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Sterile Potting Mix
January 27, 2015

I keep writing about starting our seed in sterile potting mix in various articles, having to repeatedly retell how we prepare our starting mix. So today, I decided to just do a short feature story on the subject so I can link to it from other future and how-to articles.

The need for sterile potting mix, or as close as one can get to it, becomes obvious the first time a seed starter has young seedlings begin to topple over with a faint ring or browned area on the stem at the base of the plant. The dreaded damping off disease, present in most soils and in a whole lot of commercial potting soils, has struck again.

Damping off is actually a collection of various causative agents, mostly fungi, that kill young plants around the soil line. Knowing the specific pathogenic organisms which can cause the condition isn't as important to the home gardener as is knowing how to prevent it.

First and foremost for preventing damping off is having a starting mix free of any pathogens that can cause the disorder. One can purchase starting mixes that are supposed to be free of any damping off organisms, but such soilless mixes can be quite expensive. For the number of transplants we start, we had to learn to make our own sterile soil mix.

What's Worked for Us

KettleYears ago, I bought my wife a large, stainless steel cooking pot with an aluminized base for some birthday or such. She wasn't much impressed with it, and became less so as it tended to burn along the bottom large batches of sauces cooked in it. Getting cast aside as a very bad gift idea on my part, it got repurposed as our soil sterilization pot, replacing an old water bath canner that had rusted out from years of sterilizing potting soil.

We make our starting mix with a combination of commercial potting mix and peat moss. Our current favorite potting soil for seed starting is fairly light, but we also mix it with peat moss in a 3:1 ratio, as I like a really light starter mix that has the ability to retain lots of moisture. I add a bit of lime to neutralize the acidity of the peat moss, and at times a bit of bone meal and/or dried seaweed.

We avoid commercial potting mixes for our starter mix that have perlite and/or fertilizer pellets in them. While possibly good for houseplants, perlite just seems to get in the way when working with small seeds, and commercial fertilizer pellets can actually kill nearby seeds by burning them with their fertilizer.

I fill our soil kettle, the gift that failed, with the mix of ingredients, water it well, and then pop it into the oven (covered, of course) for an hour to an hour and a half at 400° F. I generally do this process when my wife is out of the house, as it can generate some unpleasant odors. Interestingly, the level of offensive odors varies by batch, leading me to believe that commercial potting mixes aren't all that standardized.

If kept covered, the sterilized soil mix should be good for seed starting for several weeks. Left longer, one may find mold or moss starting on the supposedly sterilized soil mix, a testimony to the spores adrift in our and most other homes.

A Quick Method

Boiling water to sterilize potting mixOn occasion, I've fallen back on a quick method for sterilizing small batches of potting mix. I simply pour boiling water over the potting mix, making sure to thoroughly saturate it. This usually happens at the Senior Garden when we're moving very young plants or cuttings to large pots. The boiling water doesn't seem to hurt the plastic pots. Of course, one has to let the soil cool before moving plants into it. We've seen no evidence of damping off when using this method, although I trust the baked potting mix method more.

That's about it, other than obvious stuff like starting with clean seed, pots, and trays. Other things that can help ward off damping off and other problems are not overwatering ones started seedlings and allowing adequate air movement around them. But the real key in preventing damping off is starting with a sterile planting medium.

Here are some good links about preventing damping off:


Just because a retired, old guy has a web site on gardening doesn't mean he knows jack about it. I have absolutely no degrees in agriculture or horticulture or any other special training. At 71 years old, I'm still learning, so there could be tragic errors in the information above. It's just what has worked for us.

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From Steve Wood, the at Senior Gardening
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last updated 12/31/2019