnuts were cultivated as far north as England. During the early 1800s walnuts were taken from England to North America, and this gave rise to the name 'English Walnut'.
The Black Walnut - Juglans nigra - is native to North America. It too produces large edible nuts but these are generally considered inferior to the nuts of Juglans regia.
Nowadays cultivation of the walnut takes place in many countries worldwide. The annual harvest is around 1.5 million tonnes. China is the world's leading producer followed by the USA, Iran and Turkey. Within Europe, the main producers are Ukraine, Romania, France and Italy. Production of walnuts in England has been in decline since the 18th Century, but continues on a very small scale in a number of counties including Yorkshire, Lincolnshire, Somerset and Devon. Although commercial walnut orchards usually have a life expectancy of 60-100 years, in favourable circumstances walnut trees can live for 300 years or more. The tree has a short trunk and broad crown. Walnut trees have been recorded as producing viable nuts as far north as Altyre, Scotland (east of Inverness and 250km north of Edinburgh) at latitude 57.5º N (see links).
In the last one hundred years and fifty years, considerable research has been carried out into walnut cultivation. Worldwide there may be as many as 300 distinct cultivars of Juglans regia. Selective breeding has produced cultivars with characteristics such as cropping at a young age, early ripening of nuts, and disease resistance. These cultivars are normally propagated by layering rather than grown from seed. Walnut trees grown from seed may revert to the characterisitics of an earlier parent tree. Often the trees do not begin cropping for many years.
Yield varies considerably. In trials carried out at the East Malling Research Station, England during the period 1934-1986, the average annual yield was 16kg per tree, equivalent to 1.6 tonnes per hectare. However, the best 25 percent of the trees yielded three times the average weight per tree.
Walnuts are among the most nutritious of all nuts. On average, 100g of shelled walnuts provide:
15g of protein
Walnuts also contain the following vitamins and minerals (per 100g):
These figures derived from US Department of Agriculture data. There may be nutritional variation from location to location and among individual cultivars.
Selected named varieties of Juglans regia will fruit in the south, midlands and east of Ireland, and are worth a try in favourable locations elsewhere. Walnuts are quite intolerant of wet climates, and may not fruit successfully in locations where the the annual rainfall exceeds 1 meter (40 inches). Areas east of a line from Skibbereen to Sligo, and south of a line from Sligo to Belfast will probably do best. However, the area adjacent to the Shannon estuary, parts of North Kerry, east Clare, east Connaught, the Bann valley and the area around Lough Neagh may also be suitable. Even outside these areas, walnuts can succeed in favourable micro climates - we have visited two young walnut trees near Shrule in east Mayo that have produced table grade nuts for the last three seasons, and believe walnuts are very viable in the west of Ireland when planted in the right location and soil.
The main problem associated with growing walnuts in high rainfall areas is walnut blight (Xanthomonas campestris pv.juglandis). This causes die back of young shoots, reduces the number of male flowers and spoils the nuts. Walnut blight can be controlled to some degree by maintaining the soil pH above 6.0, cutting out affected shoots, sterilising tools, avoiding excessive feeds of nitrogen, and spraying with copper based fungicides. More information can be found in the booklet produced by the Agroforestry Research Trust (see below).
The best locations for walnut trees are sunny, relatively sheltered sites with deep, well drained loam. Frost pockets should be avoided.
One little known fact about walnuts is that they produce a growth inhibitor - juglone - that has a detrimental effect on other species of plant growing nearby. Juglone is produced in both the leaves and roots of the walnut tree. It has a particularly negative effect on fruit trees. For this reason, it is recommended that walnut trees are grown in relative isolation from other fruit or nut trees.
Some walnut varieties are partially self fertile. However, planting two or more varieties of the same pollination group will increase the likelihood of regular crops. Most named varieties begin fruiting at a young age, often in 3-8 years.