Reusing your potting mix isn’t so simple

Container plants are nearing the season’s end. Gardeners will soon be discarding colorful annuals and potted vegetables. Meanwhile, houseplants enjoying a summer vacation in the garden or porch need to come indoors.

“We recommend bringing houseplants inside before night temperatures drop below 55 degrees,” said Spencer Campbell, manager of the Plant Clinic at The Morton Arboretum in Lisle.

With all this container-emptying and houseplant-repotting, it’s an obvious time to ask: Can you reuse potting mix? Or do you have to discard it and buy fresh potting mix next spring?

After all, quality potting mix is not cheap: a 2-cubic-foot bag often costs $20. It’s also heavy and bulky, a serious consideration for people who garden on balconies and porches.

The answer is not cut-and-dried, according to Stephanie Adams of the Arboretum’s plant health care program. “It depends on a lot of factors, including your own risk tolerance,” she said.

Potting mix does deteriorate in various ways over the course of a season. “Fertilizer that was added to the mix will be used up by the plants,” she said. “If the package said it will fertilize for six months and you planted in spring, you can assume the fertilizer is gone by fall.”

The physical structure of the mix also will change.

“The texture of potting mix is very important,” Adams said. “It needs to include large particles so there are spaces between them for water to drain and air to flow.”

If the potting mix is too fine and dense, it will drain poorly, trapping moisture around plants’ roots and preventing air from reaching them. That’s an environment that encourages the growth of bacteria that can lead to various plant diseases. “Drainage is the number one problem for plants in containers,” she said.

Potting mix is made up mostly of organic matter — some kind of dead plant material, which is usually composted, or partially decayed. Manufacturers choose from a wide menu that can include peat moss; sphagnum peat; coir, a fiber from coconut shells; composted leaves and landscape waste; and agricultural byproducts such as ground wood and rice hulls. Some national brands use different materials regionally, depending on what is locally available.

A mix may also contain white flecks of perlite, a lightweight mineral material that is added to ensure there are some large particles to improve drainage and aeration.

“As your plants grow over six or eight months, the organic matter will break down,” Adams said. “By the end of the season, there will be fewer large particles and more fine particles, so the mix doesn’t drain as well.” Since brands of potting mix are so varied in their ingredients, they may change texture at different rates.

Another consideration in reusing potting mix is the possibility that disease-causing fungal spores or bacteria or weed seeds have found their way into the potting mix over the plant’s months outdoors. “If a plant looks sickly or was underperforming in the pot, I would not reuse that potting mix,” Adams said.

“If the plants were vibrant and vigorous, I’d be more likely to reuse it.” What you do will depend on your judgment of the condition of your plants and potting mix at the end of the growing season, your budget and how much you are willing to gamble.

The Arboretum, which has extensive container plantings that are centered on small trees and shrubs, repots them all in fresh potting mix each year. At home, Adams takes more chances.

“If you’re going to reuse potting mix — which I do, because I’m cheap — you have to mix it and replenish it,” Adams said.

She stirs up old potting mix with a combination of fresh potting mix and sifted homegrown compost. “I would only use a quarter to a third of old potting mix to at least two-thirds of new material,” she said.

Don’t use mushroom compost or any other product containing manure, she said, because it contains too much nitrogen, or chicken litter, because it has too much phosphorus.

To duplicate the effects of potting mix with added fertilizer, add slow-release fertilizer in spring. Check the label to make sure organic fertilizers don’t contain manure.

What can you do with all the potting mix you don’t reuse? Unless you have reason to think it contains disease organisms, she said, “you can just scatter it in your garden or under trees as a light mulch or add it as a little organic matter to your soil. It’s already composted.”

For tree and plant advice, contact the Plant Clinic at The Morton Arboretum (630-719-2424, mortonarb.org/plant-clinic, or plantclinic@mortonarb.org). Beth Botts is a staff writer at the Arboretum.