Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
May is the month where the majority of vegetable transplants are set in the field. Successful crops start with transplants that get off to a good start. The following are some considerations when handling and setting transplants.
Make sure that transplants in trays are hardened off well before transplanting. Hardening off is most commonly done by exposing plants to outside conditions by moving the plants out of the greenhouse, in a protected area, for about a week. Wagons are ideal because they can be moved into sheds at night if temperatures drop too low or cold strong winds are expected. In greenhouses with roll-up sides, hardening off can be accomplished by increasing the day-time exposure to cross winds. Reduced watering and fertilization are also a part of hardening off the plants. During the hardening off process, the cuticle of the plant thickens. The cuticle is the outermost layer that covers leaf surfaces and is composed of wax, lipids, and hydrocarbon polymers and protects the plant from water loss and desiccation.
We have thousands of plants that are shipped into the region from southern growers. When receiving transplants, make sure to inspect them well upon arrival for signs of disease or damage. For plants in trays, remove from trailers immediately upon arrival and place in a staging area that is protected from wind. Plants that have been pulled from trays and boxed should be transplanted within 2 days of receipt. Holding them longer risks severe defoliation and potential for heating in the boxes. If they must be stored longer, place in a cool but not cold place. Some plants are still shipped bare root in bundles. These also should be planting within 2 days and roots should not be allowed to dry out.
Currently, temperatures are not optimum for setting out warm season transplants. Ideally, you want to plant on a warming trend where nights do not drop below 45. 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, soils are wet, and there is cloudy weather, soils stay cool, even under plastic mulch. 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 and average air temperatures should also be above 65°F (ideally above 70°F) for good establishment of these crops. Seed and root maggots and root diseases such as Pythium can further stress transplants and reduce stands.
Make sure transplants have well developed root systems. Transplants should pull easily from trays and have full root balls. Do not rush transplants into the field. Vine crops are very sensitive to root damage during transplanting.
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 or tall 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.
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. Other crops such as tomatoes and peppers can tolerate deeper planting. 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 conditions are not favorable for planting and 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.
Provision for water at transplanting is critical for plant survival. Planting hole watering is recommended at planting. Mechanical transplanters with water tanks are ideal for this. With hand plantings, provision to irrigate overhead immediately after transplanting may be necessary. In plastic mulch systems with drip irrigation, having adequate water at planting can sometimes be difficult. Running the drip irrigation system so that the planting area is saturated often leads to leaching of fertilizer nutrients from the bed and can keep beds cold in adverse weather. Adding dilute fertilizer solutions in the transplant water is also a common practice. Follow manufacturer’s recommendations and make sure the fertilizer is dissolved well if using dry soluble sources. Fertilizers are salts and excess fertilizer or fertilizer that is unevenly mixed or dissolved can cause salt injury to transplants.