Ask a Master Gardener: Can I reuse potted dirt?
You should not reuse potting soil from plants that had disease.
Q: At the end of fall, I emptied all the dirt from my pots of flowers into a big plastic bag. Can I reuse it next year, or do I have to buy new dirt? Does it matter where I store it? Could I have just piled it up in the garden?
A: Yes, you can reuse the potting soil from your outdoor pots, with a couple of caveats.
You should not reuse potting soil from plants that had disease. And you should not use used potting soil for starting seeds indoors.
You can just pile up the potting soil in the garden, but you’ll lose some, and you’ll have to wait for it to dry out when you fill your pots in the spring. You also run some risk of having insect pests find your pile.
Keeping the used potting mix in a closed plastic bag is a good option. Remove all the leftover plant matter before you bag it up. You can store the bag indoors or out — someplace where the weather and sun won’t damage the plastic.
Potting mixes do break down over time, so you’ll still want to add some new mix to your leftovers. Some potting mixes come with fertilizer already added. If that’s what you’ve been using, that fertilizer will be used up in last year’s potting soil, so you’ll need to supplement.
If you start seeds indoors, don’t use used potting soil or garden soil. Potting medium that has been outdoors — or even been in contact with plants or pots that have been outdoors — may have picked up pathogens that can wipe out trays of seedlings. Sanitation is key to seed starting. Start with a new, unopened bag of sterile seed-starting medium and sanitized containers.
One last suggestion: Don’t leave your ceramic pots outdoors, whether they’ve got potting soil in them or not. Freezing and thawing during the winter can wreck them.
Written by U of M Extension Master Gardeners in St. Louis County. Send questions to email@example.com .