How to sterilise your potting mix

When we put a wee snippet into our video showing you how to sterilise your potting mix, we didn’t expect the response it generated. Another one of those wee things I had assumed everyone knew how to do or at least knew why they might choose to do it. Boy did I get that wrong.

So, yes, in answer to all the requests, we will do a video about it once we have a moment to breathe 😀 but for now, let’s just go over some of the basics.

Why would I sterilise my potting soil/mix/compost?

In short, because the mixes you buy may not be as free of disease, pests, mould or beastie eggs as you might think. Most commonly, fungus gnats.

Obviously, I can’t comment on individual brands and their practices, but I can tell you from my personal experience and from the feedback we get from the huge worldwide community here on the blog and youtube, we can say that you definitely can’t guarantee that using your potting mix won’t introduce something like fungus gnats into your home or greenhouse. Which trust me, is super, super annoying once it happens and very difficult to get rid of. So I operate on better safe than sorry.

It may be that there have been changes in practices around potting mix (houseplant mixes) in recent years or it may just be that there are air holes in the bags and this allows nasties to get into the mix while they are sitting in the store. Who knows, but I can say that we are hearing that our community definitely feel that they can no longer be 100% certain of the sterility of their bought potting mixes anymore.

This also applies to starting your delicate little seedlings. If you can eliminate pathogens, beasties, fungus all that jazz early on, then your seedlings have a better chance at being successful and flourishing. However, like everything else, your house, your rules 😀

So, I did a video showing you guys one method, where you can use boiling water to saturate your potting mix and let it do the work. This is a bit more labour intensive and does take a bit longer as the soil is obviously wet and needs to dry out before you can use it.

Another method of doing this is to bake your mix.

Now some people do this in the microwave, but it’s not something I have ever tried so I don’t think it’s fair for me to talk about it, but something I have done is bake this in a conventional oven (or for me the grill in the garden).

For this method, you need a large ovenproof tray or dish as obviously it’s going into the hot oven. And you want to dampen your mix, not soak it, just dampen it and then add your potting mix to this in a layer about 3 inches deep. You dampen the soil as you want the heat to generate steam, but if you make the mix too wet, it will slow the whole process down.

Get your oven preheated to about 100C ready to go and grab an ovenproof tray or dish, some foil and a thermometer.

Now cover the top with some foil, keep it loose, don’t tighten it all down securely and you are going to pop this into the oven and bake it. Keep a check on it with a thermometer, and when the temperature in the centre of the mix is 85C, bake it for 30 mins. When you are finished, leave the mix to cool to room temperature, still covered.

You may however decide to bake at a different temperature, depending on what you are wanting to target, so here are the generally recommended temperatures for a 30-minute bake.

Target Temperature (C)What you are aiming to kill when maintained for 30 mins
50water molds (oomycetes)
65most plant pathogenic fungi, bacteria, and viruses, slugs, worms, centipedes etc
75pathogenic bacteria, soil-based insects
85weed seeds
100heat resistant plant viruses and weed seeds (you know the really annoying ones)

Just a wee word of warning about this method though. It does stink, so be aware of this if you are choosing to bake your soil in your family oven. Also, baking your potting mix has the potential to be harmful if the internal temperature of the mix is allowed to get too high. So if you are worried, that may be something you want to look into.

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