How to sterilise used compost for your seed and potting mixes 

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 🇨🇦👍

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.

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.

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

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.

Given the cost of calor gas these days I think I will buy new compost 😊

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!!!

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

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!

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 😁

2lt microwave bowl with lid (ventilated) full power for 3 mins Job done

So funny I ran across your video. I actually did this for some potting soil that I was reusing but I put it on a foil lined cookie sheet in my oven at 200°F. It was a bit smelly but, I think it did the job. The reason I was watching your video is because I wasn’t sure whether I should heat up compost before using it or not. I know that my compost does not get hot enough to kill off all the weed seeds and everything else that may be in it, but I thought it might be harmful to kill off the beneficial micro organisms that were in my compost. I am planning on using it in my raised beds.

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 🙂 🍄.

I've used boiling water before,but worried about all of the soil getting hot enough. Using an outdoor grill is a great idea!!

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!

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

I scorch my soil using my weed burner. Does burn some organic matter but ash is quite good for growing. Only to be done carefully outside away from anything which might catch fire.

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

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

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!

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

I've used my oven as a soil sterilizer (once!) and now am thinking of moving my old microwave outside just for this purpose. It would mean doing many, many batches, but in between routine chores (goats and hens) it would just be one more little thing :)

Didn't think about using the BBQ, lol, I've always used boiling water and microwaved mine. Great idea if you have a gas one but a bit of a faff if you have a coal one like me. So excited by the way, my greenhouse will be put together on Fri so I can finally get cracking on my seed starting. 🙌🏾

I just used your boiling water method on some spent potting mix from houseplants, and "hacked" my way out of the sogginess issue with coconut coir: after the old mix was properly soaked in and sat in the boiling water for a half hour, I added some crumbly bits from the side of a big brick of coco coir, and it absorbed all the extra liquid super quickly. With a bit of mixing it was nice and fluffy and moist, I was able to pop seeds in right away (winter density lettuce, sown indoors with lights and a heatpad, but I couldn't wait to start *something* lol)

I use boiling water and on seedlings,vermiculite on top,helps keep mould & fungus at bay

Hi Ladies, Interesting discussion this one. I am sorry, I will be a bit controversial - although I am still quite new to growing. I can fully understand why you want to sterilise your seedling mix: seeds don’t need nutrition to germinate, just a growing medium. But by sterilising your compost you be killing the nasties but also the life that is in the compost. Yes, minerals will survive but compost (soil) is a living organism, and a lot of goodness will die. Spent compost still has loads of goodness to be had.

I used the boiling water last year. This year I will try the BBQ. Thanks for the video. Cheers 🍻

What a good idea with the barbecue! Great information, thank you for sharing!!!

Never done this before but I should definitely try this with my cuttings! I have a lot die since I just leave them outside. (Concerned about beasties in the house) but I think I would have better success if i bring them inside

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

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.

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

tried to grow strawberries from a strawberry before and out of 30 seedlings i have one left. i think fungus nats was the culprit,but had never seen them before. since then i use boiling water and never had the problem since..

So just how much spent compost are you guys hording ready to go??

I have sterilised mine on occasion using boiling water. Never the oven though, I bit concerned about doing it this year though cos ummm theres loads of wormies and I don't want to hurt them or their babies xox

Another brilliant video. Was wondering how steralise large amounts. And using the bbq so you don't get the smell indoors. Thank you Eli 😊

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

What a tip! Thank you

Sterilizing also kills off bugs and their eggs.

Really useful video. Do I sound like a sap when I say that I don’t like sterilising spend compost in case I frazzle some insects or worms 🙄. I take the hit and buy fresh. I have a cheek when I encourage my chickens to have a good root around to find tasty beasties to eat. 😙😙😙

You are always on top of things. I don’t know where you get the time for all this. I’ve not tried this before….but will. Do you think this process would kill any weed seeds that might currently be viable in homemade compost?

Thanks Eli

Have u tried chamomile tea on seedling that works well with killing bugs 🐛

Hi Eli I absolutely love all of your videos. I have been gardening for decades and started following you at the start of the first pandemic in 2020. I have learnt so much from you. I wanted to know if I could sterilise soil from pots which had tomatoes that got late blight last year. I cleared away the infected plants and emptied the soil into 2 large dustbins. Can the sterilised soil be used for seed sowing or is it better to go on the flower borders? Thanks Minni

Wish I new this a month ago thanks for the info

love this idea thank you

This is a great way to reuse. Can you tell me if this destroys vine weevil grubs/eggs?

Hey Eli, I’ve heard of solarizing soil where you lay out soil on a tarp in a sunny location then place clear plastic over top (I’m thinking a cheap clear shower liner) then leave in the sun for a few days or however long it takes to get it to desired temp to kill off everything… I don’t recall how long, sorry ..but then the sun bakes it for you. Would that ruin the soil too much or remove too many of the good microbes and nutrients? Thanks for sharing all your great ideas and helping me get smart(er) ; )

Great tips👍

Great idea about using your BBQ!

Hi, firstly great video. I did the boiling water method on a small amount of compost before and it seemed to work. I've read through all the comments, I can see why you would do this for seed starting mix (regardless if you're using bought or spent compost). I did get a little confused on one of the comments about nutrition, so wanted to clarify. Would you use this method before using in house plant potting mixes? Or has it then lost its nutritional value? Is this process useful on spent compost which is going to be reused in flower or veg beds? Or is it that unnecessary? I'm very keen on reusing - throwing away compost/soil just doesnt seem right to me. I also have ME, so I need to be clever with how I use my energy. Though at the same time, I'd much rather take the time to get it right at the beginning to avoid pests etc.

Interesting video and relevant 🙂🙂👌👌

Hi Eli. I grew some potatoes and sweet pea flowers in pots last year....i was going to dump the remaining soil on top of my flower beds in the garden....would you advise against this? Dave from Ireland.

Nice one Eli 👍 Would this method also kill off weed seeds? I get a lot of it growing through! 😖

I have a Vego and a Birdies raised bed. I have a 3rd bed that is either a Vego or a Birdies also. Anyway, I want to use my compost that I made in a compost tumbler to add to the soil mix that I'll use to fill the 2 that I have yet to get put together and fill up. Since I had gradually made the compost, adding things over time, my compost was a pretty much "cold process" vs me adding a bunch of material at once, and having it "cook" in the barrel. So, I'm worried about seeds from the plants I had added to my compost barrel. So, I want to sterilze the compost before I add it to the mix that I will fill the raised beds with. But, what I want to know is will the sterilization process kill the "good stuff" that's in the compost? Or is the temp that is used to sterilize it too low to kill the "good stuff?" Compost gets hot when you add greens to it, so, maybe it's okay to sterilize it when it comes to beneficial organisms?

Wooooooow!!!

What about the worms that are good for compost?

would microwaving work?

Hi, Do i need to sterilise seed compost, or only sterilise multi purpose?

What are you sowing now?

Hey I was trying to find the link to the video where you talked about how to dry out a small amount of compost. Could you help me with that? I can't figure out how to do it without baking it

👍

I didn't know this was a thing 😅

Dear Eli and Kate, okay - so I reckon you will be horrified, but some people (naming no names) microwave theirs. . .

I microwave mine - quicker and cheaper!