Transplanting Warm Season Crops in April

Gordon Johnson, Extension Vegetable & Fruit Specialist;

Warm season vegetables such as tomatoes, cucumbers, squash, and watermelons require special attention when transplanted in April.

It is important to plant in your highest elevation fields with the lightest soils first and avoid low areas and frost pockets.

Black plastic mulch is most commonly used for early vegetables because of its soil heating property. It is important to lay the plastic so that it is tight against the soil on a firm bed. This allows for effective heat transfer that will promote good root growth. Green/brown IRT (InfraRed Transmitting) plastic suppresses weeds nearly as well as black mulch and lets infrared light through to warm the soil beneath more quickly and to a higher temperature. It is best used with very early planted warm season vegetables.

Start planting only when a warming trend is in the forecast. This is when daytime temperatures are expected to increase during the week and nighttime temperatures do not drop below 45°F. Bed temperatures should be above 60°F during the day. Do not plant on a cooling trend and avoid planting when cold, clear nights and high winds are in the forecast.

Also avoid planting if extended cold, cloudy weather is in the forecast. It is critical to have warm soil conditions after transplanting to allow roots to grow out into the bed quickly. In cold, cloudy conditions, plants shut down physiologically, little root growth occurs, and the existing roots on the transplant do not function well, thus increasing the risk for transplant stunting or transplant losses.

Target fields with well advanced (the tallest) rye windbreaks between each row for early plantings. Windbreaks reduce wind injury and desiccation of transplants and reduces the loss of heat from black plastic mulched beds, thus allowing more heat to be accumulated during the day (to be released at night). Row covers may be required in addition to windbreaks in the earliest plantings.

In areas without windbreaks, consider using floating row covers for cold sensitive crops for the first 2-3 weeks. Use wire hoops supports over the top of plants to avoid mechanical injury. Clear perforated plastic row covers also can be used to increase daytime temperatures and heat the plastic beds. However, clear row covers do not have the same insulating effect of floating row covers.

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.

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 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.

In seedless watermelons, losses of plants in the field can be problematic, especially where pollenizers died. Replanted pollenizers will flower later and may delay fruit set. To complicate matters, many seedless watermelon growers have switched to co-planted pollenizers (pollenizers planted in the same cell with the seedless variety). Loss of both co-plants will require replacing both seedless and pollenizer. Loss of the seedless in the co-planted cell will require replanting next to the pollinizer with a seedless transplant. Loss of pollenizers in the co-planted cells may necessitate adding pollenizers between plants when replanting is straight pollenizer trays may are available. Another complication is that it may be difficult to tell which plant has died in co-planted cells. Fields with reduced numbers of pollenizers can have fruit set problems, reduced fruit sizes, and increased hollow heart, particularly in the crown set.

leggy watermelon transplant

Leggy watermelon transplant with poor root ball. This plant would not survive in early plantings.