There should be no other plants in the transplant greenhouse to harbor insects or diseases that can infect the young tomato plants. If you adhere closely to the standard horticultural recommendations for temperature, fertilizer, and spacing, your transplants should be as wide as they are tall. Because embryonic flowers are forming in the plants after they are four weeks old, it is essential to keep the temperature above 60 degrees. Cooler temperatures for more than a week can result in catfaced fruit. The irrigation frequency and fertilizer components should be noted whenever changes are made.
Balanced nutrition is critical to both high fruit production and disease resistance.
Most greenhouse tomato growers send either the spent fertilizer solution or foliage samples for analysis by a laboratory that is familiar with greenhouse tomato requirements. Water quality can have an effect on solubility of fertilizer salts, so it should be checked for carbonates and pH. If you have hard well water, it may be necessary to bring the pH down by adding acid. The components of the fertilizer should change with growth stage. It is very important to make sure that the plants are not receiving too much nitrogen. Some varieties are very sensitive to excess nitrogen, which reduces yield and causes the plants to be more susceptible to disease.