How to sterilise used compost for your seed and potting mixes

Before reusing your old or spent compost, it's a good idea to sterilise it to help prevent problems like damping off, fungal problems, kill off insect eggs like fungus gnats and aphids. It's really easy to do and can make a huge difference to the success of your seed starting and seedling growth not to mention make life easier if you have plants indoors.

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[myrustygarden]

Morning ladies, I even sterilize my new compost because I’m sick of fungus gnats etc being in the new stuff too. As I use 50 50 compost and coconut coir I put the coir on top of the soil then pour boiling water on top it swells the coir and kills the critters. Thank you for sharing as always 🐝 safe ladies 🇨🇦👍

[GardenerScott]

Using an outside grill is a great way to sterilize the mix without running everyone out of the house from the odor. Great tip!

[markcrichton]

Thanks Eli for the definition page for the word beastie. I laughed out loud. Great explanation on why you sterilize your compost. Had issues with aphids one summer and did not sterilize the compost and I had a city sized population of aphids in the middle of the following growing season in my pots. Really regret the shortcut of not sterilizing. Thanks for the good video reminder.

[knitknitsew]

I have always just added my spent compost to my garden beds and bought new seed sowing mix. Such a good money saving and environmentally savvy idea. Thank you. I'll just have to ensure I move my seedlings on quickly. They sometimes get left in their first tray for a while longer than they should!

[stevefromthegarden]

Good advice Eli. Enjoyed listening to you on the chat with Gardener Scott.

[clairep]

Thanks Eli. I'm a potter so used an old dog bowl and my kiln to sterilise mine last week, worked a treat! This is my first time mixing my own seed compost so thanks for giving me the confidence to have a go 😁

[GraeMatterz]

I just came across another method that's a little faster in a video from the Louisiana State Univerisity (LSU) AgCenter on reusing potting mix, which showed how to sterilize it in a microwave. Dr Joe added enough water to make it about 50% moisture (enough so that it will feel damp and clump together but will break apart easily and not be soggy) in a big ziplock bag, leaving it open to vent, placed the bag in a large microwave safe baking dish and nuked it for 90seconds until it reaches about 200degF/93degC, increasing by 30sec intervals until it's up to temp. (Using the bag means less clean up and since it's already in a bag, it doesn't have to be transferred into one for storage.) Dr Joe said this method is scientifically proven and it would even work for dirt right out of the garden. For those of us who don't have a grill and don't want to use the oven, the micro could be taken outside. Thought I'd pass that along.

[juliegogola]

Sorry, nevermind the next comment. I asked and posted my question before I had watched your whole video. Sorry. Great video!

[Jimsraisedbedgarden]

I did this last week to make my own seed mix for the first time, so thanks very much for the advice Eli 😀 x

[melissaschloneger]

I def made the mistake to reuse old soil and now have some gnats (and a cute centipede). I think the grill is the best idea to get it done without making my family ill.

Soooo pumped to get seeds going!!!

[jamiecurrie]

I just wanted to thank you for the inspiration to get me into greenhouse / raised bed growing, I have been watching your very informative superb videos for some time now and your great delivery / production makes for easy viewing....I hope that you don't mind but I call it 'brain porridge' viewing as its very absorbing and relaxing...well done to you both!

[denisekeable]

Thank you for the video Eli, really useful for recycling compost.

[marksallotmentplot]

Brilliant video Eli.

Not something I’ve done before, always learning something new! Will deffo be trying this one out 👍

[mariemoffett]

As a fungus gnat infestation survivor, I baked my purchased seed starting mix in the oven. (350 F for 25 mins) No strong smell. I like the grill idea for homemade compost! Thanks for that tip. Don't understand why steam is necessary. The heat really dehydrated the mix.

[robinscontainergarden]

Fab video, Eli & Kate! I probably should have mentioned I poured boiling water through mine before I dumped it in my flowerbed in my video 😂 Great video for doing a big batch! I'm glad I'm not the only person who reuses compost.

[jennyjohnson]

Wow you are so full of good ideas. Thankyou so much for this wonderful video!!

[she_sows_she_grows]

Fascinating idea! And such a good money saving tip. I usually buy new compost but it’s so expensive.

[ItsJustGardening]

Great information, now I know why there are bugs from my house potted plants.

[justine]

These videos are so useful. I bought some potting compost from B&Q a few weeks ago and every tray I've started seed in sprouts these very tiny mushrooms!!! Think I'll pour boiling water on it in future and it will hopefully kill off whatever is causing it. Thanks you 🙂 🍄.

[lindarobertson]

Thank you so much for this video Eli. I've always wondered which way to do it. I'm further north than you so I definitely wouldn't be able to get compost dry with the kettle method!