Special Note about Unvented Heaters in Greenhouses. One of the most critical features in greenhouses is a source of heat to provide appropriate temperatures. A frequent question by growers is the use of supplemental heaters in the spring. Do not use unvented heaters when growing transplants in the greenhouse or high tunnel. An unvented heater is one that is designed without a flue connection so that the heat and products of combustion are exhausted into the greenhouse. Unvented heaters can be fired with natural gas, propane or kerosene which all contain traces of sulfur. During combustion sulfur in the fuel is combined with oxygen to form sulfur dioxide. Levels as low as 0.5 part per million (ppm) can cause injury to some plants. Once the sulfur dioxide enters the plant through the stomates, it reacts with water to produce sulfuric acid that causes leaf burn, flecking and general chlorosis. Tomatoes and white petunias are very sensitive and will show damage in as little as one hour. Ethylene gas is another pollutant formed during combustion. Ethylene levels as low as 0.01 ppm can cause symptoms such as malformed leaves and flowers, stunted growth, bud abscission, epinasty (downward bending of leaves) and flower senescence.
Handwatering and overhead irrigation systems are the primary methods of watering vegetable transplants. The amount of water and frequency of watering will vary depending on container size, growing media, greenhouse ventilation and weather conditions. Water requirements change as plants grow. The weight of containers is commonly used to determine the moisture level of the media and when to water. Pick up individual containers and if they feel heavy, even though the surface is dry, do not water. If containers are light in weight, even though the surface looks wet, investigate further to make sure the water is thoroughly wetting the media. Tempered water is used by some growers to minimize shocking seedlings with cold irrigation water.
Some growers successfully use a water gauge that measures how much water is used. Some growers use a container placed among the pots to measure the amount of water applied. Experience will dictate how much water is needed to thoroughly saturate a container. Other growers use their finger to feel the media in shallow containers (the surface can be dry, while deep in the pot the media can feel wet).
It is important to water thoroughly, to moisten the entire container, which will promote root growth to the bottom of the container. If this is not done, root growth will develop in the upper part of the container and plants will be more prone to drying and drought stress. Excessive watering leads to succulent plants and restricted root growth.
Allow plants to dry down before watering, but do not let the plant wilt severely, as this will damage roots. Vegetable transplants should be watered thoroughly early in the day to allow foliage to dry before evening. If foliage remains wet overnight, foliar diseases will develop.
Most commercial mixes contain a wetting agent that provides initial hydration and improves wettability of the mix. Older mixes (stored longer than 8 months) are harder to wet and the addition of a liquid wetting agent may be needed.
Compact, uniform transplants can be easily handled for planting in the field. Leggy transplants may develop with low light levels, overwatering, overfertilizing or when plants are held for longer than anticipated in the greenhouse due to unseasonable weather conditons. Since very few growth regulators are registered for vegetable transplants, plant height is often managed by increasing light levels, using mechanical stresses such as "brushing" and adjusting temperature, water and fertilizer levels.
Maximizing Light Levels: Maximizing the amount of light plants receive helps reduce plant stretch. Giving plants adequate space, and replacing plastic coverings as needed helps prevent reduction in light levels.
Adjusting Temperature (DIF): The greater the difference between daytime and nighttime temperatures, the more plants will "stretch" (stems elongate). When the day temperature is very warm and the night temperature is cool or cold, plants will be taller. If the day and night temperatures are both the same, plants will be shorter than with warm days and cool nights. If the night temperature in the greenhouse is kept warmer than the day temperature by using heating at night and ventilation during the day, the plants will be even shorter. Keeping day temperatures cool (70º F) will help keep transplants shorter. The relationship is referred to as DIF, or difference between day minus night temperatures. The critical period during a day for height control using DIF is the first 2-3 hours following sunrise, starting about 30 minutes before sunrise. By lowering the temperature during this 3-hour period by a few degrees, plant height in many vegetables can be managed. Vegetables vary in their response to DIF. For example, tomatoes, cole crops, eggplant and melons are very responsive, while squash, cucumber and peppers are much less responsive.
Mechanical Brushing: Mechanical stress reduces stem elongation. Wind, shaking or brushing are all types of mechanical stress. Research has shown that mechanical stress reduces stem elongation and maintains plant height. For example, brushing transplants twice daily for 18 days using about 40 strokes back and forth with a cardboard tube suspended from an irrigation boom can result in as much as a 30% reduction in stem elongation. Growers have also successfully used a wand made of plastic plumbing pipe or a flat piece of polystyrene foam. Vegetable plants such as tomatoes, eggplants, cucumbers and some varieties of broccoli and cabbage have responded to this method of height control. Note that this technique has damaged some tender plant species such as peppers and could also enhance the spread of disease. Brush plants when the foliage is dry and if you see plant damage, reduce the number of times you are brushing the plants. Brushing can improve establishment of transplants in the field. They resume their normal growth about 3 days after the brushing stops. There is also little or no reported effect on yield.
Water Stress: Water stress is another tool growers can use to manage plant height. Maintaining plants on the dry side limits cell expansion and plant growth. This method requires close monitoring to avoid permanent damage such as leaf burn or even plant death. One technique is to irrigate the growing mix thoroughly and then allow it to dry to the point where plants wilt before irrigating thoroughly again. Growth is restricted during the period when the growing medium is very dry. Once watered, the plants rapidly resume growth. Experienced tomato growers have successfully used this technique.
Low Phosphorus: Withholding nutrients can also be used to prevent stretching. Low phosphorus fertilization is especially effective for tomatoes. If carefully managed, a mild to moderate phosphorus (P) deficiency may result in a desirable reduction in growth with no foliar symptoms of P deficiency. If this method is used, use a starter fertilizer when transplanting into the field.
NOTE: To produce healthy transplants for field production, water and nutrients must be carefully managed.
Plant Growth Regulator: A review of pesticide labels indicates that Sumagic (uniconazole) is the only growth regulator labeled for use on a limited group of vegetable transplants (tomato, pepper, eggplant, tomatillo, ground cherry, and pepino). Sumagic is a gibberllin biosynthesis inhibitor suppressing plant height by inhibiting internode elongation. It is a particularly active plant growth regulator, so very small concentrations are needed. Apply Sumagic only as a foliar spray at a rate of 2-10 ppm. As with any plant growth regulator, it is recommended to test growth regulator treatments on a small number of plants with a low rate before full-scale implementation. The maximum cumulative amount of Sumagic applied must not exceed 10 ppm with coverage of 2 quarts per 100 sq. feet. This means that total amount used in sequential applications can only add up to 10 ppm spray (example, two applications at 5 ppm or 4 applications at 2.5 ppm). The last spray must be no later than two weeks after the two to four leaf stage of development. Experiments have shown that sequential applications produce the best results and that the earlier that the plants receive the Sumagic spray, the greater effect it will have on the final height of the transplants. As only a limited number of tomato varieties have been tested, growers are encouraged to do their own in-house trials on a small number of plants with a low rate before full-scale implementation.
Acclimating or Hardening-Off Transplants
The transition from the greenhouse to the field involves changes in light, temperature and wind. Vegetable transplants benefit by a gradual acclimating "hardening off" period before they are transplanted into the field. Gradual exposure to outdoor growing conditions and reduced watering at the end of the growing period with some protection from wind and temperature but full exposure to light can increase the survival rate of transplants in the field. Three to six days are adequate to acclimate transplants. Larger growers may be using roll out benches but smaller growers can use wagons to move transplants into and out of the greenhouses as needed.
Care must be taken to not "over-harden" young transplants. Cool-season crops exposed to very low temperatures can result in bolting (in cabbage) or buttoning (in broccoli or cauliflower). Warm-season crops generally are hardened at temperatures higher than those of cool-season crops. Cold temperatures can set back warm-season crops and can induce disorders such as catfacing in tomatoes.