Tips for Starting Seedless Watermelon Transplants
The information below is a summary from the Mid-Atlantic Commercial Vegetable Production Recommendations https://go.umd.edu/MACVPRG
Triploid (seedless) watermelon seeds require a special regime to germinate well, since they are less vigorous than standard diploid types. Seeded pollenizers and standard-seeded watermelon transplant production do not need special germinating conditions and can be done directly in the greenhouse.
Due to their size, watermelon transplants should be grown in plug trays with cells at least 1.5 inches in diameter and 2 inches deep; anything smaller will restrict root growth and provide less protection to the transplant.
- Use a general commercial greenhouse growing medium with a starter fertilizer. Fill trays and water them to capacity. Allowed to drain excess water for 24 hours in a heated area so that the media can warm up to 85°F (29°C). Maintain this temperature during seeding. Planted seeds at 1 inch deep with the “pointed” side up.
- During germination, it is critical that trays are kept at a uniform temperature of 85-90°F (29-32°C) and at high humidity. Check plants after 44-48 hours.
- Once they have emerged, move them immediately to the greenhouse. Set your greenhouses to 72-75°F (22-24°C) during the day and 65°F (18°C) at night. Maintain this temperature cycle until transplants are ready to be hardening off in 4-6 weeks.
- Do not water until after crook emergence. Water as needed to prevent media & seedlings from drying out.
- Do not fertilize until the first true leaf emerges.
- If you used a growing medium that contains starter fertilizer, then apply 100 ppm N at the first true leaf and again at second true leaf appearance. If using a constant feed system then, apply 50 ppm N for each watering once the first true leaf has emerged.
- If you used a medium without starter fertilizer, then you will want to use fertilizers with calcium nitrate and potassium nitrate as N sources, and apply 50 ppm N every 3 days from emergence to first true leaf, and 200 ppm N every other day from first true leaf to second true leaf.
- Transplants will take 4-6 weeks from sowing to finish transplants. One week before to transplanting into the field, harden off plants. Do this by lowering day temperatures, reduce watering, and stop fertilization. If possible, place plants on wagons or move benches outside during the day and bring them in at night, but make sure the area is sheltered from high winds and avoid days where the temperature is below 60°F (16°C).
- Transplant watermelons into the field when daily mean temperatures have reached 60°F (16°C). Protected transplants from winds with row covers or rye windbreak strips.
- Tips to reduce transplant stretch: Stretched watermelon transplants that have an extended hypocotyl, the area of the stem below the cotyledons or seed leaves.), which causes them to be more susceptible to damage during and after transplanting.
- Avoid leaving plants in the germination chambers too long. Move trays to growing areas before the cotyledons emerge.
- Stretch is more likely to happen during low-light or cloudy conditions. High light reduces stretch. Make sure greenhouse films have high light transmission and replace film more often in transplant houses.
- Avoid using fertilizers with large amounts of ammonium as the N source, as this can lead to stretch. Use fertilizers with calcium nitrate and potassium nitrate instead. Avoid high phosphorus fertilizers.
- Keep houses in the 72-75°F range during the day.
- Avoid overwatering, keep plants on the dry side.