Walnut (English/Persian: Juglans regia; Black: Juglans hindsii)

Walnuts need a deep, well-drained soil (at least 5 ft.) or they will do poorly. Shoots, particularly blossoms, do not tolerate frosts. Once growth begins in the spring, rainy weather can cause severe losses due to walnut blight.

Trees range in size from very large (80 ft. tall) to medium height (40–50 ft. tall). They require a 30 to 60 ft. spacing. Walnut culture has changed drastically in the last few years due to introduction of new varieties. Production in coastal climates should be limited to the late-leafing varieties.

Walnuts are monoecious (separate male and female flowers on one tree) and dichogamous (pollen is shed when female flowers are not receptive); thus, two different varieties must be planted to ensure overlapping bloom periods, fertilization, and fruit set.

Calendar of Backyard Gardening Operations for Walnuts

Winter Dormant Season

Prune trees by thinning out crowded areas to let light into whole tree.
Remove broken, dead branches.
Spray trees with dormant oil to control scale insects, if needed.

Spring Bloom Season

Spray for blossom blight when female flowers appear (tiny nuts with feathery pistil) and at 7 day intervals until rainy weather stops. Blight appears as black blossom ends of nuts in June and later as black hollow nuts.
Remove all weeds from tree base to reduce competition and pest problems.
Fertilize mature trees with 5–7 lb. urea or 90–150 lb. manure.

Summer Growing Season

Keep tree base dry to reduce crown rot problems.
Irrigate trees at the drip line but away from trunks. Sprinkler irrigate with about 3" water every 3–4 weeks or drip irrigate daily during May-Oct. Young, small trees will need 4–12 gal. water/day; large trees about 20–40 gal/day.
Spray for walnut husk fly about Aug. 1 and Aug. 15. Damage is cosmetic, control is optional. Spray to control codling moth worms or tolerate damage.

Fall Harvest Season

Harvest nuts by shaking or poling the tree when green hulls begin to break away from the shell. Nuts are fully mature at this stage. If left on the tree or allowed to fall on their own, the hulls will rot and stick to the shell.
Hull nuts, freeze in shell nuts to kill resident worms, and store in plastic.

Resources

Walnut Links, UC Fruit and Nut RIC
[PDF] Calendar of Operations for Backyard Walnut Trees on the North Coast, Rachel Elkins and Paul Vossen
[PDF] Harvesting and Storing Your Home Orchard's Nut Crop: Almonds, Walnuts, Pecans, Pistachios, and Chestnuts, Ed Perry, Stanislaus Co. UCCE and Steve Sibbett, Tulare Co. UCCE
How to Manage Pests in Gardens and Landscapes - Fruits and Nuts, UC IPM
[PDF] Walnuts: Calendar of Operations for Home Gardeners, Pamela M. Geisel and Carolyn L. Unruh, Fresno County and Paul Vossen, Sonoma and Marin County