Gordon Johnson, Extension Vegetable & Fruit Specialist; firstname.lastname@example.org
We have seen some problems where excess bleach used for disinfecting greenhouse flats caused damage to transplants. The following is some information from the University of Massachusetts on using bleach for disinfecting flats: “When used properly, chlorine is an effective disinfectant and has been used for many years by growers. A solution of chlorine bleach and water is short-lived and the half-life (time required for 50 percent reduction in strength) of a chlorine solution is only two hours. After two hours, only one-half as much chlorine is present as was present at first. After four hours, only one-fourth is there, and so on. To ensure the effectiveness of chlorine solutions, it should be prepared fresh just before each use. The concentration normally used is one part of household bleach (5.25 percent sodium hypochlorite) to nine parts of water, giving a final strength of 0.5 percent. Chlorine is corrosive. Repeated use of chlorine solutions may be harmful to plastics or metals. Objects to be sanitized with chlorine require 30 minutes of soaking and then should be rinsed with water. Bleach should be used in a well-ventilated area. It should also be noted that bleach is phytotoxic to some plants.”
Do not use straight bleach for disinfecting flats. Bleach contains sodium and chloride. Excess chlorine can be toxic to some plants. With excess chlorine, plants may wilt when soil moisture seems adequate, foliage has an abnormal dark blue/green color and individual leaves are dull and leathery, with scorching on leaf edges and premature yellowing of the oldest leaves. Sodium toxicity is seen as marginal leaf burn on the oldest leaves.