Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
It has been a difficult week for early transplanted crops (watermelons, tomatoes). Cool temperatures and high winds have made for unfavorable conditions and there have been reports of plants wilting after transplanting.
Why do transplants wilt after transplanting when there is adequate water in the root zone (transplants are well watered and water is applied during transplanting)? In the simplest terms, more water is lost from the plant leaves and stems than is taken up from the roots. Water is lost through the stomates in the leaves during transpiration and through the cuticle of the cells on the outside of the leaf. Plant cells loose turgor (internal water pressure) and become less rigid leading to typical wilt symptoms.
There are several potential causes for the disparity in water loss from foliage and water uptake from the transplant root ball. The first is related to root activity and function. In cold soils, roots of warm season adapted plants such as watermelons, cantaloupes, peppers, or tomatoes are physiologically less active. Normal functions are slowed, root tips stop growing and movement of water across the Casparian strip, which requires active transport, is slowed. This occurs where soil temperatures are below 60°F for most of the day under the plastic mulch.
Black plastic mulched beds do not heat up as much when days are cloudy, windy and temperatures are low (night temperatures in the 30s day temperatures in the 50s). Windy conditions increase heat transfer and loss from mulched beds. Plastic that is not tight or that has been laid over cloddy soil also does not transfer heat to the soil very well. Water will heat up much more slowly so excess water at transplanting can keep beds cooler longer (applied during transplanting and with the drip system). Large transplanting holes or holes that allow wind to get underneath the plastic will also keep beds cooler.
Of course, damage to the root during transplanting can also reduce root function and cause wilting. This is most common where root balls have not completely filled the cells of the trays and as the plant is pulled from the tray, some roots separate or the root ball falls apart. Watermelons and cantaloupes are most sensitive to root damage and poor handling during transplanting.
The second major cause for transplant wilting is related to the physiological state of the plant. It is recommended that transplants are well hardened off before going to the field. The hardening off process (reduced water and fertilizer, gradual exposure to outside conditions) prepares the plant physiologically to withstand the shock of transplanting. During the hardening off process, leaf cuticles thicken, stomata close partially, and photosynthesis slows. If plants are not properly hardened off they will have higher transpirational and cuticular water loss and will be much more subject to wilting after transplanting.
Often plants are shipped up from the South and they have not been exposed to colder temperatures during hardening off. They may be taken from growing houses too soon and not have full root balls. During shipping, they are in the dark and if they remain on trailers or shelves for too long, they will become acclimated to low light. When placed back in full sunlight, photosynthesis increases and stomates open, also leading to water loss and potential wilting.
Finally, environmental conditions will increase chances of wilt. We have already described how cold soils affect transplants. Wind is another major factor for water loss. Windy conditions increases water loss from leaves. Wind blows away water vapor near the leaf surface, and increases water leaving the leaf through the stomata and leaf cuticle (diffusion of water from the leaf increases). In addition, in heavy wind, the boundary layer of water right on the leaf surface becomes smaller: the stronger the wind, the thinner the boundary layer surrounding leaves, the greater the water potential gradient from the inside to the outside of the leaf and therefore the greater the water loss from the leaf. Humidity also is important in this process. In low humidity conditions, water loss will be greater.
To reduce wilting in warm season transplants, wait until beds are maintaining temperatures above 60°F. Use windbreaks and consider using row covers for early plantings in poor environmental conditions. Harden off plants well and transition plants shipped from southern areas in a staging area for a few days to get better acclimated to our weather conditions (additional hardening off).