1 - You can solarize plants before putting them into compost if you are not sure if they are diseased or not, or solarize the compost afterwards for safety.

Yard and Garden - November 26, 2011

Q.

I know compost is good for my soil and that it is better to compost the dead plants from my garden. However, I worry that I will put diseased plants into my compost pile and create problems next year. I am not sure if my plants are diseased or healthy. Is there any way to avoid spreading disease with my compost?

- Jamie D.

A.

Books and common wisdom say do not put diseased plants into the compost pile, but as you mention, you may not know that your plants are diseased. To complicate matters, some diseases do not spread in the soil. Curly top virus and tomato spotted wilt virus are not soil-borne and will not cause problems if the diseased plants are composted. Plants with bacterial and fungal diseases are the ones that can cause problems in compost. If all the material put into the compost reach a pasteurizing temperature (130 degrees for prolonged periods of time, or 160 to 180 for half an hour), most disease organisms, weed seeds, and animal pests will be killed. Higher temperatures that sterilize the compost are not as effective because pasteurization leaves some beneficial organisms living in the soil to help keep disease organisms out. Achieving adequate temperatures through the compost pile takes properly balancing the nitrogen and carbon containing components of the compost pile, maintaining proper moisture, and proper aeration and turning of the compost. An alternative procedure is to pasteurize potentially diseased plants and seed bearing weeds before adding these materials to the compost pile. You can do this by solarizing the plant debris. Place the material to be solarized into a black plastic garbage bag (moisten if dried material is added), and place the bag and plant material in a sunny location. If the days are sunny and not too cold, the plant material should be pasteurized in a week or two. You can test the internal temperature on a sunny day using a soil thermometer. Put sticky tape over the area you will penetrate with the thermometer to seal the hole. Once the material has been treated, it can be safely added to the compost pile. It is also possible to pasteurize compost after composting by heating it in an oven (best with a portable oven in the garage because of odors produced), on the barbeque, or by solarization. If compost is to be used in making potting soil at home, it is good practice to treat the compost before making the potting soil.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

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