(seedless) watermelons have been grown in the United States for over
40 years. However, it was not until fairly recently that improved
varieties, aggressive marketing, and increased consumer demand
created a rapidly expanding market for triploid watermelons. The
seedless condition is actually sterility resulting from a cross
between two plants of incompatible chromosome complements. The
normal chromosome number in most living organisms is referred to as
2N. Triploid watermelons are produced on highly sterile triploid
(3N) plants, which result from crossing a normal diploid (2N) plant
with a tetraploid (4N). The tetraploid is used as the female or seed
parent and the diploid is the male or pollen parent (Fig.1).
Since the tetraploid seed parent produces only 5 to 10% as many
seeds as a normal diploid plant, seed cost is considerably more than
for diploid watermelon hybrids. Tetraploid lines are usually
developed by treating diploid plants with a chemical called
lines normally have a light, medium, or dark green rind without
stripes. By contrast, the diploid pollen parent almost always has a
fruit with a striped rind. The resulting hybrid triploid melon will
inherit a striped pattern. Growers may occasionally find a
non-striped fruit in fields of striped triploid watermelons. These
are the result of accidental self pollinations of the tetraploid
seed parent during triploid seed production. Tetraploid fruit are of
high quality but will have seeds and must not be sold as seedless.
The amount of tetraploid contamination is dependent upon methods and
care employed in triploid seed production.
plants normally do not produce viable seed. However, small, white
rudimentary seeds or seedcoats, which are eaten along with the fruit
as in cucumber, develop within the fruit. The number and size of
these rudimentary seeds, sometimes called pips, vary with variety.
An occasional dark, hard, viable seed may be found in triploid
The United States
Standards for Grades of Watermelons (March 23, 2006) states that
“Seedless watermelons are watermelons which have 10 or less mature
seeds, not to include pips/caplets, on the face of the melon which
has been cut into four equal sections (one lengthwise cut and one
crosswise cut).” A link to the U.S. Standards for Grades is in the
Other Links of Interest in this website.
can be grown successfully in areas where conventional seeded
varieties are produced. However, they require some very unique
cultural practices for successful production.
production of triploid watermelon transplants is essential because
of the special conditions required for seed germination, emergence,
and early plant development not found in open-field situations.
Furthermore, the extra cost of transplants is justified because
triploid watermelon seeds costs are about five times greater than
those of diploid hybrid seeds and 60 times greater than
open-pollinated diploid watermelon seeds. One seed per cell should
be planted 1 inch deep with the radicle (pointed end) up to reduce
seedcoat adherence to the cotyledons. Transplants have been
successfully produced with peat pellets or in trays containing
sterile media with 1 to 2 inch cell size. Stacked prewatered trays
are placed in a germination chamber 85-90°F for two days or until
radicles are visible in the cell drainage holes. The trays are then
arranged in a greenhouse with day temperature 70-80°F and night
temperature 65-70°F where temperature control can be achieved.
Plants are fertilized every three days with a solution containing 50
ppm N from Ca(NO3)2 and KNO3 from
cotyledon expansion until the first true leaf is fully expanded,
then with a 200 ppm N solution applied every other day until the
second true leaf is fully expanded, finally the fertilizer is
reduced for several days before transplanting to the field. Plants
are ready for transplanting when the roots are sufficiently
developed to permit removal from the cell with the entire growing
mix volume intact. This will require three to five weeks depending
on cell size and growing conditions. See
Seedless Watermelon Transplant Production Guide for further
information on transplant production.
(Microsoft PowerPoint required for viewing)
Click here for web friendly version.
Watermelon fruit set
and enlargement is dependent upon growth regulators from pollen
grains and from embryos in developing seeds within the fruit and
results in triploid watermelon fruit that are triangular shaped and
of poor quality and may increase the incidence of hollowheart.
Triploid watermelon flowers do not produce sufficient viable pollen
to induce fruit set and development. Therefore, pollen from a
diploid seeded watermelon pollenizer must be interplanted with the
triploid watermelon plants to provide additional pollen.
pollenizerers have been developed by several major seed companies.
The entire field is transplanted with the triploid production
variety and then the dedicated pollenizer is interplanted according
to company recommendations; usually one pollenizer for every two or
three triploids. When the field is drip irrigated, the triploids
usually are arranged on one side of the tape and the pollenizer on
the other side.
A desirable diploid
dedicated pollinizer will produce pollen over an extended period of
time; will not unduly compete with the triploid; will have soft
fruit that will be crushed when stepped on; and have fruit that are
readily distinguished from the triploid. Special care in selection
of the dedicated pollenizer for use in miniwatermelon production is
necessary since certain combinations may be difficult to distinguish
pollenizers currently available are Companion (Seminis), Jenny (Nunhems),
Minipool (Hazera), Pinnacle (Southwestern), Side Kick (Harris
Moran), and SP-1 (Syngenta). Others will become available from
information on pollenizers see
requirements vary depending on variety selection, growing area, time
of planting, and soil type. In general, early growth of triploid
plants is slower than that of diploid plants. However, triploid
plant size eventually exceeds that of diploid plants. Seed
development in fruit of diploid varieties inhibits further flowering
and fruit set. This inhibition does not occur in triploids;
therefore, plants continue to produce fruit as long as viral
infection does not occur, insects and foliar diseases are controlled
and environmental conditions are favorable. Triploid plant
population density may be 10 to 20% less than that recommended for
production of diploid watermelon varieties. Triploid watermelon
production has been successful with 25-30 sq. ft. per plant.
Triploid miniwatermelons (producing fruit 8 lbs. or less) are grown
with about 10 sq. ft. per plant.
All methods of
irrigation including overhead, drip, seepage, and furrow are used
successfully in producing triploid watermelons. Maintaining soil
moisture at optimum levels is critical for triploid watermelon
production. Water stress (drought) can increase the incidence of
blossom-end rot and result in poorly shaped, bottle-neck fruit.
Excessive field moisture has been associated with hollowheart, a
disorder which seems to be more severe in some varieties of triploid
melons than in diploid varieties.
Pollination of Cucurbits
Cucurbit plants have
separate male (staminate) and female (pistillate) flowers (Fig.
2). Male flowers generally appear on the plants several days
before female flowers. The female flower is easily recognized by the
presence of a miniature fruit below the flower petals. Pollen from
the male flower must be transferred to the female flower for
pollination and subsequent fruit development to occur. Pollen must
be transferred from the dedicated pollenizer male flower to the
seedless watermelon female flower when this production system is
sufficiently high honeybee population is necessary to insure that
each flower is visited at least eight times. How does this translate
into hives per acre? Recommendations from various sources range from
two hives per acre to one hive per 5 acres (Fig.
3). Under most conditions, however, one strong hive per 2 acres
should result in sufficient bee activity to effect needed
open shortly after sunrise and remain open until late afternoon or
early evening. Accordingly, each flower is open for only a few
hours. The period of maximum honeybee - the most common and
effective pollinator of cucurbits - activity closely coincides with
the period when the flower is open. Honeybee visitation begins
shortly after sunrise and continues until mid-afternoon. If
temperatures are very warm, bee activity may decline about noon.
Research on cantaloupe pollination conducted in California showed
that bee visitations increased until 10 a.m. and then declined until
3 p.m. when activity almost ceased.
watermelon at the Central Florida Research and Education
Center-Leesburg showed that the number of bee visitations was more
important than the length of time that each bee stayed on the
flower. Well-shaped, fully expanded fruit occurred following eight
bee visitations to a female flower. Fruit set was significantly
reduced when only four or two bee visitations were made. Hives
should be spaced around the perimeter of large fields to provide
distribution of bees over the entire field. To maintain the health
and activity of the bee colonies, pesticide applications to the crop
should be made when bees are not present in the field, usually at
dusk or after dark.