Can bleach be used to sterilize potting mix?

Oklahoma_Tim(z7a OK)May 15, 2009

I remember reading here a few years ago where someone recommended using diluted bleach to kill soil-borne fungi. The new bag of Miracle-Gro Potting Mix that I bought this spring has a white, fuzzy mold growing in it, and I was wondering if I could use bleach to kill it. I tried using some of the moldy mix to repot a spider plant a couple weeks ago, and within a couple days all the new soil was covered in a dense, white fungus which resembled that which you usually find on rotting wood. I've been keeping it on the dry side, and the mold seems to be slowly dieing off, but I would prefer to kill the mold before its growth takes off like that.

What I was thinking was that I could fill an empty pot with the mix, and soak it with the diluted bleach. After giving it enough time to kill the mold, I could rinse it out with plenty of water, and let it dry a few days before I try to use it. I was also thinking I could simply let the soil dry out before using it--would that kill the mold? Or is the mold even worth worrying about, meaning that's it's harmless to houseplants, and that if I ignore it it'll go away? One more question: Is diluted bleach harmful to potted plants? The other idea I had was using the moldy potting mix like normal, and then soaking it with diluted bleach instead of plain water.

Any suggestions?

(I think my mold could be the same thing described at the top of the page I've posted a link to.)

Here is a link that might be useful: Mold in the Lawn

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Mentha(9 CA)

Miracle grow is going to get mold because it is designed to hold water, I would add perlite to it to make it drain better. Boiling water woauld also kill the fungus with no need to rinse.

    Bookmark   May 15, 2009 at 10:35PM
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tapla (mid-MI z5b-6a)

The solution of sodium hyperchlorite (I think that's right) has a pH of nearly 13.0, which is close to as caustic as you can get (14.0). Though it may kill the fungus, it will also raise the pH of the soil quickly and cause other problems with the soil chemistry - even when diluted. It is usually used in a 10% solution as a DIP, to rid bare-root plants or cuttings of fungal infections. It is then normally rinsed from the plant & the plant potted or the cutting stuck. I'm afraid you'll likely turn the soil phytotoxic and end up with a greater problem than that with which you started.

Allowing the soil to dry will only make the fungal spores go dormant. Plants live in harmony with thousands of species of molds and fungi, so unless you have evidence it's the mold/fungus causing a problem & not something else cultural (like over-watering), I wouldn't be too concerned.


    Bookmark   May 16, 2009 at 11:12PM
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Oklahoma_Tim(z7a OK)

Okay, so I can't pour the diluted bleach in the soil and leave it, so what if I rinse it out? I didn't really think I could simply water a potted plant with bleach, so thanks for confirming that, Al. I also kinda figured that the fungi would simply go dormant if allowed to dry out.

As for using boiling water, I considered asking about that when posting my question last week, but thought that my message was long enough already. That seems easier than trying to rinse bleach out of the dirt, but I wonder if it would really kill the mold....

I'm about ready to see if I can use the mix "as is," since the spider plant I potted with it seems to be doing fine. Something I had forgotten about when I posted my question was the mint that I had recently potted in the soil, and I don't think there's even any mold growing in it.

When I bought the mix I was also able to buy a bag of perlite, and I've been adding that to pretty much everything I've potted/repotted this spring.

    Bookmark   May 20, 2009 at 10:31PM
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albert_135(Sunset 2 or 3)

I agree with above posts that bleach should probably not be used to sterilize potting soil. I would fear too much residual. I should think that rinsing soil will rinse out nutrients too and perhaps not get rid of the residual bleach.

I usually don't worry about a bit of mold.

If, for some reason I feel I need a sterile soil and the volume I need is not bigger than a turkey I get a turkey cooking bag from the food store, fill it with soil and put it in the oven at a temperature a bit below 200°F for a hour or so. This procedure was used in microbiology labs years ago. As I think about it I seem to recall that we set the oven to 178°F for some reason.

    Bookmark   May 21, 2009 at 11:51AM
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Hi Jim,

Do not put bleach into the soil at all. You can never rinse it out. It would be a horrible death for the plant.

Albert is correct. The way to sterilize soil is to cook it. I have always been told 175 degrees F for 2 hours. If you don't want to put soil in a nice pan, then line the pan with foil or do as I do and use left over pie tins. I wet the soil first and put them in the pie plates and cook. It works really well. No fungus or any nasties after that.

Good luck,


    Bookmark   May 22, 2009 at 2:09AM
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Sorry Tim, I called you Jim.


    Bookmark   May 22, 2009 at 2:11AM
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Oklahoma_Tim(z7a OK)

That's okay, Larry. I'm just glad you didn't call me Timothy!
('Cause that ain't my name, either.... My third-grade teacher insisted on calling me "Timothy," despite the fact that my mother showed her where it said "Tim" on my birth certificate. It's been nearly 20 years, and I still get annoyed when I remember that.)

Okay, so I get the point: No bleach.

I've also considered sterilizing the mix in the oven, but I'm starting to think that won't be necessary. (Plus, I've read that it stinks!) I still think I'll let the stuff dry out before I use it, since that should kill the mold that's actively growing in it. I don't think there's enough mold to worry about, and since the stuff I've already planted in it seems to be doing fine I think I'll just stay with the "ignore it and hope it'll go away" strategy.

The reason I posted this question in the first place was that I was worried the mold would hurt my houseplants. Even though it appears that this mold is harmless I still think I'll mostly use this bag for outdoor potted plants, and try to get a fresh bag this summer.

    Bookmark   May 29, 2009 at 12:14AM
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I understand Tim. I am a Larry, not a Laurence. Larry is on my birth certificate.

As far as the mold is concerned, I really don't think you need to worry about it either. I only sterilize soil if I am going sow gesneriad seeds. They are so small that the mold can be a little rough on them. Gesneriad seeds are extremely small and the new seedlings are smaller than a pin head.


    Bookmark   May 29, 2009 at 3:21AM
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how bout vinager salotion?

    Bookmark   February 25, 2011 at 11:34AM
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MojaveLove(5 - IL)

If you can cook it in a pan I might have a better, quicker solution though admittedly I've never tried it. You can sterilize a sponge by cooking in a microwave for 1 minute. I'll be the same thing will happen with dirt if you throw it in for a minute or two.

    Bookmark   February 25, 2011 at 2:03PM
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I tried cooking potting soil many years ago. It stunk and I couldn't get the smell out of the oven forever! I've microwaved bark and that smelled a bit better.

I suppose you could fill a tin-foil tray with the dirt and cook it on your outdoor grill.

I sterilize flower pots with bleach and leave them outside in the sun for a few days before potting. It doesn't hurt the plant, but I wouldn't pour bleach on the soil.


    Bookmark   February 26, 2011 at 12:37AM
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dellis326 (Danny)

Vinegar can kill your plant too if it isn't removed well enough. the pH is something like 3 and I don't think it would be practical to rinse soil so much the you'd get enough of it out.

There are thousands of species of Fungus and some probably will live in soil too acidic for plants anyways. Beside that only a few types of fungus are harmful to plants and those normally only become a problem because the soil is too muddy.

I've had plants growing in pots with Fungus growing and eventually if goes away without harming the plants. again, it all depends on the species of fungus. At least in my experience, molds and fungus that you can see generally aren't the types you need to be concerned with but the types you can't see and can sometimes smell are the ones you need to make some changes in the way you're doing something(usually watering or your grow mix).

I do like the idea of using an oven bag with a little bit of water in it to steam the soil but if I needed a sterile grow mix I wouldn't use soil to begin with.

    Bookmark   February 26, 2011 at 8:41AM
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Isn't one of the good things about a proper compost pile that it kills off organisms when it goes through its "heat?" It gets really hot....granted not as hot as a pile of sawdust shavings at a sawmill, where it gets so hot it smolders.

I've read about heating the soil in the oven, and the word was the smell would be quite enduring long after. But it would be dead dead dead.

In another thread on GW somewhere, I read that you could sort of dilute hydrogen peroxide (kind you buy at the store), and then pour it over plant roots which had problems. Just like with human infections, where the H2O2 encountered the plant infection, it would boil up. Have not tried that myself, did not quite understand all that was being discussed, but they were trying to sterilize a plant and soil infection.

    Bookmark   March 4, 2011 at 5:25PM
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albert_135(Sunset 2 or 3)

When I wanted sterile soil for my propagation experiments I ''cooked'' it in those Reynolds oven bags sold for cooking turkeys. I sealed the bag tightly and did not puncture it. I do not recall any unpleasant smells. The spouse, who grew up in a family of restaurateurs, and has a keen nose, never came home and said ''What is that smell?'' as she often did when I attempted to prepare a meal.

The proper time and temperature for Dry-Heat sterilization is 320ðF (160ðC) for 2 hours

    Bookmark   March 5, 2011 at 2:54PM
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I know this is extremely old, but I found that if you get a spray bottle and fill it with water and a tenth hydrogen peroxide, it kills the mold. I was having a problem with the white mold in almost of my potted plants, I sprayed them twice, and it went away. I even did it on seedlings.

    Bookmark   May 27, 2012 at 6:12PM
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I've had varying results with spraying different things on soil recently.

Mold came up on some gritty mix (which I salvaged and sterilized from some cuttings tapla sent me) I was (am) using to root some (I think philodendron or the like).

I'd treated the cuttings with cinnamon, so I tried that. No dice, though it may have suppressed it slightly. I also tried diluted (chamomile) tea solution, vinegar, insecticidal soap, and hydrogen peroxide solution.

Combined with increased ventilation, some combination of those wiped out most of the mold--the soap (castile peppermint soap) probably did the most, although once it dissipated, the mold came back if I didn't ventilate enough. Vinegar seemed to work OK too, although unless you flushed it out, an already-flourishing plant might suffer if your pH gets too low.

I did the same type of thing for some peat/perlite/bark mix I'm starting some seeds in. For some old seeds that all just rotted (whoops), the mold came back eventually, depending on moisture level. Only very small amounts survived in other containers, and don't do much unless I add too much moisture.

I guess the moral of the story is: it's probably not worth the effort :)

Note that some of the stuff I was working with, was already sterile. Fungus spores of one kind or another are ubiquitous, so if you wipe out one population, that just makes room for another to move in.

BTW, I haven't had trouble with spraying undiluted H2O2 (hydrogen peroxide--what you buy in the US is usually 3% solution) on or around plants, so that may work. Trouble is it breaks down rapidly and then the mold can come back (I've found it to be less than satisfactory for mold control, by itself).

PS, Al: it's sodium hy*po*chlorite ;) Just in case anyone was wondering...

    Bookmark   May 28, 2012 at 5:54PM
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rhizo_1 (North AL) zone 7

What the heck are you people growing that fungi and mold are such a problem, lol? I have NEVER had such issues...even when I am propagating cuttings or seeds. I also do not (ever) sterilize my potting mix by chemical or steam or dry heat.

I think we need to get to the bottom of why you are all having these problems and eliminate the source.

    Bookmark   May 28, 2012 at 11:22PM
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tapla (mid-MI z5b-6a)

Plants live in happy communion with thousands of species of fungi & mold. A few species can be a problem, and usually surface as such when the grower is trying to root cuttings or start seeds in a medium too wet - damping-off. To avoid damping-off, avoid a soggy medium. For cuttings, you can gain an edge by rooting in a sterile, well-aerated medium, like perlite, Turface, crushed granite, or any combination of the three. Other things like very coarse sand (at least 1/2 BB-size) work fine, too. I've rooted so many plants in the gritty mix I'd never be able to count them, and while mold & fungi spores are everywhere, I'd leave the bark out of the gritty mix if I was using it exclusively for important cuttings; that, simply because it makes sense that there will be a more concentrated population of mold/fungi spores on products that offer them sustenance. I also can't remember ever seeing mold on the gritty mix. I often see mychorrhizal colonization in the root zone, and I know there are other fungal populations at play in the pots, but I never see anything at the surface. Also, while I occasionally lose cuttings, it's usually due to my laziness or inattention and not damping-off/rot issues.

I'd be a lot more concerned about what white salt deposits portend on a soil's surface than I would be at the sight of any mold or fungal colonization. It's just not a big deal unless it's to be taken as a message you're keeping the medium too wet - THEN it needs attention.


    Bookmark   May 29, 2012 at 2:26PM
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