Sterilizing Potting Mix or “Die Pythium, Die”

(After two years, I’m starting to realize that this blog is a ten month blog – I practically forget about it in November and December. Oh well, it’s January so it’s time to start things up once again!)

Last year I posted African Violet Disaster – Pythium in which I described how my attempts to propagate african violets all failed because some potting mixed that I’d purchased must have contained pythium spores. This soil-born fungus is fatal to african violets. While most potting mixes that you purchase are sterilized, this experience made me a little wary of what might be in the potting mix that I bring home from the store.

003This fall I ordered a couple of violets from Lyndon Lyon Greenhouses in Dolgeville, NY. The plants were packed well and arrived in great condition. But when I saw them, I knew that sooner rather than later I’d have to move them into larger pots since they arrived in small 2″ pots. That got me thinking about my last african violet disaster and hoping I could find a way to not repeat it.

While I’ve never had a problem with potting mix purchased at a particular garden center, I wanted to take an extra step to sterilize the soil just in case and to calm my concerns! I went online and found a number of different ways to sterilize potting mix, the majority of them involving baking the mix in the oven.

As I looked closer into this approach, I noted a few warnings that made me think twice about trying it. The first is that if you over-heat the mix, chemicals that are toxic to plants can develop. Talk about defeating the purpose of sterilizing. The other thing that put me off to the oven technique is that all of the sites say that baking potting mix smells terrible! If it were spring or summer and the windows were open I wouldn’t have given it much thought. But in the cold of winter with everything sealed up tightly, I wasn’t too excited about smelling up the house.

So I tried a different approach – the hot water method of potting mix sterilization. With this technique, you put the mix into a pot, pour boiling water over it and let it drain and then repeat the process two more times. This should be enough heat to kill any pathogens without overheating the mix. Also it limits the smell of sterilization. The only downside is that you can only sterilize small amounts of potting mix.

Since I was looking to transplant two african violets, a small amount of mix was fine with me. I took a clean 8″ clay pot, put a piece of paper towel over the drainage hole and then filled it about half full with potting mix. The first tea kettle of boiling water drained well but the second kettle didn’t. The paper towel had gotten clogged so I poked a small hole in it and the water drained easily. After allowing the third application of boiling water to drain, I had a clay pot about one-third full of potting mix that should be sterilized. The only problem was that it was too wet to use. So I loosened it up a little bit and put the pot aside to allow the mix to dry – the in the dry winter air of a heated home, it didn’t take long for the excess water to evaporate. In my next post I’ll show you how I used it.

While the process is a little tedious, if boiling water can limit pythium, I’ll be using this technique to sterilize all of the potting mix that I use with african violets.

The experiment has begun and only time will show how well it works…


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