Emmalea Ernest, Extension Fruit & Vegetable Specialist; emmalea@udel.edu
Some vegetable transplants for spring planting have already been seeded in the greenhouse and others will be started soon. Efficiently producing healthy transplants on the right schedule for timely planting is an important part of successful vegetable production. The information below is an adaptation of Table A-4 from the Mid-Atlantic Commercial Vegetable Production Recommendations.
Vegetable Transplant Production: Germination and Transplant Growth Temperatures, Time to Transplant and Recommended Trays
| Crop | Optimum Germination Temp (°F) | Optimum Day Temp (°F) | Minimum Night Temp (°F) | Weeks to Grow | Recommended 1020 Tray Sizes |
| Broccoli | 60-85 | 65-70 | 60 | 4-7 | 72 |
| Cabbage | 60-85 | 65 | 60 | 6-7 | 72 |
| Cauliflower | 65-85 | 65-70 | 60 | 6-8 | 72 |
| Cucumber | 65-90 | 70-75 | 65 | 2-4 | 50, 72 |
| Eggplant | 80-90 | 70-85 | 65 | 6-9 | 50, 72, 128 |
| Lettuce | 60-70 | 60-65 | 40 | 4-6 | 96, 128, 200 |
| Melon (cantaloupe, honeydew, etc.) | 75-85 | 70-75 | 65 | 2-4 | 50, 72 |
| Onion | 65-85 | 65-70 | 60 | 9-12 | 288, 338 |
| Pepper | 80-90 | 70-75 | 60 | 7-9 | 72, 128 |
| Summer Squash | 85-95 | 70-75 | 65 | 2-4 | 50, 72 |
| Tomato | 70-80 | 65-75 | 60 | 5-6 | 50, 72, 128 |
| Watermelon (seeded/pollenizers) | 80-90 | 70-75 | 65 | 3-4 | 50,72 |
| Watermelon* (seedless) |
85-90 | 72-75 | 65 | 4-6 | 50, 72 |
* See the watermelon section of the Mid-Atlantic Commercial Vegetable Production Recommendations for detailed instructions on seedless watermelon transplant production.
To decrease transplant production time and increase germination uniformity, place transplant trays in a germination chamber or on thermostatically controlled heat mats for the first 48-36 hours after seeding. Use the optimal germination temperatures in the table as a guide for setting the thermostat. For crops with a wide optimal range, germination will occur more quickly at the high end of the range. Trays should be removed from the germination chamber or heat mats turned down to the optimal day temperature as soon as the root emerges from the seed and before the seedlings emerge. Continuous high temperatures will produce leggy transplants.
The time to produce a well rooted transplant depends on the crop and the growth conditions in the greenhouse. To produce transplants more quickly, use the smallest recommended cell size and keep temperatures within the recommended optimal range. Low light conditions (caused by cloudy weather or weathered greenhouse film) will slow transplant growth.
Transplant cell size should be chosen to balance the goals of timely transplant production (smaller sizes establish more quickly), successful establishment (larger cell sizes are generally easier to establish), greenhouse space efficiency (smaller cell sizes take up less greenhouse space) and greenhouse management time (smaller cell sizes require more timely watering). Generally, you should use the smallest cell size that you can successfully establish in the field to reduce costs associated with growing media and greenhouse time and space.