Triploid hybrid watermelons are commonly called seedless watermelons. Occasionally, a few normal seeds occur in these watermelons. Completely sterile triploid hybrids are produced by crossing normal diploid (2n) lines with tetraploid (4n) parental lines.
Observance of a number of technological recommendations is a prerequisite for successful production of triploid watermelons. The elements of watermelon growing technology given below have been developed based on the experience of growers, suppliers, seedling producers, university research workers and Petoseed experts. Regional, seasonal and climatic conditions and technological recommendations for growing watermelons may vary and affect the end result. Watermelon growers should be aware of the need to adjust the present recommendations to the local growing conditions. This information is no more than a set of familiar growing technology elements used by watermelon growers to formulate their own cultivation programs.
Triploid hybrid watermelons FAIL TO PRODUCE ENOUGH POLLEN to ensure normal pollination. Therefore, a regular (seeded) variety should be planted nearby to ensure the production of the pollen needed. A salable variety should be used as a pollinator. To avoid confusion during the harvesting and to be able to distinguish between triploid seedless watermelons and seeded ones, some experienced growers prefer to use as pollinating varieties those ones which markedly differ in external color from the triploid hybrid. The proposed set of pollinating varieties has been made up with due regard for plant vigor, pollen producing potential, etc. The recommended triploid hybrid-pollinator ratio is 2:1 (two rows of the triploid hybrid and 1 row of the pollinating variety).
In watermelons, pollen is transferred from the pollen plants by insects, predominantly bees. Use of bees for pollination is, therefore, advisable and, in some cases, a necessity. Watermelon growers may examine their fields for bee activity. It is normally done by walking around the field when the watermelon plants have come into bloom and counting the bees. The recommended number of healthy and active bees varies between 3.75 and 7.5 swarms per hectare.
An important feature
Triploid hybrid watermelons can be grown by both transplanting and direct seeding. In either case, it should be borne in mind that triploid hybrid watermelons have particularly high TEMPERATURE and MOISTURE requirements during germination. The triploid watermelon seeds require HIGHER SOIL TEMPERATURESand OPTIMUM SOIL MOISTURE for germination. That is why most growers cultivate these watermelons by transplanting.
Cultivation by transplanting
Transplants are recommended because the seeds of triploid watermelons will germinate poorly if you start them in the field. Triploid watermelon seeds have special needs for good germination: Use soil mixture of peat moss and vermiculite. The main characteristic of this soil mix is that the balance between water retention and drainage is optimal for the triploid seed requirements. Don’t use a soil mix that dries out too soon or one that is ‘to heavy’ and stays saturated with water for long time.
- Soak peat pellets thoroughly and let them drain a few hours before planting. Be more than generous with the amount of water supplied. After you let the soil trays drain; you may want to re-water to make sure the moisture in the soil mix is adequate for the germination of your seeds. (Unlike ordinary watermelons, these seeds have an air space inside that can pick up water and rot the seeds if seeds are kept too wet.) Never soak triploid watermelon seeds in water (not overnight, not even for a few hours).
- Do not add any other chemical to the seed. Do not put any treatment. The seed has been milled and conditioned to germinate under optimum conditions.Don’t leave seed in contact with high temperature or high humidity.
- When the trays are wet and drained make the holes in the soil mix a 1,5 cm depth.
- Plant 1 or 2 seeds per pellet at least 1,5cm deep. If seeds are inserted pointed side up, you will have less trouble with seed coats that clamp around seedlings. This step may be too laborious but if you can afford to it the results will be more satisfactory. (Seedless watermelons have a problem with seed coats that clamp around sprouting plants, stopping further growth. Seedcoats that don’t come off during germination must be removed by hand immediately after emergence).
- After seeding, cover flats with a fine layer of coarse vermiculite and in top of that a sheet of clear poly plastic to maintain uniform moisture. You can also use standard paper towels. The goal is to maintain uniform moisture for at least the first 36-48 hours. This is important!
- Maintain a temperature of 85°F (or 30°C) for 36-48 hours day and night. This is a critical period for germination. Do not allow pellets to become dry but do not soak them if water is needed. The plastic sheet or paper towel covering should hold an adequate moisture level for this critical period, but check by feel to be sure.
- As soon as some seedlings start to emerge – say 10% – remove the plastic. Begin watering as needed. Drop temperature to 75°F (or 24°C). Remove seed coats by hand as mentioned earlier, immediately after emergence. Thinning to one plant per pot is done at this time to improve size of melons at harvest.
- When an 80 to 90% emergence has been achieved you can drop the day and night temperatures even further to allow the plants to harden out and be ready for field transplant.
- Plants will be ready for transplanting in about 3-4 weeks. Do not grow plants larger than 3 true leaves. Warmer temperatures stimulate spindly growth. Shading or cloudy weather may also cause excessively spindly growth. Fine tune the details on water, temperature, and shade according to your growing conditions. You don’t want plants to be very stretched but don’t want to stress them in the opposite direction either
Cultivation by direct seeding
Direct seeding into the open ground can be performed when the midday temperature of soil at a depth of 10 cm has been 21°C and higher for three days running. Excess soil moisture can delay germination or result in complete nongermination.
Pollen production time
Not infrequently, the first fruits of triploid hybrid watermelons develop in stress conditions resulting from exposure to low temperatures, temporary soil dryness, from bad fertilizer application timing, etc. On frequent occasions, these fruits are of low quality, misshapen, sutured and contain some seeds. The growers can partly resolve the problem by guarding against the presence of pollen from the pollinating variety during early flowering of triploid watermelons, thus preventing early fruit-setting on triploid plants. This can be effectively achieved by: 1) rows of the pollinating variety being sown simultaneously with the transplanting of triploid seedlings; 2) rows of the pollinating variety being sown two weeks after the direct seeding of the triploid hybrid into open ground. This strategy ensures that mature pollen is available from the pollinating variety by the time the first female flowers of the triploid hybrid have fallen off. Many watermelon growers send farm hands out to the field to remove the first fruits.
The plant density depends strongly on the growing area, planting date and soil conditions. One possible planting pattern is 300 – 500 cm х 40 – 80 cm. Each grower should try various planting patterns to be able to identify the most suitable one.
Fruits of the triploid hybrid King of Hearts F1 PS are similar to those of Crimson Sweet in rind color. Therefore, the hybrids Royal Majesty F1 PS (markedly elongated fruit) and Pata Negra F1 PS (dark green color of rind) are recommended to be used as pollinating varieties. The same hybrids make an effective pollinator for the yellow-fleshed triploid hybrid Honeyheart F1 PS.
Irrigation and fertilization
Triploid hybrid watermelons may develop an abnormality called “hollow core” or “cracked core”. This is most commonly the case where growers cultivating triploid hybrids use the same irrigation and fertilizer schedules as with regular varieties. Observations by specialists involved in triploid hybrid watermelon production allow the following conclusions to be drawn: 1. The rate of nitrogenous fertilizer application can be brought down to a somewhat lower average. The triploid hybrid watermelon plants are highly vigorous and respond well to fertilization. However, if the grower believes he is using an optimal fertilizer application rate, he can proceed as before. In either case, it is strongly recommended that lower rates of nitrogenous fertilizer application be considered for individual fields or field plots. 2. Irrigation immediately prior to harvesting should be avoided, because watering at this point appears to be the primary cause of the “cracked core”. Irrigation immediately after harvest seems to be a much better option. Monitoring the soil moisture content at a depth of 30 cm is highly desirable.
Watermelon growers should be able to identify the signs of ripeness of triploid hybrids by cutting fruits prior to harvesting and noting 1) the rind color and waxy condition; 2) the state of the watermelon tendril; 3) other characters by which the degree of ripeness can be judged. Triploid watermelons should be harvested at economic maturity since, unlike seeded varieties, they do not ripen after being harvested. Triploid watermelons often outyield seeded varieties, producing a higher marketable yield per hectare. Triploid hybrid watermelons harvested at economic maturity exhibit higher shipping quality and keeping ability. Chilling of harvested fruits or picking fruits at cool hours of the day can increase the quality and extend the shelf-life of triploid watermelons.