Gordon Johnson, Extension Ag Agent, Kent Co.; gcjohn@udel.edu
I have seen several cases of heat necrosis of transplants on black plastic mulch in the last two weeks. This is a common problem in later plantings of peppers and tomatoes in particular. Black plastic can heat up to well over 110°F on hot days in the late spring and summer. Vegetable transplants are exposed to these high soil temperatures at the soil line around the transplant hole. 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 necrosis is that there is little or no shading of the mulch with the leaves of small transplants.
There are a number of practices that can reduce heat necrosis in later planted vegetable transpants:
● Avoid using tender transplants that have not been hardened off.
● 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.
● Plant in the evening once the plastic has cooled down or in the very early morning.
● Avoid transplanting on very hot days or when extended hot, sunny weather is forecast.
● 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.