Gordon Johnson, Extension Fruit & Vegetable Specialist; gcjohn@udel.edu
Watermelon transplants for early production are being started this week. Those destined for later field plantings will be started throughout the month of April. Growing quality seedless watermelon transplants requires attention to detail and conditions required are different than for seeded types. Common problems include incomplete germination, uneven growth, weak plants, and stretching (leggy plants).
Seedless watermelon production can be broken into 6 phases: seeding, initial germination, emergence, seed leaf stage to first true leaf, first true leaf to second true leaf, and hardening off.
Seeding
Trays with square cells at least 2 inches deep and 1 inch square should be evenly filled with a general greenhouse growing medium like Pro-Mix BX , Fafard #2, or Sunshine #1 (these all have a starter fertilizer charge). Do not use fine seed starter or plug mix types. Do not compress the media. Trays should be watered to capacity and then allowed to drain off excess for 24 hours. During this 24 hour period, trays should be placed in a heated area so that the media reaches a temperature of 85°F. Make planting holes 1” deep with a dibber and plant seeds with the pointed side up. Cover with a small amount of warm moist media just enough to fill over seeds in the holes. Do not water after seeding. Seeding should be done in a way that trays stay at 85° F (do not allow trays to get cold).
Initial Germination
Germination should be done in a room or chamber where temperatures can be maintained at 85-90°F and where there is high humidity. Uniform tray temperature is critical. This phase will last 2 day – trays should be kept in this high temperature growth area for 48 hours and no more. To insure even germination, it may be necessary to move trays around after 24 hours (trays on bottom shelves moved to top shelves and vice versa). In this phase the seed root will emerge but the crook that will carry the seed leaves above the surface should not be visible. If you see crooks, you have left trays in the germination area too long and you may experience plant stretch during emergence (if plants have emerged you are too late – stretch has already occurred).
Emergence
After initial germination, it is critical to move plants immediately from germination areas to the greenhouse for emergence. If you are having another grower germinate your seeds, it is important to schedule pickup or delivery so that there are no delays. Greenhouses should be set for 72-75°F day temperatures and 65°F night temperatures. Do not water until after you see emergence and even then water sparingly as needed to keep trays and emerging seedlings from drying out. Excess water and high greenhouse temperatures during the emergence phase will lead to stretch.
Seed Leaf Stage to First True Leaf
Maintain greenhouse temperatures in the 72-75°F day and 65°F night range during this period. Water moderately to keep plants from drying out but do not fertilize during this period (this assumes that the media you are growing in has a starter fertilizer charge). You want the plants to grow slowly for highest quality.
First True Leaf to Second True Leaf
Continue maintaining greenhouse temperatures in the 72-75°F day and 65°F night range during this period. You can fertilize once the first true leaf emerges. Generally 2 fertilizations of 100 ppm nitrogen concentration one at first true leaf and one at second true leaf will be all that is needed. If a constant feed system is used, set for 50 ppm nitrogen each watering once the first true leaf has emerged. These fertilization rates are for the media listed in the seeding section that contain a starter fertilizer charge. Avoid using fertilizers with high amounts of ammonium N as the nitrogen source as this can lead to stretch (use fertilizers with calcium nitrate and potassium nitrate as the main nitrogen sources). Avoid over-watering. Again, you want plants to grow slowly for highest quality.
(Some growers will use a media with no starter fertilizer charge. If that is the case, a different fertilizer program will be needed. Use fertilizers with calcium nitrate and potassium nitrate as nitrogen sources. Use 50 ppm N from emergence to first true leaf every 3 days, 200 ppm N every other day from first true leaf to second true leaf)
Hardening Off
It will take 4-6 weeks to finish transplants. Prior to transplanting, harden off plants starting one week before setting in the field. This is accomplished by lowering day time temperatures in the greenhouse (if greenhouses have side curtains roll them up during days if temperatures are not too cool). Reduce watering and stop fertilization. Some growers have the ability to place plants on wagons or move benches outside during the day, bringing them in at night. This is advised where possible but make sure the area is sheltered from high winds and avoid days where the temperature is below 60°F.
Pollenizers
The above information is for growing the seedless watermelons. Seeded pollenizers do not need special germinating conditions and can be grown directly in the greenhouse. The key is to time the production so that plants are produced and hardened off at the same time as the seedless types. They also should be grown slowly and attention should be paid to avoid stretch. Follow the same recommendations from seed leaf stage through hardening off.