Hybrid triploid (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 colchicine.

Tetraploid parental 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.

Sterile triploid 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 melons.

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.

Triploid watermelons can be grown successfully in areas where conventional seeded varieties are produced. However, they require some very unique cultural practices for successful production.

Stand Establishment

Containerized 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.

Field Arrangement

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.

Dedicated 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 at harvest.

Some dedicated 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 time-to-time.

For more information on pollenizers see http://edis.ifas.ufl.edu/HS333

Cultural Practices

Plant spacing 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 used.

Therefore, a 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 pollination.

Cucurbit flowers 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.

Research on 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.