Potting soil puzzlement

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).

Comments

[Naomi]

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.

[Ayse]

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.

[rainymountain]

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.

[Rachel]

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.

[Michele Owens]

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.

[Elizabeth]

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.

[Brie]

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.

[chuck b.]

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.

[Sheila]

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!

[Renee Beaulieu]

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.

[Colleen]

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.

[firefly]

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.

[Michelle Derviss]

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.

[susan harris]

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.

[David E. Perry]

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.

[Barry Prince]

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."

[Rosella]

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.

[Mr. McGregor's Daughter]

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.

[Genevieve]

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.

[Janit Calvo]

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!

[greg draiss]

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

[Sally]

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.

[Marte]

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.

[Layanee DeMerchant]

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!

[tai haku]

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