Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
Watch for heat damage in transplants on black plastic mulch planted recently. This is a common problem in later plantings of peppers and tomatoes in particular.
Vegetable transplants are exposed to high soil temperatures at the soil line around the transplant hole. I took soil surface temperatures in black, white and metalized plastic mulch in mid-afternoon this Thursday (June 9). The air temperature was 97ºF. In open planting holes at the soil line, the temperature was >130ºF in the black plastic mulch, near 120ºF in the white plastic, and less than 110ºF in the metalized (aluminum) mulch.
The stem tissue just at or above the level of the plastic will be killed at these high temperatures and the transplants will then collapse and die. Small transplants do not have the ability to dissipate heat around the stem as roots are not yet grown out into the soil and water uptake is limited. Another factor in heat damage is that there is little or no shading of the mulch with the leaves of small transplants.
In addition, high bed temperatures have the potential to damage roots.
There are a number of practices that can reduce heat damage in later planted vegetable transplants:
● Delay planting until a cold front goes through if possible.
● Use larger transplants with greater stem diameters and more leaves to shade.
● Make a larger planting hole, cutting or burning out the plastic.
● When transplanting into the plastic, make sure the stems of transplants do not touch the plastic once set.
● Water sufficiently in the hole to reduce heat load, the more water the better.
● Plant in the evening, once the plastic has cooled down, or in the very early morning.
● Switch to white or aluminized plastic mulch for later plantings. This will reduce the heat loading significantly.
● In smaller plantings you may paint the planting zone on the black plastic mulch white with latex paint and then plant through this white strip once dry. You can also mulch around the planting holes with wet straw to reduce heat loading.
● Use overhead irrigation after planting to keep the plastic cooler.