Shut Up and Dig

Potting soil puzzlement

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It’s come to this. Thanks to a perfect storm of insane late-season bulb purchases and a rather unseasonable late season freeze (usually I can do a bit of planting around Thanksgiving—but not when there’s frigid sleet), I find myself having to pot a few bulbs up to be stored in the root cellar or garage until they can be planted in the spring. This should work. If not, at least I tried.

So I’ll be buying and using potting soil soon, which isn’t unusual, since I do a lot of winter bulb forcing (as shown above) as well. But here’s the thing. I’ll probably want to reuse this mixture for summer containers after the bulbs are all either gone or safely in the warm ground. The wisdom has always been that one not only does not reuse potting soil, one actually sterilizes the empty pot once the soil has been discarded. Used potting soil is said to harbor all kinds of nastys, including disease pathogens and tiny unwelcome pests that can hurt any new plants you pot it with. But—of course—sustainability means we want to reuse as much of any gardening material as we can. There is an interesting article on this topic in a Fort Myers paper. (Not many Northeastern garden writers would be talking about potting soil now.)

I will probably reuse my potting soil, as I do every year when the container bulbs are done. And I have never sterilized a pot in my life. Nothing bad has happened yet, and I can’t stand the thought of tossing that expensive soil. I can buy a flat of plants more cheaply than I can buy some of the fancier big bags of organic potting soil.

What is your wisdom on this, knowledgeable Rant readers? I have long been reading and hearing all this about used potting soil and used pots, and I have pretty much ignored it (other than hosing out pots now and then—oh yeah, and regularly buying lots of new pots cause they’re pretty).

Posted by on December 3, 2008 at 9:00 am, in the category Shut Up and Dig.
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32 responses to “Potting soil puzzlement”

  1. Naomi says:

    I’m with you on this one. BOth the reusing of soil (over and over and over), and the sterilization of the pots. Can’t be bothered, and I have no interest in using bleach while gardening, no matter what. Never got that plants need to live in sterile conditions. Just found your site, and LOVE it. Gardening is a passion and a pain up here in the California mountains.

  2. Ayse says:

    Oh, geez, that old saw about sterilizing pots. I’ve been reusing potting soil (mixed with new compost as required and occasionally steam sterilized for seed starting) for ages. I never sterilize a pot (though I wash them out between uses). I worked in a greenhouse and we never sterilized pots as a matter of course; that would have been ridiculous. It’s like somebody once heard that there are times when you want sterility and decided that if a little was good, a lot was better.

  3. rainymountain says:

    I agree with you that potting soil has become very expensive and in my experience is not very good quality either so I mix it with soil from the garden and/or compost. It usually goes back onto garden but may be reused depending on whether I can persuade someone with wheels to get me some more bags of potting soil. Like you I have never sterilized a pot although Lee Valley Tools suggested a wipe around with 50/50 vinegar and water as a disinfectant.

  4. Rachel says:

    I agree with the lack of sterlization- unless I know that something horribly susceptible to root or stem rots will be planted in the pot. Especially in my super foggy Northern California climate. This includes basil and the solonaceous plants. Otherwise, to quote my British friends, “I can’t be bothered.” If I feel unsure about reusing the soil, then there is always the compost pile or just add it to the garden. I suppose that means it depends on the condition of the plant that was in the pot.

  5. Michele Owens says:

    I usually throw my old potting soil into the compost bin, but I like your attitude, Elizabeth. It is ferociously expensive, so why waste it?

    I’ve never noticed any diseases arising out of my slovenly failure to sterilize a pot. Another bit of make-work gardening advice that has nothing to do with reality.

  6. The sterilization advice does seem so ridiculous as to not even deserve a mention. Yet, it is so prevalent! I’ve read it everywhere, consistently.

  7. Brie says:

    I reuse potting soil, never sterilize, and haven’t had any problems as a result (that I’m aware of). And I have lots of potted plants that are in queue for going in the ground.

    I tried sterilizing some soil for seed starting once using the oven method (cooking the mix). It didn’t seem to produce results any better than if I hadn’t sterilized.

  8. chuck b. says:

    I spray all my pots with dilute alcohol between use and wipe them down with a clean rag, just like I do my hand pruners between plants. It doesn’t take that long to do. The goal isn’t to totally “sterilize” everything. That’s ridiculous–I would need an autoclave to do that.

    I only use compost as a container mix. When I’m done with the pot, I throw the mix back in the compost bin or use it as a mulch in the garden.

  9. Sheila says:

    I have never sterilized pots or soil that I reuse and have never been besieged with any awful diseases as a result. After all, it is DIRT!

  10. Renee Beaulieu says:

    Sterile potting mix is a good idea for seed-starting, to reduce damping off. Otherwise, like most commenters, I’ve never sterilized a pot in 25 years of gardening, and never regretted the decision. I do wash out seed-starting pots, though. And I started re-using potting mix a couple of years into my gardening career, when the expense started to add up. It’s a lot more fun to spend the money on new plants than on soil.

  11. Colleen says:

    I reuse my potting soil, and I used to sterilize my pots until I neglected to do it one year and, lo and behold, nothing terrible befell my plants. Since then, I haven’t bothered.

  12. firefly says:

    What everybody else said. I brush old soil out of pots with a vegetable brush, but that’s about all. Now that I have a compost bin, I throw the potting soil in it, but when my gardening was all containers, I used to reuse the soil repeatedly for morning glories and sweet peas.

    From that, the only caveat I can think of is potting soil eventually “wears out.” I got into the habit of tossing the top half of it and replacing with fresh stuff.

    One other thing worth pointing out is not all organic potting soil is equal. The Miracle-Gro stuff seems to be based on shredded wood fiber (blech) and I’m noticing the first fungus gnats in months around two flats of kitty grass I just planted.

  13. For the home gardener who hasn’t noticed any diseases lurking in their pots or in their previously planted plants, the act of sterilizing a container is not necessary unless you have a rare specimen or are planting for trials.

    For the professional gardener who earns their living potting up plants for their clients then taking the effort to wash down and sterilize the pots and adding new fresh potting soil should be considered sound professional practice.

    I got in the habit of sterilizing pots and using professional quality potting mix when I worked at the Arnold Arboretum and the Filoli Estate Garden.

    Some old habits are hard to let go of.

    Sometimes I go thru the extra steps of sterilization in my own garden, sometimes not.
    It just depends on the plant, the container and the previous conditions that existed.

  14. susan harris says:

    I”m in the never-sterilized, always-re-use camp, too – and isn’t it wonderful to find so many other rule-breakers? Though as Michelle points out, it just depends.

  15. I picked up an autoclave from a hospital that was going out of business a year or so ago and have sterilized everything ever since, soil, pots, pruners: the whole nine.

    Which lead me gradually to the realization that once sterilized, I could eat the dirt too. Why bother with all the fuss of trying to grow plants?

    So, now I just eat the dirt. Simple!

    NOT!

    Count me in that group that re-uses and does not ask for dirt to be less than it is. I want my dirt to be dirty, and germ-y and full of the beautiful unseen. I do not want to imagine a world with no yeast breads and no smelly cheeses. Or wines or any of the other foods that require something far, far, far from sterile to make them magical.

    Now I’m hungry. Thanks for a thought-provoking post.

  16. Barry Prince says:

    I agree with all the other non-sterilizers. FYI: To me this whole discussion has a faintly Hitleresque / final solution ring to it…
    I suggest this topic rolling into a new Rant Category called “Anal-retentive Gardening” or “I am not an Obsessive Compulsive, but I play one in my garden.”

  17. Rosella says:

    Oh, am I relieved to discover that I am not alone in not sterilizing pots, and in re-using potting soil! It has been a guilty secret with me for a long time, but now it can be told. Yes, I re-use it! And no, I don’t sterilize pots. I usually throw about half the soil in the pot on the garden or the compost heap and replace it with new fresh stuff, thus reducing my total potting soil purchase per season to something less than outrageous. I mix it well, and thing seem to grow just fine.

    For seeds and cuttings though I buy Pro-Mix or something similar and use that, because I do worry about damping off.

  18. I’ve tried to feel guilty about not dumping or scooping out the potting soil from my large containers each spring. I just can’t. I’ve got perennials growing in some of them & it just seems like way more trouble than it’s worth. Put another way, it’s just not justified by a cost/benefit analysis.

  19. Genevieve says:

    I only sterilize when planting something expensive or if there are signs of disease.

    I do reuse soil to a point – I think after about a year or two it just doesn’t grow things in there as well, even with organic fertilizer added. Then it goes into my raised veggie beds and gets mixed in.

  20. Janit Calvo says:

    For the pots that I sell, I start with a new pot and new soil.

    For my garden, I’m always reusing the soil – if it looks good and it’s less than a year old, I mix it in with new soil for my containers. If the soil has been in a pot, and is a few years old, then it gets tossed into the garden bed. (Although I don’t like to see the vermiculite in the garden bed for some reason.)

    When I’m reusing a pot for my garden, I brush out the soil and leave it out in the sunshine for a day or two – when there is sunshine that is. The sun is supposed to sterilize and I guess it does – I haven’t had any problems!

  21. greg draiss says:

    perfectly ok re-using potting soil. If you are worried about the “quality” of used soil compost it for a few months.

    Cleaning the pot/container is ok. Simple 1 part bleach to 9 parts water will do the trick.
    Do I do this? NEVER unless the pot is clay and has been sitting in the basement (damp, mildewy etc is what the pots look like in my basement)

    The TROLL

  22. Sally says:

    It has never occurred to me to sterilize my pots! I think that if something I planted does not thrive in my reused potting soil (which is often mixed with compost if reused) maybe it’s because the soil is used up, dried up.
    Also wondering if that sort of mixture sits outside during a freezing winter, if that prevents the spread of bacteria.

  23. Marte says:

    My pots sit outside in freezing winter and I usually reuse some of the soil in the next season, mixing with new stuff and dumping some in the garden. I never sterilize pots, and I have never had any problems that I’m aware of reusing old frozen potting soil.

  24. This is a question which comes up fairly frequently and, as Michelle above states, the professional should always follow the sterilization procedures but for the home gardener, where time is at a premium, do what you will. I find that spent or used potting soil can be put in the garden and tilled in to great advantage as the formulations have an abundant amount of peat moss, perlite and sometimes vermiculite. I usually take out half of the mixture and replace it with professional potting mix (buy the 3.8 cf compressed bale for economy) and add some compost to it for added weight. The professional mixes are designed for professional growing and compost adds missing micronutrients and stability to the container planting. Great question and great discussions!

  25. tai haku says:

    I can totally see the wisdom of sterilising secateur blades/cutting knives etc but pots? meh.

  26. M Wilkie says:

    Potting soil

    I have given up buying potting soil when I garden in containers. I have also given up mixing my own, preferring to use compost. Pure unmixed compost is what I use. I have heavy clay soil, it is no good for mixing in. Sometimes I buy sand, or take it from former children’s sand boxes.

    I figure the variety of microorganisms in the soil will make a nice environment for the plants. By the house is the paved part where I grow in pots. This is the only place broccoli and lettuce don’t attract the ground hog. Carrots and parsley get tucked in here, also flax seeds itself. I also have luck with potatoes, sweet potatoes, tender herbs, many seeds I like to toss in the pots like Love in a mist, Nasturtums, vining annuals like morning glories and patio tomatoes. Staking and letting the vines spill over the sides of the pots keeps me entertained.

    I think the compost gets heavy and I know it dries out, has to be watered everyday in July and August. Potting soil has sphagnum moss or peat or perlite to lighten it up and absorb water. Sphagnum and peat are acid, will kill nasties. Compost usually has a balance of bacteria and will support a heathy colony, prevent overgrowth of cooties (I stole the term cooties from a landscaper who was talking about not putting mulch within an inch of a tree trunk in order to avoid unbalance of microorganisms and kill the tree. This situation seems analogous to Candida albicans in the gut).

    I like the idea of balance of microorganisms. Alternately, an acid environment can be maintained. Potting soil with perlite is best added to the compost after it is used, in the compost pile, it will acquire a healthy balance of microorganisms. Being sterile, perlite potting soil has no defense against any cootie that decides to take up residence in your pot. Sometimes it will work, sometimes it won’t.

  27. Vera Patrick says:

    I got this answer from my soils teacher this morning in lab: the reason commercial growers sterilize is due to the volume and space limitations in a greenhouse. The spores that settle in the dirt can find host homes and kill a crop in no time flat. Spores will not die in a pot of soil during the winter but they will not usually affect your plants the following year due to the diverse culture of plants that the home gardener grows. So the best answer is do what works for you and if it is not working then try something else.

  28. John says:

    I grow a lot of plants in pots, and I buy a lot of plants from various sources. So on occasion I end up with some form of garden nasty. My solution is to bag up the offending plant, pot and all and toss it in the trash. I would be spending all my time sterilyzing and not gardening otherwise.

  29. I try not to add anything from the outside to my Southern California garden, except native plants and vegetables. I compost almost all of my kitchen and garden green waste. My goal is: nothing in, nothing out.

    The only addition I make for potted plants is perlite. I dump old potting soil in a bin, add garden soil, compost, and perlite to lighten the mix.

    Vegetable gardening, of course, is a bit different. Tomatoes and other veggies require some added fertilizer, though here too I am going to try to use more composted green waste and less fertilizer, organic or chemical.

    I never sterilize pots – just hose them out – and like others, I have few problems and pests. Fussy plants are not welcome in my garden.

    I feel safe and clean in my garden and would not dream of adding poison to it.

  30. Peg says:

    I dump old potting soil into my flower beds. Is this a problem? I wash ots with water and maybe a bit of soap if they’re slimy.

  31. Faith says:

    I have two old whiskey barrels outside that I dump all my used potting soil into and let the rain, sun and air “recharge” it for a season or two. Then I reuse it for the next round of potting and re-potting.

    I used to mix the renewed potting soil with regular compost, but now that I’ve discovered vermicomposting, that’s my preferred source of food and soil amendments for potted plants. Plants love vermicompost and it’s it’s fun to raise a herd of worms indoors in winter!

  32. Robin says:

    I know I’m late to the game, as I came across this discussion while Googling for information on gardening. But seriously, Barry, did you really just invoke Hitler in a discussion about gardening? I’d advise that you go look at some pictures from the Holocaust, because you’ve apparently forgotten just what it was about. Hitler and his Final Solution are way, way, WAY too important and serious a topic to be casually used when people are discussing non-Holocaust matters. “Final Solution” is not just words, we’re talking about a concept that was the philosophical backing for the deaths of more than six million people. Please don’t trivialize it by using it lightly.

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