Gordon Johnson, Extension Fruit & Vegetable Specialist; gcjohn@udel.edu
The first seven days in May in 2010 saw temperatures in the high 80s and even some night temperatures in the low 70s in Delaware, and while windy, it was not excessively so. Then came the weekend of the 8th and 9th where average wind speeds doubled or tripled followed by low temperatures the middle of this week with scattered frost at night. These weather conditions illustrate the perils of setting out warm-season transplants in the beginning of May and to the need to take actions to avoid transplant shock.
Transplant shock will be evident by severe wilting, drying of leaves and stems, and, in severe cases, full plant collapse and death. This should not be confused with diseases such as Pythium damping off or damage from seed corn maggot or other soil insects.
Many of our transplants come from southern producers this time of year and there is always the potential for transplant shock when they are removed from southern greenhouses with little or no hardening and then are shipped up to Delaware in unheated trucks, especially when temperatures drop in the 40s or below during transport. Locally grown transplants are also susceptible to transplant shock if taken directly from greenhouses to the field without a hardening off process. Even with good hardening off, 40 mph winds can quickly desiccate plants if set in the field, especially without adequate windbreaks.
As a reminder, 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.
Soil temperatures are a key factor in the establishment of warm season crops. It is important to lay plastic well ahead of planting and to have adequate soil moisture to absorb and then retain heat. When soil temperatures are too cold, root growth is minimal in these crops and root function is impaired. Water uptake is limited by low root activity and new growth and rooting-in is slowed. Root zone insects and diseases can further stress transplants and reduce stands in cold conditions.
To avoid transplant shock, make sure transplants have well developed root systems. Transplants should easily pull from trays and have full root balls. This is critical to avoid transplant shock. Do not rush transplants with poorly developed roots 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. 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.
It is important to plant so that soil covers the root ball and that the root ball is not exposed to drying. However, for crops such as watermelons and cantaloupes, make sure that soil does not surround the stem. Deep planting in cold wet soils will result in additional stress on melons. 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. If plants are not pulling well from trays and do not have intact root balls, plants will not survive adverse weather.
Avoid planting if weather conditions are unfavorable. Look at extended forecasts and plant on a warming trend where winds are not excessive. If heavy winds or very cold nights are expected, 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. If weather conditions are unfavorable, you may also consider using row covers to protect plants.
Windbreaks are critical for early plantings. Use windbreaks between every bed for the early plantings and have windbreaks between multiple beds for later plantings. This year, many of our windbreaks offer minimal protection due to poor fall and winter growing conditions. Where windbreaks are not adequate, delay planting until favorable weather is in the forecast.