Like many ‘English’ plants, the English walnut, Juglans regia, is not actually from England. It is Persian, so is quite comfortable here in California. However, the foliar litter and husks contain a natural herbicide that can make nearby seed grown plants and annuals uncomfortable. Planted trees are typically grafted onto native California black walnut understock (roots) that is more resistant to disease, and improves stability.
Mature trees can be as tall as seventy five feet with trunks as wide as five feet, but are almost always significantly smaller, and even proportionate to urban gardens, although notoriously messy with bloom, leaves and nuts. The pinnately compound leaves are generally less than a foot long, with five to nine leaflets that can be between two to six inches long, and about two inches wide. The bark is notably smooth and gray until trees get quite old and furrowed.