The Walnut tree genus (juglans) has 21 species, but there are two main species in common garden or landscape use; the Black Walnut (Juglans nigra) and English (or Common or Persian)Walnut (Juglans regia). I’ve recently written about the poison they both contain and what this means for growing other plants underneath or nearby.
Common name: ‘Walnut’ or Black/English/Common/Persian Walnut. The word walnut derives from the Germanic wal– and Old English wealhhnutu, literally “foreign nut”, wealh meaning “foreign”. The genus name “Juglans” comes from Latin jūglans, meaning ‘walnut, walnut tree’; jūglans in turn is a contraction of Jōvis glans, ‘nut of [the god] Jupiter’
Native areas: The English Walnut (J. regia) originated in Persia, and the Black Walnut (J. nigra) is native to eastern North America.
Historical notes: The Black Walnut was introduced to Europe in 1629 from north America, whereas the English Walnut is thought to have been grown in Britian since Roman times. The worldwide production of walnuts has been increasing rapidly in recent years, with the largest increase coming from Asia. The husks of the black walnut Juglans nigra are used to make an ink for writing and drawing. Walnut ink has good archival properties, and was used by several great artists including Leonardo da Vinci and Rembrandt. Walnut husks are used as a brown dye for fabric. Walnut dyes were used in classical Rome and in medieval Europe for dyeing hair. The U.S. Army used ground walnut shells for the cleaning of aviation parts because it was inexpensive and non-abrasive. However, an investigation of a fatal helicopter crash in 1982 (in Mannheim, Germany) revealed that walnut grit clogged an oil port, leading to the accident and to the discontinuation of walnut shells as a cleaning agent.
Features: The Black Walnut is of high flavour, but due to its hard shell and poor hulling characteristics it is not grown commercially for nut production. The commercially produced walnut varieties are nearly all hybrids of the English Walnut. Walnuts are late to grow leaves, typically not until more than halfway through the spring. The blossoms also normally appear in spring. The male cylindrical catkins are developed from leafless shoots from the past year; they are about 10 cm (3.9 in) in length and have a large number of little flowers. Female flowers appear in a cluster at the peak of the current year’s leafy shoots. Both main species contain a chemical called “juglone” which can be poisonous (or allelopathic) to other plants. This can be a particular issue for growing other plants underneath a Black Walnut.
Uses: As garden trees, Walnuts have some drawbacks, in particular the falling nuts and the release of Juglone.However, they are grown and both species make a stately subject for parkland and avenue plantings, or as specimen trees where space allows. The English Walnut develops a broad crown at maturity (and a height of 15- 20 metres). It also has delightfully aromatic young foliage, from which a wine can be made, followed by a crop of delicious nuts. It is smooth barked when young. Both species are also grown for their timber. The Black Walnut is fast growing and was awarded the RHS Award of Garden Merit in 2002. It can reach 20 metres or more in height and makes a large tree with a pyramidal crown. It produces an abundance of nuts over a long period, but they are rather difficult to extract from their very hard shells. It is rough barked from a young age.
Growing conditions: Walnuts are light-demanding species that benefit from protection from wind. Walnuts are also very hardy against drought. Walnuts grow on most soils but the English Walnut does not favour water logged conditions. The Black Walnut favours deep loam. Walnut trees are easily propagated from the nuts. Seedlings grow rapidly on good soils.
Walnut through the seasons….
Old School Garden 
We recently saw specimens of both sorts at the Wells Cathedral Bishops Palace Garden. Very stately trees and lovely old wrinkled bark.