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how to reuse potting soil

Yes, You Can Reuse Your Potting Soil Instead of Tossing It at the End of the Season

Bagged mixes for containers aren’t “dirt cheap,” so here’s how to make the most of it.

When your flowers fade, and the temperatures drop, it’s time to empty your containers and put them away for the winter. It’s tempting to keep and reuse the old potting soil, which can be pricey, especially when you have a lot of potted plants like I do. But this lightweight mix of compost, peat, perlite, and other materials doesn’t last forever. Watering and rain leaches out the nutrients in it, and plants use them up as they grow. The mix can become compacted and filled with roots. And sometimes pests, diseases and weeds can take up residence, ready to pop back up when you replant in the mix. However, you can remedy each of these issues and get another use out of your potting soil if you don’t mind doing a little extra work.

How to Reuse Potting Soil

It’s generally fine to reuse potting soil if whatever you were growing in it was healthy. But even if your plants seemed problem-free, or if you did notice pests or diseases popping up, it’s best to sterilize the mix before reusing in it to avoid infecting next year’s plants. First, remove any roots, grubs, leaves and other debris from the old potting soil. Then, decide on the best method for banishing microbes and insects.

One technique for sterilizing soil is called solarizing. It involves putting old potting soil in lidded, five-gallon buckets ($5, The Home Depot) or black plastic bags that are tightly tied shut and leaving them in the sun for four to six weeks. The heat builds up inside the buckets or bags just enough to kill pathogens and bugs.

You can also sterilize old potting soil in your oven, by baking it in an oven-safe pan, covered with foil, at 180 to 200 degrees F for 30 minutes. (I tried this once, but didn’t like the earthy smell it created.) It’s also important to check the soil temperature with a candy or meat thermometer ($20, Williams Sonoma) to make sure it stays below 200 degrees. Higher temperatures can release toxins. When it’s done, take the soil out of the oven and keep it covered until it cools.

Microwaving is another option. Put old, moistened potting soil in quart-sized, microwavable containers. Cover them with microwavable lids (never use foil) that you can poke ventilation holes in or can leave cracked to allow steam to escape. Heat at full power for about 90 seconds per two pounds of soil. Remove the containers, cover the vent holes with tape, and let the soil cool completely before using it.

Once your old potting soil has been sterilized, you’ll need to replenish its nutrients. You can do this by combining equal parts of new potting soil with the old and adding a dose of slow-release fertilizer pellets according to package directions. Or, you can mix in one-part compost to three or four parts of your old potting soil. Besides adding nutrients that plants need, both the fresh potting soil and compost will help keep the mix from compacting.

How to Store Reused Potting Soil and Containers

If you’re storing your refreshed potting soil until it’s time to plant again, keep it in covered buckets or large, clean trash cans ($20, Walmart) or tubs with lids. As for your emptied containers, keep them in a protected spot over the winter, especially if your area experiences frigid temperatures that can damage pots. And before reusing those containers, make sure they’re clean, too.

Mix some warm water with a little mild dish detergent and a cup of vinegar in a large bucket or another container. Scrub the inside and outside of terra cotta pots with a stiff brush. Use a soft brush or nylon scrubber ($2, Amazon) to clean pots made of other materials that might show scratches. If the pots have a white build-up of salts, scrape them off with an old, dull knife or putty knife.

Next, fill a large tub or sink with one part household bleach to nine parts water. Soak the pots for 10 minutes to kill any last bacteria or fungi. Soak terra cots pots in clean water for 10 more minutes to remove the bleach. Rinse plastic pots with clean water. Let all the pots air-dry before filling them with your refreshed potting mix.

Can Old Potting Soil be Reused in the Yard and Garden?

If you’re not up for sterilizing and refreshing old potting soil, you still can put it to use instead of throwing it out. It can be dumped directly out of your containers and into established beds and borders. I like to use mine in my raised beds or wherever I need to fill in holes or eroded areas in my yard. It can also be mixed into compost piles. The old potting soil you reuse can help you save money for what all gardeners want: more plants.

Bagged mixes for containers aren't "dirt cheap," so here's how to make the most of it.

Can You Re-Use Potting Soil From Your Containers?

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Q. Mike: My wife likes to plant flowers and vegetables in large pots. Come winter, we empty the pots. The used potting soil usually contains an extensive root structure and often is beginning to sprout weeds. What is the best way to store and recondition the potting soil for reuse the following year? Thanks!

    —Dr. Mitch; The Children’s Hospital of Philadelphia

Both my wife and I thoroughly enjoy your programme for the sound, useful advice and the humorous and boisterous tone in which it is delivered. Would you please settle a long-standing disagreement between us: Can one re-use potting soil after having grown plants/flowers in it for one season? The husband says: “Yes! You bet-your-potting-soil!” The dear wife (who, admittedly has spent 100 hours gardening for every hour the husband has dabbled) claims that such soil is spent after one growing season and may now harbour insects and/or disease. Who is right? Please do let us know…

    —Michael (the husband) in Spokane, Washington

Q. An excellent question, and one that deserves an intelligent discussion. But instead of that, I’ll tell you what I do with mine.

First, let’s review the basics of container contents. Because you are going to trap these poor plants in a finite space, as opposed to the great outdoors where they can send their roots out much further in search of help, you have to supply them with a light growing mediumthat drains exceptionally well. That means no garden soil in the mix. Instead, the ideal medium for containers is three-quarters soil-free mix and one-quarter compost.

“Soil-free mix” is the term I use for high-quality potting soil; it may also be called professional mix, seed-starting mix, sterile growing medium or some other synonym. It’s generally composed of milled peat moss (with a little lime to adjust the pH), perlite and/or vermiculite (naturally occurring minerals that are ‘popped’ in big ovens) and some compost or “composted forest products”. Some packagers substitute coir (shredded coconut fiber) for the peat; and some companies add nutrients to the mix (which is bad if the nutrients are man-made chemicals, but wonderful if they’re natural things like worm castings).

Mix one of those mixes up with some high-quality compost, and you’ll have a growing medium that retains moisture and drains well, contains a nice amount of organic matter, and is light enough for you to move the containers around fairly easily.

So—what do you then do with this wonderful stuff at the end of its first season? Because the soil will expand and contract greatly over a harsh winter, those who grow where the ground freezes hard should empty out plastic, ceramic and clay pots to protect them from cracking. Or you can just bring the whole schmageggie inside to a place that will remain above freezing. (If you do empty them out in the fall, remove any roots or weeds and add them to your compost pile. If you store the pots full, plan to remove this debris when you freshen up the mix the following Spring.)

In my opinion and physical reality, the only hard-core issue of re-use here is The Tomato Rule. Potting soil that was used to grow tomatoes should not be used to grow tomatoes the following two years. BUT that soil can be used to grow flowers, bush beans, peppers, salad greens—whatever you want, as long as it’s not tamatas. Conversely (like the sneakers), soil that hasn’t ever been used for tomatoes (or that hasn’t seen their roots for a few seasons) can be used to grow this year’s love apples.

One way to achieve this noble end is to have two big galvanized or hard plastic trash cans, label one with a T and one without, and use these to store your soils over winter. Don’t worry about otherwise mixing the soil from different pots; I actually prefer to combine mine to mitigate any potential nutrient imbalances and such.

The following season, buy some fresh soil-free mix and use it to freshen up every pot that gets filled with old soil. How much? Up to a third new mix if your old soil is really old or if it seems to be bulking up on you; less if your old stuff is still light and fluffy. Always add fresh compost to the tune of one-quarter of the container.

Now the risks. Insect carry-over is fairly remote, as is the risk of keeping a disease alive other than the soil-borne wilts that attack tomatoes. Weeds could be an issue, especially if you don’t mulch the tops of your containers with shredded leaves (which I highly recommend as the leaves also retain moisture, a very important consideration for pots in direct sun or during an especially hot dry summer).

But those weeds (and any tricky diseases) will still be much less of an issue than in outdoor gardens, and the weeds can be even further avoided by layering the new season’s compost a couple of inches thick on top of the old soil-free mix instead of mixing it in.

And if, like me, you garden in ground and in containers, it’s a wonderful idea to give one or two of your containers a completely fresh set of clothes every few years and mix their old potting soil into your garden, where its mix of lightweight ingredients will be welcomed by the roots of your plants—especially if those poor rooties have to try and fight their way through the misery of clay.

Read these Previous Questions of the Week on CONTAINER GROWING BASICS and GROWING TOMATOES IN CONTAINERS for more info on those important topics.

Q. Mike: My wife likes to plant flowers and vegetables in large pots. Come winter, we empty the pots. The used potting soil usually contains an extensive root structure and often is beginning to sprout weeds. What is the best way to store and recondition the potting soil for reuse the following year? Thanks! —Dr. Mitch; The Children’s Hospital of Philadelphia Both my wife and I thoroughly enjoy your programme for the sound, useful advice and the humorous and boisterous tone in which it is delivered. Would you please settle a long-standing disagreement between us: Can one re-use potting soil after having grown plants/flowers in it for one season? The husband says: Yes! You bet-your-potting-soil! The dear wife (who, admittedly has spent 100 hours gardening for every hour the husband has dabbled) claims that such soil is spent after one growing season and may now harbour insects and/or disease. Who is right? Please do let us know… —Michael (the husband) in Spokane, Washington Q. An excellent question, and one that deserves an intelligent discussion. But instead of that, I’ll tell you what I do with mine. First, let’s review the basics of container contents. Because you are going to trap these poor plants in a finite space, as opposed to the great outdoors where they can send their roots out much further in search of help, you have to supply them with a light growing mediumthat drains exceptionally well. That means no garden soil in the mix. Instead, the ideal medium for containers is three-quarters soil-free mix and one-quarter compost. Soil-free mix is the term I use for high-quality potting soil; it may also be called professional mix, seed-starting mix, sterile growing medium or some other synonym. It’s generally composed of milled peat moss (with a little lime to adjust the pH), perlite and/or vermiculite (naturally occurring minerals that are ‘popped’ in big ovens) and some compost or quote composted forest products. Some packagers substitute coir (shredded coconut fiber) for the peat; and some companies add nutrients to the mix (which is bad if the nutrients are man-made chemicals, but wonderful if they’re natural things like worm castings). Mix one of those mixes up with some high-quality compost, and you’ll have a growing medium that retains moisture and drains well, contains a nice amount of organic matter, and is light enough for you to move the containers around fairly easily. So—what do you then do with this wonderful stuff at the end of its first season? Because the soil will expand and contract greatly over a harsh winter, those who grow where the ground freezes hard should empty out plastic, ceramic and clay pots to protect them from cracking. Or you can just bring the whole schmageggie inside to a place that will remain above freezing. (If you do empty them out in the fall, remove any roots or weeds and add them to your compost pile. If you store the pots full, plan to remove this debris when you freshen up the mix the following Spring.) In my opinion and physical reality, the only hard-core issue of re-use here is The Tomato Rule. Potting soil that was used to grow tomatoes should not be used to grow tomatoes the following two years. BUT that soil can be used to grow flowers, bush beans, peppers, salad greens—whatever you want, as long as it’s not tamatas. Conversely (like the sneakers), soil that hasn’t ever been used for tomatoes (or that hasn’t seen their roots for a few seasons) can be used to grow this year’s love apples. One way to achieve this noble end is to have two big galvanized or hard plastic trash cans, label one with a T and one without, and use these to store your soils over winter. Don’t worry about otherwise mixing the soil from different pots; I actually prefer to combine mine to mitigate any potential nutrient imbalances and such. The following season, buy some fresh soil-free mix and use it to freshen up every pot that gets filled with old soil. How much? Up to a third new mix if your old soil is really old or if it seems to be bulking up on you; less if your old stuff is still light and fluffy. Always add fresh compost to the tune of one-quarter of the container. Now the risks. Insect carry-over is fairly remote, as is the risk of keeping a disease alive other than the soil-borne wilts that attack tomatoes. Weeds could be an issue, especially if you don’t mulch the tops of your containers with shredded leaves (which I highly recommend as the leaves also retain moisture, a very important consideration for pots in direct sun or during an especially hot dry summer). But those weeds (and any tricky diseases) will still be much less of an issue than in outdoor gardens, and the weeds can be even further avoided by layering the new season’s compost a couple of inches thick on top of the old soil-free mix instead of mixing it in. And if, like me, you garden in ground and in containers, it’s a wonderful idea to give one or two of your containers a completely fresh set of clothes every few years and mix their old potting soil into your garden, where its mix of lightweight ingredients will be welcomed by the roots of your plants—especially if those poor rooties have to try and fight their way through the misery of clay. Read these Previous Questions of the Week on CONTAINER GROWING BASICS and GROWING TOMATOES IN CONTAINERS for more info on those important topics. Ask Mike A Question    Mike’s YBYG Archives    Find YBYG Show