Sterilizing potting mix helps prevent problems

Frederick, MD(Zone 6b)

A lot of problems (damping off, fungus gnats, weed seeds) can be prevented or reduced by starting seeds in sterile potting mix. Just because the bag is marked sterile or labeled as seed starting mix doesn't mean its sterility hasn't been compromised -- especially not once you've opened the bag.

I don't sterilize all of my potting mix, because I go through too much of it, but I do sterilize smaller amounts of mix used for seed starting or for putting down African violet leaves, etc.

You can sterilize potting mix in the oven, in roasting pans or bags with a thermometer to monitor temperature, but it does make an odor that many people (or their spouses) object to.

I sterilize my mix in the microwave. I have a big tupperware bowl that holds nearly 2 gallons, and I fill this with potting mix, first un-compressing any lumps or chunks of mix. It is fine to add moisture crystals, time-release fertilizer pellets, extra perlite, etc. before microwaving the mix. Be sure to add enough water to thoroughly moisten the potting mix, at least 1 cup per quart of mix. *** This is very important. Dry mix will catch fire in the microwave! ***

Cover the container loosely, and microwave on high for 8 to 12 minutes, or until you see steam starting to condense on the underside of the lid. Seal the lid down onto the container, and let the steam continue to sterilize the mix for another 10 minutes.

As an added benefit, the steam will "open up" the peat and get it nicely moistened.

Greensboro, AL

critter: just what I needed to know. Tons of seeds to start. Found some old bags of potting mix. Probably no longer sterile.

I was thinking about re-sterilizing the mix in old seed starting domed flasts with my steamer. They haven't been opened, but they are a few years old.


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