SUMMER SQUASH
PRODUCTION IN CALIFORNIA
Richard Molinar, University of California Cooperative Extension
Farm Advisor, Fresno County; Jose Aguiar, University of California
Cooperative Extension Farm Advisor, Riverside County; Mark
Gaskell, University of California Cooperative Extension Farm
Advisor, Santa Barbara and San Luis Obispo Counties; Keith
Mayberry, University of California Cooperative Extension Farm
Advisor, Imperial County
University of California • Division of Agriculture and Natural Resources
Publication 7245
PRODUCTION AREAS AND SEASONS
Fresh market summer squash (Cucurbita pepo) produc-
tion can be found in almost all areas of California, partic-
ularly in the southern desert areas of the Imperial Valley,
the Central Valley from Kern to Merced Counties, and the
south-central coast from Santa Barbara north. Summer
squash, which are eaten immature, include zucchini,
crookneck, straightneck, and scallop types. The main pro-
duction occurs in the spring and summer months. Acreage
is significantly reduced in the fall when plants are dam-
aged by aphid-transmitted viruses and silverleaf whitefly
problems. Counties with the largest acreage of summer
squash in 1996 included Santa Barbara, Fresno, Riverside,
San Luis Obispo, Monterey, and Orange.
SUMMER AND WINTER SQUASH ACREAGE
AND VALUE
Year Acreage Average yield Gross
(tons/acre) value/acre
1996 7,338 9.8 $3,836
1995 5,478 11.2 $3,983
1994 7,905 9.9 $3,498
Source: Annual California Agricultural Commissioners’ Report
(Sacramento: Calif. Dept. of Food and Agriculture, 1994–96).
CLIMATIC REQUIREMENTS
Summer squash is a warm-season crop that will grow in
nearly all climates of California. Since summer squash is
a short-season crop, it thrives in somewhat cooler cli-
mates better than other cucurbits such as cantaloupe and
watermelon. Although summer squash production often
slows in cool climates, virus pressure is often lower, with
improved yield and quality. The seedlings are subject to
frost damage. For early-spring squash in the southern
desert valleys, the seed can be planted in December and
January. The optimal germinating temperature range is
70° to 95°F (21° to 35°C), and the maximum germinating
temperature is 100°F (38°C). Below 60°F (15.5°C) germi-
nation may take as long as 2 weeks. In the Central Valley,
planting may start as early as mid-February with cold pro-
tection (plastic tunnels) and from March 1 to March 15
without cold protection. The optimal growing temperature
range is 65° to 75°F (18° to 24°C).
VARIETIES
Zucchini, the most popular type of summer squash, can be
found in dark, medium, and light green colors, as well as
yellow-orange. Zucchini grows to maturity in about 60
days. The fruit vary greatly in length but are generally
long and cylindrical with the stem end about equal in
width to the blossom end. Dark green types include
Ambassador, Aristocrat, Black Jack, Dividend, Elite,
Onyx, Raven, and Revenue. Medium green types include
Embassy, Spineless Beauty, Senator, and President.
Greyzini is a light-green type, and yellow types include
Gold Rush, Golden Dawn, Gold Finger.
Other summer squashes, which take about 55 days to
grow to maturity, include yellow straightneck, yellow
crookneck, and scallop squashes. Yellow straightneck
types include Multipik, Superpick, and Golden Girl; these
fruits have yellow skin and generally have a straight
crook only slightly narrower than the blossom end. Some
yellow straightneck varieties have a more pronounced
crook than others. Yellow crookneck types include
Destiny, Freedom, Goldie, Sundance, Supersett, and
Sunrise; these fruits are generally cylindrical at the blos-
som end, narrowing to a slender neck at the stem end.
Scallop types include Early White Bush, Peter Pan,
Scallopini, Sunburst, and Bennings Green Tint; these
fruits are white, yellow, or green, and they resemble a
scallop shell, wider than thick. This type of squash is
often referred to as “summer squash,” especially by
Spanish-speaking people.
Some squash varieties, such as Dividend, have resis-
tance to certain viruses. This resistance may be the result
of natural resistance through normal hybridizing or
through bioengineering (gene insertion on a protein coat).
Other varieties of summer squash, such as Multipik, are
listed as “precocious.” These fruits start out yellow
instead of green. The precocious gene also provides for
some degree of tolerance to watermelon mosaic virus II
(WMV II) by masking the mosaic symptoms for a short
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time. Eventually the fruit displays the virus symptoms and
later becomes unmarketable. Fruit stems are also yellow
instead of green.
CULTURE
Squash plants have an extensive root system, up to 4 feet
(1.2 m) deep, with the majority of the roots in the top 12
to 18 inches (30.5 to 45.5 cm) of soil. Direct seeding is
preferred over transplants since squash is very sensitive to
transplant shock, but transplants can be used successfully
if they are not allowed to remain in the greenhouse for too
long. Squash seed remains viable for as long as 4 years if
stored in a cool, dry place. Growers commonly plant 4 to
6 pounds of seed per acre (4.5 to 6.7 kg/ha). This allows
for one seed every 9 to 12 inches (23 to 30.5 cm) on beds
that are 5 feet (1.5 m) wide. Plants are later thinned to 12
to 15 inches (30.5 to 38 cm) between plants. The seed is
planted 1 to 1.5 inches (2.5 to 4 cm) deep.
In the desert areas of California, spring-planted squash
is grown on slanted beds 60 inches (1.5 m) wide. Rows
are oriented east to west and the beds are slanted facing
south. Heat is further directed to the seedline with brown
kraft wrapping paper that is supported by stakes and slant-
ed to reflect heat toward the seed. The paper also acts as a
windbreak. Black plastic mulch can also be used to warm
the soil. With black plastic mulch, the beds are not slant-
ed, and kraft paper is also used.
In the Central Valley seed are planted in clear plastic
tunnels or hotcaps in beds spaced 5 feet (1.5 m) apart for
early- spring crops. It is generally considered to be safe to
plant seed after March 15 with no frost protection. Instead
of 5-foot beds, some growers make beds on 36- to 40-inch
(0.9- to 1.0-m) centers and plant every other row. This
allows them to use the same bed spacing for other veg-
etable crops. Tunnels of clear plastic polyethylene film,
usually 1.5 mils thick, are stretched over frames con-
structed of heavy-gauge wire hoops. The tunnels must be
vented on warm days to keep the plants from scorching.
Drip irrigation is often used in conjunction with tunnels.
Fall crops in the San Joaquin Valley run the risk of virus
problems jeopardizing potential profits.
SOILS
Well-drained soils favor squash production. The optimal
pH is 5.8 to 7.0; limestone or sulfur can be used to adjust
the pH if needed. Sandier soils require more frequent
watering and fertilization than heavy-textured clay soils.
IRRIGATION
Squash roots develop rapidly, with roots in the top 18
inches (45.5 cm of soil. Irrigations should be scheduled to
avoid excessive moisture or water stress. In the San
Joaquin Valley, if the clay or loam soil is moist to at least
4 feet (1.2 m) at planting, the soil has sufficient moisture
to carry the crop well into the growing season. In the
Coachella Valley, spring squash that is grown on black
plastic mulch is usually drip-irrigated. Spring squash
grown on slanted beds is usually furrow-irrigated.
Early-season irrigation tends to cool the soil and slow
plant growth. At least 18 acre-inches (1,854 m) of water is
required for the season. The California Irrigation
Management Information System (CIMIS) can help pro-
vide evapotranspiration (ET) rates to help with irrigation
scheduling. Generally, 1 acre-inch (103 m) of water is
applied at each irrigation. Sandy soils require more fre-
quent irrigations than clay soils. Lack of adequate mois-
ture at harvest can result in misshapen fruit; too much
moisture can aggravate root and stem rot diseases.
FERTILIZATION
Nitrogen (N) fertilizer is needed to produce maximum
summer squash yields. A soil test will help to determine
specific nutrient needs, but the following general guide-
line can be used. If the soil is cold (50° to 60°F [10° to
15.5°C]), as is often the case with early-spring plantings,
apply about 60 pounds per acre (67 kg/ha) of nitrogen (N)
and 20 to 25 pounds per acre (22.5 to 28 kg/ha) of phos-
phorous (P) before planting. Broadcast the preplant fertil-
izer and till it into the bed or band the fertilizers 3 to 4
inches (7.5 to 10 cm) below the seedline and offset slight-
ly toward the water furrow. The total recommended
amounts of fertilizer for the crop are 80 to 150 pounds per
acre (90 to 168 kg/ha) of nitrogen; 60 to 120 pounds per
acre (67 to 134 kg/ha) of phosphorous (P2O5); and 0 to
150 pounds per acre (0 to 168 kg/ha) of potassium (K2O).
Sidedress with the remaining nitrogen when the plants are
3 to 5 inches (7.5 to 12.5 cm) tall. Do not apply more than
60 pounds per acre (67 kg/ha) of nitrogen in any fertilizer
application. In drip systems, the nitrogen amount can be
broken into three to four smaller applications.
POLLINATION AND FRUIT SET
Squash is monoecious (the male and female flowers devel-
op on the same plant). During the main growing season the
ratio of male to female flowers is usually 3:1 or higher. The
female flower is distinguished by the presence of an ovary
at the base; female flowers are borne on very short stems,
and male flowers are borne on long stems. Honey bees are
the primary pollinators, and 1 to 2 hives of bees per acre
(2.5 to 5 per ha) should be provided for a good fruit set.
Poor pollination results in small young fruit that turn yel-
low, shrivel, and fall off. Incomplete pollination may also
cause misshapen fruit that are unmarketable. Squash fruit
grow about 0.75 to 1 inch (1.9 to 2.5 cm) per day. Cross-
pollination between the same species of squash can occur
(for example, between zucchini and crookneck), but this is
not a concern unless the crop is being grown for seed pro-
duction. Cross-pollination does not occur between cucur-
bits of different species (for example, between zucchini and
cucumber).
Summer Squash Production in California • 2
INTEGRATED PEST MANAGEMENT
Detailed information about integrated pest management
(IPM) for squash is available from the UC Davis IPM
World Wide Web site at http://www.ipm.ucdavis.edu, by
contacting your local county Farm Advisor, or by consult-
ing the cucurbit guideline in UC IPM Pest Management
Guidelines (DANR Publication 3339).
Insect Management. Seedcorn maggot larvae (Delia
platura) feed on germinating squash seed. High levels of
decaying organic matter in the soil may encourage seed-
corn maggots. Wireworms (Limonius spp. and others) can
kill young plants and weaken older ones by feeding on the
root system. Squash bugs (Anasa tristis) have sucking
mouthparts and can kill squash leaves and vines. Eggs of
squash bugs, which are barrel shaped and reddish-brown,
are laid on the undersides of leaves.
Whiteflies can also destroy squash plants with their
feeding. The silverleaf whitefly (Bemisia argentifolii) was
named for the physiological disorder it causes in various
cucurbits, a “silvering” of the leaves caused by the feed-
ing of only a few of the young. Feeding whiteflies suck
plant sap, resulting in silvering, defoliation, stunting, and
poor yields. Sticky honeydew is excreted by the whitefly,
promoting the growth of a black sooty mold. The green-
house whitefly (Trialeurodes vaporariorum) can also feed
on and damage squash. The melon aphid (Aphis gossypii)
and the green peach aphid (Myzus persicae) also cause
problems by weakening the plant through feeding and are
also vectors of many virus diseases (see below).
Leafminers (Liriomyza spp.) may damage newly
emerged cotyledon leaves. The population of natural
predators is usually adequate to control leafminers if not
disrupted by insecticide applications. Western spotted
(Diabrotica undecimpunctata undecimpunctata) and
striped (Acalymma trivittatum) cucumber beetles can
attack squash in large numbers and feed on young and
older plants. Their larvae feed on the roots and under-
ground parts of the stem. Larvae of several species of
armyworm (Spodoptera spp.) and the cabbage looper
(Trichoplusia ni) can also feed on squash foliage.
Disease Management. Powdery mildew (Erysiphe
cichoracearum) is common in squash fields throughout
the growing season. This fungal disease can be a problem
at any time in coastal growing areas and can be a problem
in late summer and fall in the inland valleys. Gummy
stem blight (Didymella bryoniae) can cause serious prob-
lems on stems, leaves, and fruit of squash. This disease is
often first noted as a developing rot in the lower vine stem
area, but it also may affect foliage. Charcoal rot
(Macrophomina phaseoli) affects squash and other cucur-
bits. Leaves of the crown area turn yellow and wither. The
causal organism, a soilborne fungus with a wide host
range, is common in most soils in the Central Valley.
Phytophthora root rot (Phytophthora capsici, P. spp.) is a
serious disease of squash, especially where soils are wet
for extended periods. Phytophthora may cause root rot,
stem lesions, or foliar blight. Fusarium crown and root rot
(Fusarium solani f. sp. cucurbitae) may affect the lower
stem or crown area, causing plants to wilt and die. The
fungus survives on soil and seed and is most common on
the Central Coast. Fungal diseases of squash are con-
trolled by combinations of cultural controls, such as long-
term rotation out of cucurbits (4 years or more), the use
of clean seed, and chemical controls; for more informa-
tion, see the sources listed at the beginning of this section.
Some of the most serious disease problems with
squash, as with many cucurbits, are caused by viruses.
Viruses cause mottling and distortion of leaves and fruit.
Aphids and whiteflies can transmit viruses from surround-
ing fields and weeds to the very young emerging cotyle-
don leaves. Virus transmission is vector-specific: for
example, a virus such as cucumber mosaic virus (CMV) is
transmitted only by aphids. Viruses such as CMV, water-
melon mosaic virus (WMV), zucchini yellow mosaic
virus (ZYMV), and papaya ringspot virus (PRV) are
transmitted by aphids in a nonpersistent manner (requir-
ing only seconds to a few minutes for the aphids to probe
the leaf surface and transmit the virus). It is very difficult
to visually identify the particular virus strain, and positive
identification is possible only through laboratory analysis.
To complicate the identification process, it is not uncom-
mon to find two, three, or four viruses infecting a squash
field at the same time. There are no chemical tools avail-
able to control viruses. Some control is possible through
management of the insect vector. Systemic insecticides
can be applied at planting to control insect pests, and
reflective plastic mulches have been used with some
degree of success. Some squash resistance to one or sev-
eral viruses can be found in some varieties, and the possi-
bilities of resistance are increasing each year.
Weeds. Weeds can cause yield reductions, especially if
left uncontrolled early in the season. In addition, weeds
interfere with harvest by making fruit difficult to find.
The vigorous growth of many cucurbits makes integrated
weed management feasible. An integrated approach is
necessary because of the limited availability of registered
and effective selective herbicides. Avoid fields that have
high populations of certain weeds such as common
purslane (Portulaca oleracea), field bindweed
(Convulvulus arvensis), or nutsedge (Cyperus spp.), as
these weeds are not adequately controlled by registered
chemicals or nonchemical methods. When squash is sown
during cooler seasons of the year, it grows more slowly
and is less competitive against weeds. During cooler sea-
sons squash needs a more diligent weed control program
for optimal yield and quality. Just before planting cucur-
bits, preirrigate the field to germinate weed seeds and cul-
tivate to destroy them. Black plastic mulch is frequently
used to warm the soil and help control weeds.
Nematodes. Nematodes are microscopic roundworms
that live in soil and plant tissues. Plant-parasitic nema-
Summer Squash Production in California • 3
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todes feed on plants by puncturing cells and sucking their
contents with a needlelike mouthpart called a stylet. All
cucurbits are susceptible to root knot nematodes
(Meloidogyne spp.). Damage is typically greatest in warm
regions with light, sandy soils. Nematode-infested plants
may have reduced growth and lower yield and fruit quali-
ty, and they tend to wilt earlier under moisture stress. For
information on control, see the sources listed at the begin-
ning of this section.
HARVESTING AND HANDLING
Summer squashes are usually eaten at immature stages, as
opposed to the hard or winter types, which are eaten when
the fruit is mature; the male flowers are also harvested
and sold. Summer squash fruit is generally harvested
when the rind is still tender and the seeds are immature.
Care should be taken not to injure the soft skin. Wearing
gloves can reduce damage to the fruit by fingernails. A
clean, dry rag may be used to clean dirt and dust off the
fruit. Depending on the variety, fruit reach harvestable
size 4 to 8 days after pollination. If fruit is allowed to
remain on the plants too long before picking, the plants
tend to set fewer fruit. Harvests every other day (depend-
ing on the temperature) are common for up to several
months. Zucchini and straightneck fruits are more desir-
able at 5 to 6 inches (12.5 to 15 cm) long for western mar-
kets and 7 to 8 inches (18 to 20.5 cm) long for eastern
markets. Crookneck, straightneck, and zucchini fruits
should be 1.25 to 2 inches (32 to 51 mm) in diameter.
Specific sizing and maturity may vary depending on the
market.
Fruit is hand-picked into containers and usually sorted,
graded, and packed in the field. Fruit should be cooled to
50°F (10°C) as soon as possible. Delaying cooling
adversely affects fruit quality and shortens the shelf life.
The yield depends on the number of harvests and the size
of the fruit; selecting small squash reduces the overall
yield. Most fields exceed 1,000 cartons per acre, but
yields of 2,400 cartons per acre have been reported.
Zucchini are usually packed in 28-pound cartons, while
scallop squash are packed in 26-pound cartons.
POSTHARVEST HANDLING
Fresh market summer squash should be stored at 41° to
50°F (5° to 10°C) and
