Heat Damage in Transplants

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

We have recently seen late planted watermelon transplants die due to excess heat on black plastic mulch. Heat necrosis is a common problem on black plastic mulch in summer plantings in clear weather and at high temperatures. This is a common problem in later plantings of peppers and tomatoes grown in smaller cell sizes; however, all transplants can be affected. Black plastic can heat up to well over 120°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 may be killed at these high temperatures and the transplants will then collapse and die. Similarly. vine crop transplants laying on the hot plastic may be damaged. 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.

Heat necrosis on pepper stem next from excessive temperatures from black plastic mulch.

There are a number of practices that can reduce heat necrosis in later planted vegetable transplants:

  • 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. Another option is to spray on white particle film at the plant base. You can also mulch around the planting holes with wet straw to reduce heat loading or cover around the transplant stem with sand or clean soil.
  • Use overhead irrigation after planting to keep the plastic cooler.