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Why is houseplant compost so expensive and what's so special about it?

I've numerous large houseplants to repot and finally 'freshen' their very old compost they've been sitting in for years, although they still flourish! The houseplant compost I bought last year from Homebase is very poor quality more like seed compost. Can I use multipurpose with added John Innes 3?

Comments (12)

  1. Grower

    Jeremy Wright

    Hi Jane - my personal view is that there is nothing very special about 'houseplant' compost. It's more a case that manufacturers will charge more per litre for small bags and they'd like you to think that you 'need' special houseplant compost in the small bags! Multi-purpose compost is fine ... and go for the bigger bags, which are always better value. Peat-free preferably as 90%+ of peat habitats have been destroyed by abstraction.

  2. Grower

    Rachel the Gardener

    Hi Jane, oddly enough I have just answered this exact question on my blog - if you'd like to skip over to my blog, you can read all about it!

  3. Grower


    Hi Rachel
    Once again thank you so much for your response. Having read your blog you've given me the 'green light ' to .use my recently bought new compost from Wyevales(Which approved)

  4. Grower

    Helene U Taylor

    Hi Rachel and Jeremy
    I am opening up this thread again as I am searching high and low online for a company which can deliver guarantied indoor compost - compost that is stored indoors from production to delivery. So far I have had no luck.

    I had a delivery from a well known retailer last spring with a product from one of the largest producers - supposed to be indoor compost. After about 3 months I had an explosion of fungus gnats in my house and I spent the whole autumn trying to get rid of them with all online remedies I could find online, with no luck. Finally I had to buy a product from US (BTI - Bacillus thuringiensis subspecies israelensis, via Amazon) to try to kill off the blasted gnats, they are still not completely gone, although much improved.

    I would now like to replace the compost on the top layer of all my houseplants to try to get rid of the rest of eggs and larva together with a new round of treatment, but I don't dare use just any normal compost that has been stored outside as it would most likely contain fungus gnats.

    I have read that there is no treatment that is really effective - you can put the compost bag in the freezer for a week - the gnats will still survive. You can pour vinegar or hydrogen peroxide on them, they will still survive (I have tried that too). I have not tried putting the compost in the oven and bake it, but apparently it stinks like hell, you can only do it in small portions so for as many plants as I have it will take days or weeks - and all the nutrients will die too, so it is a bit counter productive....

    I buy everything I need online so I can't go to a shop and ask " have you stored these bags safely indoors and made sure they have never been outside?" So that's a dilemma...I am nowhere closer to get new compost for my plants. I bought a brick of coir and had a trial with one plant to see if I could use that instead, but it was useless - soggy wet when wet, and dried very quickly and was impossible to re-wet just by watering the plant.
    Suggestions to where I can safely buy compost from would be greatly appreciated - or a recipe for how I can make compost safe to use - if that's possible.

  5. Grower

    Rachel the Gardener

    Hi Helene (waves), wow, that's really annoying. I have a couple of suggestions, which you might have already tried: firstly, watering your plants from the bottom can help, as the fungus gnats live in the top inch of compost, and they like it really moist: so if you water from below, they should be unable to hatch.

    I wrote about the principles of bottom watering in this article a year ago.

    Secondly, you could try using an ornamental mulch on the tops: those expanded clay pellets, or decorative shingle, or small pieces of slate -anything that can cover the soil surface completely. I'm not a big fan of this, as it makes it such a faff to re-pot them, and once a year or so you have to scrape it all off, wash it clean, then replace it. But it might help!

    Thirdly, a friend of mine recommends Neem oil for just about everything, including the killing of fungus gnats and fruit flies on house plants.

    Hope this helps!

  6. Grower

    Helene U Taylor

    Hi Rachel, I think I have tried all the tips and tricks in the book, including yours, and yes I have shingle on some of my bigger plants and yes, they are annoyingly already looking rather grubby and dirty.

    Best tip so far has been strongly mixed orange juice in small cups dotted around the plants. Fungus gnats are attracted to yellow, fly in, smell the orange, dive in and drown, job done! I have killed many hundred that way, but you need many cups and top them up ever other day. But nothing has killed off all larva, eggs and flies so they keep coming back - although since the biological treatment last autumn, numbers are low.

    My question was really more about finding a safe way to get fungus gnat free compost for my house plants. I am still looking for that :-)

  7. Grower

    Helene U Taylor

    Sorry! Was doing this on my phone and the site keeps logging me out and I thought I had lost my answer as I was asked to log in again so I wrote it agin - and then it came twice! Not sure if I can delete this one myself.

  8. Grower

    Rachel the Gardener

    Hi Helene, I've asked the question of my PGG colleagues (Professional Gardeners' Guild) many of whom have glasshouses/potted plants for the house etc to deal with - so maybe they'll have some suggestions about a source of clean compost.

  9. Grower

    Helene U Taylor

    Thanks Rachel, that's great!
    Many companies produce so-called indoor compost, but as soon as it leaves the producer they have of course no control over how and where it is stored. Those little gnats are very clever and can manage to climb through the ventilation holes in the compost bags to lay their eggs while the bags are stored outdoors so an unopened bag is no guarantee for clean compost.
    I think perhaps the solution would be if I can somehow treat the compost myself so I can be assured I have clean compost and don't have to rely on the retailer/seller - and I could then just use cheap normal compost.
    Hope you get good response :-)

  10. Grower

    Rachel the Gardener

    Hi Helene,

    Right, responses from my PGG colleagues: they one and all said that it was FAR more likely that the gnats came into your house via another plant, or through open windows, than in the compost: commercially-made compost is indeed sterilised (many of them grumble about this, because it lacks any sort of natural eco-system on arrival) and although you are correct that gnats "could" crawl in through the vent holes, it is highly unlikely, as they lay their eggs in very moist compost and would not be attracted to the dry compost.

    I would, at this point, say that I buy my commercial compost in big 145litre bags which are stuffed full to bursting, not the smaller bags where there is enough room to shake the compost down to the bottom of the bag, This helps, as there is no room for anything to get inside, and the compression of the compost also keeps invaders out, even once I've opened the bag.

    My colleagues all agree that the super-expensive houseplant compost in small bags is a great big rip-off, and is no better for your plants than any other compost, and is over-priced due to the small quantity packs. Good to know, eh?

    "They find their way in from anywhere and everywhere; they're outside too." said one head gardener.

    You can sterilise your bought-in compost by blasting it in the microwave, apparently: you might like to try that, quicker than oven-ing it, and less likely to destroy the small nutritional content.

    "The whole life cycle of sciarid fly is only four weeks so definitely not in the compost when bought." said another head gardener.

    You can get nematodes to treat them, but I think you said you'd already tried that: and it does take a long time to work. You have already worked out an organic version of yellow fly-paper, which does at least get rid of the adults.

    Basically, they all said "keep things dry", with bottom-watering, mulches, and so on: keep each bag of compost covered as you use it (don't leave the top of the bag open), and it seems that no-one thinks the compost is what is actually bringing them in... sorry!

  11. Grower

    Helene U Taylor

    Hi Rachel,
    Thank you for taking the time to write the responses you have had.
    I realise that it is difficult to give full and correct answers to questions like this without all information at hand - and that is difficult without things becoming very lengthy. But since this thread might interest other readers I will try to pick up on a few things.

    The compost I normally buy as 'multipurpose compost' for outdoor use is always stored outdoors at retailers and is always moist on arrival to me - 125L compacted bags. Still possible for fungus gnats to crawl into and lay eggs in should they choose to. Once it is here and opened, I would never take it indoors (or possibly I would with a second microwave, see below).

    The compost I have occasionally bought from a few retailers (online) as 'indoor compost' or 'houseplant compost' have also always arrived to me as moist, the most recent one was a small, 8L bag from Wilkinson's bought last week - I was desperate and needed to pot up some cuttings and had to take a chance. I agree it is a rip-off in price, I don't expect it to be any different to the compost I buy for outdoor use, I just hope it has been stored indoors and come without fungus gnat eggs. Hope I don't regret it.

    I have never bought 'dry' compost from anyone - except my one and only attempt with coir. I will not repeat that, unless I find a source of clean indoor compost so I could perhaps mix 50/50. Using it neat made my plants dry out far too quickly and it was very difficult to re-wet just by watering the plants. I normally water my plants once every 2 weeks or so except for my huge stephanotis which require a bit more frequent watering. The 2 plants I potted up with coir as an experiment wanted water every 2-3 day. No way.

    As for sterilising compost in the microwave - I have thought about that and would love to have a second microwave in my shed for this purpose, as after reading stories about how much it stinks when (and after) you do this I am reluctant to use the one in the kitchen :-)

    And finally - the life cycle might be only 4 weeks for a fungus gnat, but both eggs and adult flies are incredibly good at surviving just about anything you throw at them - they just hibernate and come back. That's why they survive anything from extreme winters to blazing hot summers with not a drop of rain - anywhere on the globe.

    I never had a problem with fungus gnats in the past until I invited them in with the compost I bought. I am reasonably clued up on watering my about 40 houseplants and I have not tried nematodes. What I did use though - after having tried just about everything else - was BTI - Bacillus thuringiensis subspecies israelensis which is a bacteria that only attacks mosquitoes and fungus gnats. It was expensive and I had to buy it from US, If I do a second treatment I would possibly get rid of the gnats for now, but I want to replace all the top layers of compost in all the pots at the same time. That's a lot of compost to sterilise if I need to do it that way - but maybe that's the answer - a cheap microwave in the shed where it can stink away as much as it wants to! :-)

    Thanks for your help Rachel, I hope all this will come in handy for other users too.

  12. Grower

    Rachel the Gardener

    Thanks for all the detail Helene, and I hope you find a way round the problem soon - maybe you could find an old microwave on Freecycle?

    And I should say, when I referred to the compost being "dry", I didn't mean bone dry - no compost should ever be bone dry - I meant "not sopping wet" . And when I said "moist compost" I meant, sopping wet. My fault for not making that clear, sorry!

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