Gordon Johnson, Extension Ag Agent, Kent Co.; gcjohn@udel.edu
Plant quality is an important factor in obtaining good yields from transplanted vegetable crops. Issues such as transplant age, size, stretch or height, health, and hardiness can be determining factors in the ultimate success of a crop.
One of the most important considerations is managing stretch or height of transplants. The goal is to have a transplant of a size that it can be handled by mechanical transplanters without damage. Transplants should be compact but not stunted. Plants that are leggy or stretched are not desired, due to the potential for damage during transplanting and by wind after transplanting.
Managing transplant height can be a challenge. Growth regulators that are used for bedding plants are not registered for vegetable transplants so other methods must be used. One method that is successful is the use of temperature differential or DIF. DIF is the difference between day and night temperatures in the greenhouse. In most heating programs, a greenhouse will be much warmer in the daytime than nighttime. The greater this difference, the more potential for stretch. Cloudy days and low light levels also encourage stretching. By reducing the day-night temperature difference, or reversing it (nights warmer than days), you can greatly reduce stem elongation. The critical period during a day for height control is the first 2 to 3 hours following sunrise. By lowering the temperature during this 3 hour period plant height in many vegetables can be controlled (50-70% height reduction). Drop air temperature to 50° – 55°F for 2-3 hours starting just before dawn, and then go back to 60° – 70°F. Vegetables vary in their response to DIF. For example, tomatoes are very responsive, squash is much less responsive.
Mechanical movements over transplants can also reduce size. You accomplish this by brushing over the tops twice daily for about 40 strokes with a pipe or wand made of soft or smooth material (plastic pipe works well). Mechanized systems have been devised to do this. Crops responding to mechanical height control include tomatoes, eggplant, and cucumbers. Peppers are damaged too much with this method.
Managing water can be a tool to control stretch. After emergence when plants have sufficient size, allow plants to go through some stress cycles, allowing plants to approach wilting or start to wilt before watering again. This is an effective method for some vegetables. Be careful not to stress plants so much that they become damaged (avoid severe wilting).
Managing greenhouse fertilizer programs is another tool for controlling height. Most greenhouse media comes with a starter nutrient charge, good for about 3-4 weeks. After that, you need to apply fertilizers, commonly done with a liquid feed program. Greenhouse fertilizers that are high in ammonium forms of nitrogen will cause more stretch than those with high amounts of nitrate nitrogen sources. Fertilizers that are high in phosphorus will also tend to lead to stretch. By fertilizing with high nitrate and low phosphorus sources, you can reduce stretch considerably. Using a 20-0-20 formulation for the mid-growth period would be an example. Consider using media custom blended with a starter charge that is low in phosphorus and that uses potassium nitrate as the nitrogen source. Watch the transplant crop carefully to avoid too much growth reduction or phosphorus deficiency symptoms and switch to higher phosphorus fertilizers prior to transplanting.
Exposing plants to outside conditions is used for the hardening off process prior to transplanting. You can also use this for height control during the production period. Roll out benches that can be moved outside of the greenhouse for a portion of the day when temperatures are warm and wind conditions are not too severe can help to maintain plant size and create more compact transplants. Growing on wagons that can be moved into and out of the greenhouse is another option. This also creates a system that can be used to harden off transplants prior to setting plants out in the field.