Green Thumb: Pasteurize Your Compost

Seedlings that have succumbed to "damping-off". Image from Oregon State.
We've run into some issues in the last few years when compost being used as potting soil for new seeds has led to all kinds of errant seedlings sprouting (we're looking at you, cherry tomatoes!). The compost has harbored seeds of all kinds that were thrown out, only to hang around until spring when we try to grow other seeds out of the newly formed dirt.

While this may be welcome in some instances, in others it's important that your compost not sprout unwanted weeds or plants you don't intend to take care of. We've also learned that "damping-off" (a horticultural condition caused by pathogens killing or weakening seeds and seedlings)
can zap your seedlings before they have a fighting chance to grow. 

These issues can be solved in one fell swoop by pasteurizing your compost before using it for potting soil.

About "Damping-Off"
Most prevalent in wet and cool conditions, damping-off happens when pathogens kill or weaken seeds and seedlings.All symptoms result in the death of at least some seedlings in any given population. Groups of seedlings may die in roughly circular patches, the seedlings sometimes having stem lesions at ground level. Stems of seedlings may also become thin and tough ("wire-stem") resulting in reduced seedling vigor. Leaf spotting sometimes accompanies other symptoms, as does a grey mold growth on stems and leaves. Roots sometimes rot completely or back to just discolored stumps.

Seeds that are infected with damping off will not germinate and plant stems shrivel causing seedlings to topple over and die. If you have waited an unusually long time for a particular seed to germinate, brush the soil away and carefully take a peak. If it is dark and mushy it has damping off and the only thing left to do is start over, this time with clean potting soil.

This problem happens everywhere things grow, no matter where you live and there is absolutely no remedy once plants and seeds are infected. The answer is prevention.

Damping off can be prevented or controlled in several different ways. Sowing seeds in a sterilized growing medium can be effective, although fungal spores may still be introduced to the medium, either on the seeds themselves or after sowing (in water or on the wind). Maintaining drier conditions with better air circulation helps prevent the spread of the disease, although it can also prevent or slow down germination. Spraying or drenching the soil with a recommended anti-fungal treatment (such as copper oxychloride) also helps suppress the disease. Homemade solutions (including ones made from chamomile tea or garlic) are used by some gardeners for this purpose.

Pasteurizing Compost

Note: the following tips were gleaned from Aradacee.
Many people choose the safest route to prevent hitchhiking seeds and damping-off by buying a pre-sterilized package of potting soil, if you have a large amount of pots and flats to fill, this could be expensive. By taking a couple of extra steps before you begin, you can use your own rich, organic compost.

Some people "bake" their soil in their oven to kill micro-organisms. But this process of sterilization kills everything, even the healthy organisms that you have worked so hard to create.

The answer is simple: Instead of sterilizing compost and garden soil, pasteurize it. While sterilizing kills virtually all surface-dwelling microorganisms, when you pasteurize your potting mixture, it is only heated to a temperature that kills harmful organisms and leaves beneficial organisms alone.

To pasteurize, take a large aluminum-baking pan and cover it with three to four inches of potting soil, insert a meat thermometer in the center and place in a preheated oven, at 200°F., once the center reads 160°F., bake for 30 minutes. Allow mixture to cool thoroughly before using.

Nicole Caldwell

Nicole Caldwell is a self-taught environmentalist, green-living savant and sustainability educator with more than a decade of professional writing experience. She is also the co-founder of Better Farm and president of betterArts. Nicole’s work has been featured in Mother Earth News, Reader’s Digest, Time Out New York, and many other publications. Her first book, Better: The Everyday Art of Sustainable Living, is due out this July through New Society Publishers.