Walnuts: Calendar of Operations
for Home Gardeners
Pamela M. Geisel is UC Cooperative Extension Farm Advisor in Environmental Horticulture for
Fresno County; Carolyn L. Unruh is UCCE staff writer for Fresno County; and Paul Vossen is
UCCE Farm Advisor in Fruits, Vegetables, and Marketing for Sonoma and Marin Counties.
University of California
Agriculture and Natural Resources
Publication 7263
Walnuts make lovely shade trees, in addition to pro-
ducing a delightful nut. Various types of walnuts are
grown in California, but the most popular are the
English varieties. They have a moderate chilling
requirement of about 500 to 1,000 hours. Shoots and
flowers of the early varieties can be damaged by late
spring frosts in cold areas. They do best in deep, well-
drained, medium-textured soils. Walnut trees require
regular irrigation for a quality nut crop, but once
established they can tolerate periods of drought. They
generally do poorly when planted in lawns: frequent
irrigations promote root diseases to which walnut
trees are susceptible.
Winter Dormant Season
1.Prune mature trees by thinning out limbs in
crowded parts of the canopy to let light into the
whole tree. Remove broken and dead branches.
2.Remove and destroy old nuts on trees and on
the ground to kill resident larvae of codling
moths and navel orangeworms and to help
reduce damage from these insects the following
year.
Spring Bloom Season
1.Spray for blossom blight when female flowers
appear (they appear as tiny green nuts with
feathery pistils) and at 10- to 14-day intervals
until rainy weather stops. Use bordeaux or
another fixed-copper material and provide thor-
ough coverage of the whole tree. Blossom blight
appears in June as black discoloration at the
blossom end of nuts, and later as black, hollow
nuts.
2.If scale is a problem, monitor the scale popula-
tion and treat with narrow-range oil when the
crawlers appear. Do not apply oil when temper-
atures go above 90°F, and do not use it in trees
that have been stressed by drought, heavy scale
populations, or other factors. Irrigate the trees
well prior to oil application.
3.Remove all weeds from around the base of the
trees, and keep this area dry in summer to
reduce root and crown rot problems.
4.Fertilize mature trees with about 8 to 10 pounds
of ammonium sulfate (depending upon tree size)
just before or during a rain or the first irrigation.
5.In very hot regions, paint the trunks and lower
branches of newly planted young trees with a 1:1
mixture of white interior latex paint and water to
prevent sunburn injury and to reduce borer
infestations.
Summer Growing Season
1.To reduce the incidence of codling moth and
navel orangeworm in the current growing sea-
son, gather and destroy any damaged immature
nuts that fall to the ground or remain on the tree.
Clean-up is especially important in May and
June.
2.If you use drip irrigation, apply just the amount
of water needed to replace what is used by the
tree and lost from the soil through evaporation. If
you use sprinkler or flood irrigation, water about
every three to four weeks during the summer
months. Young, small trees will need about 4 to
12 gallons of water per day, and large trees about
30 to 50 gallons per day. In garden settings some
of this demand is usually met by water applied
to nearby plants. Water requirements will vary
depending on your soil type and environmental
conditions. Irrigate at the drip line, well away
from the trunk of the tree.
3.Walnut husk fly larvae may cause considerable
damage to the green outer hull of walnuts, but
rarely do they affect the kernels inside the shells.
If walnut husk fly attacks have a history as a
recurring problem, spray with malathion at 21-
day intervals beginning about August 1. The
effectiveness of walnut husk fly sprays is signifi-
cantly increased if you add molasses to the spray
at 1 to 1.5 times the amount of malathion. For
example, if the label recommends 2 teaspoons
per gallon of malathion, add 3 teaspoons of
molasses as well. Always follow the label-recom-
mended rates for malathion and other chemicals.
When using a malathion-molasses mix, it is gen-
erally sufficient to spray just the lower 1/3 to 1/2
of the tree.
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This publication has been anonymously peer reviewed for technical accuracy by University of
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ANR Associate Editor for Pomology, Viticulture, and Subtropical Horticulture.
Autumn
1.To keep worm damage to a minimum, harvest
nuts as early as possible. Shake or pole the trees
when the green hulls begin to crack and separate
from the shells. Remove the hulls as soon as pos-
sible after harvesting and discard them. Wear
gloves to avoid staining your hands.
2.Place the hulled walnuts in a warm, well-venti-
lated area to dry. Store in-shell nuts in a cool, dry
place in sealed plastic bags after they have been
frozen for a few days to kill any insect eggs that
may have been present. You can shell walnuts
when the kernels have dried enough so that they
break easily when bent. Shelled nuts may be
stored for several months to a year in the refrig-
erator or freezer. In general, storage life for
shelled nuts is shorter at room temperature than
in a refrigerator or freezer.
For More Information
Cousult these UC IPM Pest Notes online at
http://www.ipm.ucdavis.edu:
Aphids
Bordeaux Mixture
Codling Moth
Scales
Spider Mites
Walnut Husk Fly
Youll also find detailed information on many aspects of
fruit and nut tree care in these titles and in other publi-
cations, slides sets, and videos from UC ANR:
California Master Gardener Handbook, publication 3382
Drip Irrigation in the Home Landscape, publication 21579
Pests of the Garden and Small Farm, publication 3332
Pruning Fruit and Nut Trees, publication 21171
Sweet Cherries for the Home Grounds, publication 2951
The UC Guide to Solving Garden and Landscape Prob-
lems, CD-ROM 3400
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Publication 7263
© 2002 by the Regents of the University of
California, Division of Agriculture and Natural
Resources. All rights reserved.
2 Walnuts: Calendar of Operations for Home Gardeners
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WARNING ON THE USE OF CHEMICALS
Carefully follow all precautions and safety recommendations given on the container label. Store all chemicals in
their original labeled containers in a locked cabinet or shed, away from foods or feeds, and out of the reach of chil-
dren, unauthorized persons, pets, and livestock.
Confine chemicals to the property being treated. Avoid drift onto neighboring properties, especially gardens con-
taining fruits and/or vegetables ready to be picked.
Mix and apply only the amount of pesticide you will need to complete the application. spray all the material
according to label directions. Do not dispose of unused material by pouring down the drain or toilet. Do not pour
on ground: soil or underground water supplies may be contaminated. follow label directions for disposing of con-
tainer. Never burn pesticide containers.
PHYTOTOXICITY: Certain chemicals may cause plant injury if used at the wrong stage of plant development or
when temperatures are too high. Injury may also result from excessive amounts or the wrong formulation or from
mixing incompatible materials. Inert ingredients, such as wetters, spreaders, emulsifiers, diluents, and solvents,
can cause plant injury. Since formulations are often changed by manufacturers, it is possible that plant injury may
occur, even though no injury was noted in previous seasons.
Funding for this publication was made possible through a grant from the Elvenia J. Slosson Fund.
Walnuts: Calendar of Operations for Home Gardeners 3
Introduction
Winter Dormant Season
Spring Bloom Season
Summer Growing Season
Autumn
For More Information
WARNING ON THE USE OF CHEMICALS
Text7: ISBN 978-1-60107-054-8