Gordon Johnson, Extension Ag Agent, Kent Co.; gcjohn@udel.edu
The frost we had this week should remind growers that as you try to get a jump on the growing season, cold weather effects need to be considered. Over the years, many of our early plantings of summer vegetables have suffered because of early cold damage and inadequate provisions to protect plants.
There has been a tendency to risk earlier and earlier plantings as growers try to hit the early market.
Earliest plantings of watermelons, cantaloupes, summer squash, and tomatoes will begin in the next 10 days. First transplanting of crops such as peppers and eggplant will begin in early May. One of the characteristics that all of these crops have in common is that they are warm season vegetables that are sensitive to cold temperatures, both in the root zone and above ground.
Considerations for early transplanted warm season vegetables:
1. Choose the lightest ground that warms up quickly for early plantings. Plant higher sections in the field first. Avoid areas that receive any shade from woods or hedgerows. Early fields should be protected from extreme wind and should not have frost pockets.
2. Lay plastic mulch well ahead of time to warm soils. Black plastic mulch should have excellent soil contact. Loose mulch is much less effective in warming soils.
3. Consider using IR plastics that trap heat (green and brown plastics). Clear plastics can be used but weeds are an issue and a good herbicide program will be needed
4. Make sure that there is good soil moisture when forming beds and laying plastic because soil water will serve as the heat reservoir during cold nights.
5. Careful attention needs to be paid to hardening off warm season vegetable transplants that will be planted early. Gradual acclimation to colder temperatures will reduce transplant shock. Do not transplant tender, leggy plants or plants coming directly out of warm greenhouse conditions for these early plantings.
6. Use vegetative windbreaks such as rye. This will reduce heat transfer by wind. Consider using windbreaks between each plastic bed in early plantings.
7. Consider using covers to protect from cold and wind and to increase accumulated heat. This includes slitted and perforated row covers and floating row covers.
8. Watch extended weather forecasts and plant at the beginning of a predicted warming trend.
9. Monitor soil temperatures in plastic beds and do not plant if they are below 60°F. Soil temperature in beds should be measured at the beginning of the day when at the coolest. When soil temperature conditions are not favorable, wait to plant.
10. Avoid planting in extended cloudy periods, especially if plants have come out of the greenhouse after an overcast period. These plants will not perform well.
11. When transplanting, make sure that there is good root to soil contact and there are few air pockets around roots.
Transplanted warm season vegetables vary in their ability to tolerate adverse weather after being set out. Tomatoes will stop growth but will grow out without much damage once warm weather returns. Summer squash also handles adverse conditions fairly well. Watermelons will hold if they have been hardened off properly. Cantaloupes can be permanently stunted if exposed to excessively harsh early conditions. Peppers and eggplants will not put on any root growth until temperatures are warm enough. Remember that all of these vegetables are susceptible to frost damage and will be killed by a late freeze.
In years with cold, cloudy, windy weather after transplanting, we have had large losses of transplants in the field. In many fields considerable hand labor was used to replace dying plants and in some cases whole fields were replanted. It is critical to have warm soil conditions after transplanting to allow roots to grow out into the bed quickly. What happens in cold, cloudy conditions is that plants shut down physiologically. Little root growth occurs and the existing roots on the transplant do not function well. If there is any wind, plants lose more water than they can take up and they die due to desiccation. This is accelerated when the sun does come out – the first sunny day after an extended cold, cloudy period is when you will see the most wilting of weakened transplants.
Later on in the growth cycle, cold weather during flowering can lead to problems with pollination and fruit formation resulting in reduced fruit set and malformed fruits.