Weather Conditions and Setting Out Transplants

Gordon Johnson, Extension Ag Agent, Kent Co.;

This past week has been challenging and growers have been trying to set transplants between the rains. Weather conditions currently are not favorable for the growth of warm season vegetable transplants (watermelons, cantaloupes, tomatoes, peppers, eggplants, cucumbers, squash). We had some unusually warm weather from April 25-28 with average air temperatures in the 70s that allowed early plantings to go in on plastic mulch with promise of good establishment. From April 29 onward, average air temperatures have been mostly in the 50s and we have had rainfall every day from May 1 to May 7. This weather is expected to continue until Sunday. Next week promises some sun but temperatures will still be moderate.

Warm season vegetable transplants vary in their ability to withstand sub-optimal conditions depending on how well they have been hardened off and their inherent ability to withstand stress. Tomatoes, cucumbers, and squash are better able to handle early season stresses than cantaloupes, watermelons, or peppers. When temperatures are cool and soils are wet, growth is minimal in these crops. We often see problems, especially the first few days when sunny weather returns, with plants wilting. This is because root systems have not established or are not functioning well. Root growth is slowed in cold soils and low oxygen in water soaked soils will also limit root growth. Average soil temperatures need to be 65°F or higher under the plastic and average air temperatures should also be above 65°F (ideally above 70°F) for good establishment of these crops. Seed corn maggots and root diseases such as Pythium can further stress transplants and reduce stands.

The following are some considerations when transplanting warm season vegetables under suboptimal conditions:

● Make sure transplants have well developed root systems (transplants easily pull from trays and have full root balls); do not rush transplants into the field.

● Make sure transplants have been hardened off well by exposing them to outside conditions, eliminating fertilizer, and controlling watering well ahead of planting.

● In seedless watermelon systems, time production of pollenizer transplants so that they coincide well with the seedless transplants. Pollenizers are often planted a number of days after seedless because they emerge quicker. However, pollenizer root balls may not be well formed compared to the seedless transplants and they can suffer excessive losses in the field when planted in stressful conditions. The opposite can also be true if pollenizers are ready but the seedless plants do not have good root balls.

● Leggy plants will be a problem in stressful conditions and should not be used if at all possible. Leggy plants are more susceptible to damage in transplanting and wind damage after planting thus subjecting them to additional stress. Unfortunately, cloudy overcast weather often leads to stretch in transplants.

● Transplants should be planted at the proper depth. This is particularly critical for watermelons and cantaloupes.  There should be enough soil to cover the root ball of these crops but they should not be planted so deep so that the stem is covered. Deep planting in cold wet soils will result in additional stress on melons. Watermelons and cantaloupes should not be set deeper even if they are leggy.

● Extra care should be taken during transplanting during stressful periods to reduce injury to plants, particularly to root balls. Damage to roots will reduce establishment success especially in melons, cucumbers, and squash. Train planting crews so that they do minimal damage to transplants.

● Target lighter sandy soils that are well drained for planting in cold and wet periods. Leave out fields or sections of fields with low areas or areas that are excessively wet and plant them when more favorable weather conditions return.

● If plants will hold, it is best to wait until more favorable weather returns. Often there is no earliness gained by planting in the stressful period; or gains are negated by stand losses and the need to replant areas.