I decided to add several containers to my back patio last year and would like to save some money on potting medium this year. Can I just use the potting soil from last year?
— Michelle Lee, Arlington Heights
I’m a big fan of adding containers to a garden, and I regularly reuse the growing medium in my containers at home for growing annuals. A good growing medium is important for plants, since it provides them with moisture and nutrients. It’s best to use a growing medium designed for use in containers, not soil from your garden, which is too heavy and won’t drain well in a container. I’ve seen some bags labeled as potting soil that I thought were much too heavy and seemed more appropriate for amending garden beds or seeding lawns. A large 40-quart bag of a high-quality growing medium will be lightweight and easy to pick up.
Your containers should have a drainage hole in the bottom; no need to put a layer of gravel at the bottom of the pot for drainage. Peat- and bark-based mixes will naturally decompose and break down into finer particles, so over time they’ll compress down and reduce the volume in the container. Peat-based mixtures can dry out and be difficult to remoisten, so that water will quickly run down the sides of the pot and escape out the bottom.
During the growing season, plants will use up the nutrients in their growing medium; I like to replenish the container each year by removing a third to half of the old medium and replacing it with fresh growing medium that’s mixed with the old. This will save some money and should give you good results. The old medium you removed can be used in garden beds if disease isn’t a concern or added to a compost pile that’s active enough to generate heat that can kill weed seeds and pathogens. If your pots are small, it might just be best to start with new medium each year—and if they’re large, replenish them. I put small used containers (generally 4-inch pots) and cell packs in the bottom of large containers to reduce the amount of medium I need to fill the pots.
If you’re planning to grow vegetables like tomatoes, peppers, and eggplants that are from the same plant family, you should ideally completely change the growing medium each year to help prevent disease. Also, change the growing medium if you had any problems with soil-borne diseases the previous year. You should also use fresh, sterile medium each year if you’re germinating vegetable or flower seeds under lights inside.
For more plant advice, contact the Plant Information Service at the Chicago Botanic Garden at plantinfo@chicagobotanic.org. Tim Johnson is senior director of horticulture at the Chicago Botanic Garden.
