SWEETPOTATO PRODUCTION
IN CALIFORNIA
C. SCOTT STODDARD, UC Cooperative Extension farm advisor,
Merced and Madera Counties; R. MICHAEL DAVIS, UCCE plant
pathology specialist, UC Davis; and MARITA CANTWELL,
UCCE postharvest specialist, UC Davis.
PRODUCTION AREAS AND SEASONS
In 2011, California sweetpotato (Ipomea batatas) produc-
tion for processing and fresh market consumption was
estimated to be a $130-million industry, with production
of about 582 million pounds on 18,200 acres. In an aver-
age year, California produces 20% of the sweetpotatoes
grown in the United States and is the second-largest-
producing state, behind North Carolina. While sweetpo-
tatoes are usually regarded as a Southern crop, California
production exceeds that of Louisiana and Mississippi
combined. Merced County accounts for approximately
90% of the commercial acreage planted to sweetpotatoes
in California, but they can be grown in many of the warm
agriculture production areas of the state.
The primary production area for sweetpotatoes is in
sandy soils between Atwater and Turlock in Stanislaus
and Merced Counties, centered on the Highway 99 cor-
ridor in the center of California. Additional commercial
production occurs in Kern County. Typical highly pro-
ductive soils are sandy to loamy sand in texture. Yield
and quality are poorer in heavy soils.
Hotbeds for producing transplants are typically
installed from the first week of February through early
March. Transplanting from the hotbeds begins in late
April and continues into June, though transplanting
for seed fields may occur as late as July. Commercial
harvest from early plantings begins in mid-July, but
crop yields are low at that point so the amount of the
harvest is limited. Regardless of location, the bulk of
the harvest occurs in September and October, when the
roots are dug out and placed into storage. Harvest usu-
ally is complete by early November.
CLIMATE
Sweetpotatoes are a warm-season, frost-sensitive
crop. Daily maximum air temperatures between 85°
and 95°F (29.7° and 35.3°C) are ideal for root produc-
tion, but temperatures above 100°F (>38°C) are not
harmful so long as the crop is adequately irrigated
and temperatures drop at night. Recent growth cham-
ber research has shown that high day/night tem-
peratures (95°/80°F [35°/27°C]) result in decreased
storage root production, but in production areas of
California elevated temperatures like this rarely occur
at night. Because they are sensitive to even a light
frost, sweetpotatoes are planted in spring after any
chance of frost has passed. For the same reason, the
crop must be harvested before the autumn onset of
heavy frost and cold rains, since the roots may sustain
chilling injury if they are subjected to temperatures
below 50°F (10°C) for even a few hours.
VARIETIES, PLANTING TECHNIQUES,
AND SOILS
The edible sweetpotato is an enlarged storage root that
grows in various shapes, sizes, and colors. Varieties,
particularly those grown for market use, fall into four
categories based on their skin color and flesh char-
acteristics: red skin with orange flesh (so-called “red
yams”), copper-rose skin with orange flesh (“yams”),
cream-tan skin with yellow flesh (“sweets”), and
burgundy-purple skin with white flesh (“Oriental”).
Most packers have a supply of all four types, though
some will specialize in certain types more than others.
Orange-flesh sweetpotatoes have a moist texture after
baking, whereas sweets may not.
Diane is the main cultivar for the red-yam cat-
egory. It is characterized by dark red, smooth skin
with deep orange flesh. These qualities make it the
premium yam-type sweetpotato. At the retail level,
it is often labeled incorrectly as Garnet, which was
the red-skinned variety that established this mar-
ket class in the 1970s. Diane is marketed mainly in
California, where red-skinned sweetpotatoes are
University of California
Agriculture and Natural Resources
http://anrcatalog.ucanr.edu • Publication 7237
Vegetable Production Series
VRIC.UCDAVIS.EDU
UC Vegetable Research
& Information Center
SWEETPOTATO ACREAGE AND VALUE
Year Acreage
Average yield
(tons/acre)
Gross
value/acre
2007 13,300 14.00 $7,000
2008 14,500 14.75 $8,968
2009 17,400 17.00 $9,112
2010 18,500 17.25 $8,627
2011 18,200 16.00 $7,072
Source: USDA–National Agriculture Statistics Service
http://anrcatalog.ucanr.edu
vric.ucdavis.edu
2 • Sweetpotato Production in California • ANR 7237
popular and often command a higher price. The
yam types Beauregard and Covington have copper-
colored skin with deep orange flesh. For 20 years,
Beauregard was the most commonly grown variety
in California and the United States, but in 2009 it
was displaced by Covington. Beauregard, while sus-
ceptible to nematode damage, has yield and storage
characteristics that are superior to those of Covington,
but Covington is adaptable to a wider range of grow-
ing environments and has better resistance to diseases
and nematodes.
Golden Sweet and O’Henry are major varieties in
the sweets category. They are characterized by their
cream-colored outer skin and yellow interior flesh.
Dry matter content for Golden Sweet is typically 30 to
35%. O’Henry has recently displaced Golden Sweet in
both acreage and production. O’Henry is actually an
off-type Beauregard that naturally mutated to have
white skin and yellow flesh. It contains about 20 to
22% dry matter and does not have the dry-flesh flavor
and texture profile of Golden Sweet.
Oriental types, and most commonly Japanese
yams, make up an important and growing part of the
California sweetpotato industry. These include a vari-
ety of flesh and skin colors, including white, purple,
and red, but their flesh is typically dryer than that of a
Beauregard, with a more subtle flavor. The most com-
mon varieties are Kotobuki and Murasaki-29, both of
which have burgundy skin and white, dry flesh.
Recently released sweetpotato cultivars have been
granted patent protection, and growers are legally
restricted in the sale and distribution of these roots
for propagation. Both Covington (released from
the North Carolina State University Agriculture
Experiment Station) and Murasaki-29 (from
Louisiana State University Agriculture Experimental
Station) are patented. Future releases of new variet-
ies, like the sweet variety Bonita released in 2012,
will carry similar restrictions. Proceeds from the sale
of plants and roots from these varieties support the
breeding programs that developed them.
Sweetpotatoes are vegetatively propagated from
plant cuttings, called “slips,” from hotbeds. “Hotbed”
is the name used for the nursery area where sweetpo-
tato roots saved from the previous year are used to
produce new plants for the production fields. These
“seed roots” for the hotbeds typically are placed on
the ground in February and covered with a shal-
low layer of soil. Medium-sized potatoes are typi-
cally used for this purpose, but any size root can
be used to produce sprouts. Commercial hotbeds
in California are typically 8 feet (245 cm) wide and
may be several hundred feet long. For warmth, clear
plastic is stretched over iron rods bent into half hoops
over the bed to form an elevated plastic tunnel run-
ning the entire length of the bed. Hotbeds are so
named because they are warmed by the decomposi-
tion of cotton gin trash that is buried under the roots.
Cold beds, which do not use gin trash, are also used
for propagation. It takes approximately 500 to 800
pounds (225 to 365 kg) of roots, depending on the
variety, to furnish cuttings for a 1-acre (0.4-ha) plant-
ing of the crop.
Sprinklers are used to irrigate hotbeds until the
plants are ready to be transplanted—when they are
about 12 inches (30 cm) tall. The plants are then cut
at or slightly below the surface of the soil. Cuttings
from the hotbeds are mechanically transplanted in
two rows on 80-inch (200-cm) prepared beds from
mid-April through the end of June. Most fields are
drip irrigated, but some are furrow irrigated. Cuttings
are watered heavily at transplanting and then surface-
applied drip tape is set on top of the beds between the
two rows. Cuttings are planted 9 to 15 inches (23 to 38
cm) apart within the row. Typical plant populations
are 13,000 to 17,000 plants per acre.
Soils selected for sweetpotato production gener-
ally range from sand to loamy sand in texture, are
well drained, and are low in salts. While the plants
can grow in heavy soils, root yield and quality will
be reduced. Acidic soil conditions (pH 5.5 to 6.5) are
preferred, as this helps suppress pox, a root disease.
Sweetpotatoes are moderately sensitive to soil salts: a
10% reduction in yield can be expected if soil electri-
cal conductivity (EC) exceeds 2 dS/m. Furthermore,
roots grown in salty soils do not store well and are
more likely to develop the abiotic disorder “tip rot”
after a few months in storage.
IRRIGATION
More than 95% of sweetpotato fields use drip irriga-
tion; the remaining acreage uses furrow irrigation.
Unlike many transplanted crops, no sprinklers are
used to establish the crop, nor are the fields typically
pre-irrigated. Producers rely on copious amounts of
transplant water (> 2,000 gallons per acre [> 19,000
L/ha]) and the expeditious installation of drip sys-
tem lines to keep water stress at a minimum for the
newly planted crop. Maintaining adequate moisture
in the soil immediately surrounding the plants for the
first 17 days after transplanting is critical to promot-
ing the initiation and development of storage root
cells. Surface drip is used because of root intrusion
problems in buried lines. A typical installation has
one drip line running down the center of an 80-inch
bed, irrigating two rows of plants. Occasionally the
drip line will be moved from the center of the bed
and placed directly in the plant row to facilitate early
root growth, but this is generally avoided because it
stimulates weed germination in the plant row.
Irrigation frequency depends on the crop’s evapo-
transpiration (ET) requirements and the water-
3 • Sweetpotato Production in California • ANR 7237
holding capacity of the soil, but may be done daily
once the plants are fully established. Total water
use typically ranges from 2.5 to 3.5 acre-feet per
acre. Irrigation cut-off dates vary depending on
crop development and harvest schedule. Irrigation
should be halted when jumbo-sized roots exceed
33% of total root production, unless the crop is slat-
ed for processing and maximum total tonnage is
desired. There is no evidence to support the practice
of discontinuing drip irrigation 2 to 4 weeks prior to
harvest in order to toughen the skin and minimize
harvest losses, but such practices do reduce the
development of scurf if the roots have already begun
to develop this disease (see Pest Management,
below).
FERTILIZATION
Sweetpotatoes have an extensive root system and
make efficient use of soil nutrients. The vines and
leaves respond better to applied fertilizers than do
the roots. On average, the crop removes 2.7, 1.0, and
4.5 pounds of nitrogen (N), phosphate (P2O5), and
potash (K2O) per 1,000 pounds of harvested roots.
Phosphorus (P), potassium (K), zinc (Zn), and boron
(B) fertilizers should be applied preplant or immedi-
ately after transplanting, as determined by soil tests.
Up to one-half of the crop’s N requirements can also
be applied at this time, with the remainder applied
later through the drip tape to match the growth char-
acteristics of the field. Because the cuttings used for
transplants are rootless, they are very sensitive to fer-
tilizer burn, so inclusion of fertilizers in the transplant
water is not recommended.
High-yielding, drip-irrigated fields require N
rates of 125 to 175 pounds per acre (140 to 200 kg/
ha), one-half of which is applied through the drip
tape during the first half of the season when the
plants are rapidly growing. If P is needed, 50 to 100
pounds of P2O5 per acre (56 to 112 kg/ha) is applied
before planting. Potassium (K) is readily taken up by
the leaves and roots of the crop, but has been shown
to have only modest impacts on sweetpotato root
yield. If required, 200 to 250 pounds of K2O per acre
(225 to 280 kg/ha) should be applied to replace what
was removed at the previous harvest. Unlike phos-
phorus, potassium is sometimes applied through the
drip tape. Preplant fertilizer should be placed in a
band 9 inches to the side of the plant row at a depth
of 8 to 10 inches. Alternatively, a banded applica-
tion of potash directly beneath the drip tape in the
middle of the bed can be used. This latter method
relies on irrigation water to move the potassium to
the plant.
INTEGRATED PEST MANAGEMENT
Hotbeds are a distinct and separate part of the whole
production system for sweetpotatoes, and as such
they require different pest management techniques
than are used for the production fields. The prima-
ry method of pest control for the past two decades
has been preplant soil fumigation with methyl bro-
mide plus chloropicrin, followed by hand weeding
as needed. This fumigation regimen provides nearly
100% control of weeds, nematodes, and fungal dis-
eases. However, with the implementation of laws that
require the phase-out of all but the most critical uses
of methyl bromide, alternative management tech-
niques are needed. These include 1,3-D (Telone) plus
chloropicrin (Pic) applied under plastic tarp, metam
sodium/metam potassium, preplant fungicides and
herbicides, and solarization.
Weeds are the main pest problem in sweetpota-
to hotbeds, and hand weeding remains an impor-
tant component of hotbed weed management.
Nonselective foliar herbicides (glyphosate, pelargonic
acid) are occasionally used postemergence on weeds,
but before the crop plants’ emergence. Annual grasses
can be effectively controlled with postemergence grass
herbicides such as fluazifop (Fusilade), sethyoxydim
(Poast), and clethodim (Select). Other occasional pests
include aphids, which may need to be controlled in
order to limit their impact on the growth of plant cut-
tings. Fungicides are available to help control fungal
diseases, but in general these can be managed more
effectively with good seed selection and irrigation
management. If fungicides are used, applications are
made to the seed roots prior to their being covered
with soil. Both thiabedazole (Mertect) and dichloro
nitroanaline (Botran) can help reduce the incidence of
scurf development on seed roots in the hotbed.
Weed management. The primary method for weed
control in sweetpotato production fields is mechan-
ical cultivation, supplemented by hand weeding.
Hand weeding, though heavily used, is not the pre-
ferred method due to the expense and the amount of
time it requires. Because sweetpotatoes are a small-
acreage specialty crop, only a limited number of
effective chemical control options are available for use
in the United States, and preplant herbicides are not
commonly used in California. Perennial and annual
grasses can be controlled with postemergence grass
herbicides such as fluaziflop (Fusilade), sethyoxydim
(Poast), and clethodim (Select). A crop oil concentrate
is often required to maximize effectiveness. Annual
and perennial grasses should be of the proper size
and actively growing for good control. Consult your
farm advisor or PCA for details and follow label
directions with regard to chemical use restrictions.
4 • Sweetpotato Production in California • ANR 7237
Since the main method of irrigation is through
drip tape set on top of the bed, the bulk of weed
germination occurs in this wetted zone, so mechani-
cal cultivation down the center of the bed (with the
drip tape temporarily moved aside) is an effective
way to control many weeds, provided that the cul-
tivation is done when the weeds are still small and
before the crop begins to vine, typically around 5
to 6 weeks after transplanting. Directed sprays of
glyphosate (Roundup) with shielded sprayers can
also provide effective control of most weeds down the
drip line, and eliminates the need to move the drip
tape. Chemical contact with the crop foliage should be
minimized in order to avoid damage to desirable root
development.
Disease identification and management.
Sweetpotatoes are vegetatively propagated: roots are
sprouted and those sprouts are transplanted to the
field to produce more roots. True seed is not used in
commercial production because sweetpotatoes rarely
flower. An unfortunate consequence of not using true
seed, however, is that viruses and many soilborne dis-
eases can accumulate in the plants, greatly diminish-
ing both yield and quality. Common diseases include
russet crack (caused by the russet crack strain of
sweetpotato feathery mottle virus [SPFMV]), soil rot
(pox, Streptomyces ipomea), charcoal rot (Macrophomina
phaseoli), scurf (Monilochaetes infuscans), stem rot
(Fusarium oxysporum f. sp. Batatas, race 1), and vari-
ous forms of soft rot (multiple causal organisms,
including Erwinia, Pythium, Rhizoctonia, and Fusarium
spp.). The fungal diseases are best controlled by using
disease-free seed (roots) and by cutting transplants
above the ground. Fungicide dips can offer addi-
tional protection from stem rot, but because recently
released varieties have been bred to be resistant to this
disease, this practice becomes less and less common
every year. Soil pox is controlled with field fumiga-
tion and through the use of resistant varieties; manag-
ing soil pH and avoiding late season plant dates can
also lessen the severity of this disease. Crop rotation
is an important component of disease management in
sweetpotatoes; a given field should not be planted to
sweetpotatoes for more than three years in succession.
Once planted in the field, sweetpotatoes imme-
diately start to acquire viruses. Research has shown
100% infection by the end of the first season, albeit at
low levels within each plant. As these roots are saved
year after year, virus levels build up and yield and
root quality decline. The only effective way to control
virus diseases is through the use of virus-tested plants
or roots. The process, developed by UC Davis in the
1960s, involves aseptically cutting the meristem (usu-
ally 0.5 mm long) from the very tip of a sprouted root.
The cut meristem is placed in a test tube and grown
on synthetic nutrient agar to produce a new plant,
which is then transplanted in the greenhouse and
grown out for virus testing. The whole process takes 6
months or longer, depending on the sweetpotato vari-
ety. Once verified as virus-free, the new plant is prop-
agated through cuttings, which are sold to growers.
Insect and pest management. The main insect
pests impacting sweetpotatoes are wireworms
(Limonius spp.) and grubs (various species, but most
likely the larvae of scarab beetles such as the ten-lined
June beetle [Polyphylla decemlineata and P. sobrina]).
Preplant soil fumigation with products that contain
1,3-D or metam sodium is the most effective way to
control these pests. Preplant insecticides offer some
pest suppression, but their degree of efficacy is more
variable. Nonetheless, preplant insecticides are used
in areas where no fumigation is allowed, such as
in buffer zones. Occasionally, western yellowstripe
armyworm (Spodoptera praefica) can become a prob-
lem that will need to be controlled. Sweetpotatoes
have a very high threshold for leaf damage before
yields are affected, but control measures are generally
warranted when the loss of leaf canopy exceeds 30%.
Several good insecticides are registered for this use;
consult your farm advisor or PCA for detai
