UC Master Gardeners

California is an ideal place for walnut trees

UC Master GardenerSeptember 11, 2013 

Walnut trees are easily dry-land farmed, don’t have many pests and can be prolific producers of good-for-you nuts.


Q: I have a large walnut tree in my yard. How should I take care of it? — Fred, Paso Robles.

A: The California climate is ideal for growing walnut trees. In 1867, Joseph Sexton planted the first known walnut orchard in California. Today’s acreage exceeds 222 million. Demand has outperformed production since the walnut’s health benefit was discovered.

Walnut trees have long been dry farmed in the state; its long tap root is ideal for summer survival. Over-irrigation often causes oak root fungus and a wet trunk promotes gall diseases. If you have good loamy soil, no additional water is needed if you keep the root zone under its canopy weed-free. Here are some tips to identify your tree:

• Black walnut: features a deep furrowed bark and leaves with 15 to 23 leaflets.

• English walnut: has a smooth bark and leaves with five to seven leaflets.

Combinations of a black walnut trunk and rootstock with an English walnut tree grafted on top are common in Paso Robles. If you have a black walnut tree, you need to consider that it hinders the growth of other plants by root competition and by secreting a growth-inhibiting substance.

Regarding insect pests, the walnut husk fly is a small fly that deposits its eggs inside the nut by piercing the green husk. The emerging larvae feed on the nut and will ruin the nut meat. A good way to monitor and catch the fly is to use yellow sticky traps, which, by the addition of an added pheromone, attract the female fly. Putting these traps out by June and monitoring them weekly will help avoid serious infestations.

Other less common insect pests are scales, spider mites, aphids and codling moth. Ask your local Master Gardener office to help you identify these insects. And unless you like to share your harvest with rodents, consider trapping ground squirrels and tree squirrels. Apparently they also know about the health benefits of the walnut; a tree squirrel can squirrel away 104 pounds of nuts a year!


Contact the University of California Cooperative Extension Master Gardeners: at 781-5939 from 1 to 5 p.m. on Monday and Thursday; at 473-7190 from 10 a.m. to noon on Wednesday in Arroyo Grande; and at 434-4105 from 9 a.m. to noon on Wednesday in Templeton. Visit the UCCE Master Gardeners website at http://ucanr.org/sites/mgslo or email mgsanluisobispo@ucdavis.edu  .

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