How to sanitize garden containers with Sam Schipani. Credit: Linda Coan O'Kresik / BDN

Summer may feel like it’s in full swing, but Maine is mere weeks away from cold weather and, more importantly, the end of the growing season. As we pack in our containers to use for next year, it is important to sanitize them to make sure the problems that you had this growing season don’t persist next year.

“It’s a good insurance policy,” said Caleb Goossen, Organic Crop and Conservation Specialist at the Maine Organic Farmers and Gardeners Association. “When we’re starting our seedlings, they’re very susceptible to damping off diseases and those are typically soilborne. If you’re reusing pots or containers, there’s a chance that you’re carrying over some of these species disease organisms and your brand new seedling baby.”

Even if you didn’t experience any issues with the plants grown in your containers this growing season, they may still be harboring disease.

“In a scenario where you have a good year and did everything right, the disease organisms may have still been there but outcompeted by good organisms and had healthy seedlings that could withstand them,” Goossen explained. “You can never assume that you’re free of it.”

Another important element that people often forget is to sanitize trays for seedlings. In fact, Alicyn Smart, assistant extension professor and extension plant pathologist at the University of Maine Cooperative Extension, said that seedling trays and containers are perhaps the most important things to sanitize because fledgling seedlings are most sensitive to the diseases that unwashed containers can harbor.

The first step to sanitizing garden containers of any kind is to brush off any remaining dirt or debris.

How to sanitize garden containers with Sam Schipani. Credit: Linda Coan O’Kresik / BDN

“Anything that’s dirty, the sanitizer won’t be able to have an impact,” Goossen said. “Anything that’s on them — old roots, leaves, potting mix — is usually really dry, [so] the first step is to brush it out, get as much out as you can. If it looks visibly clean, move on to disinfecting but if it looks dirty you want to clean maybe with soap and water. A quick scrub or wipe depending on how dirty it is.”

Then, prepare your solution: a 10 percent bleach solution is recommended, so mix one part bleach with nine parts water in a large enough container for your pots and trays.

“There’s of course the normal safety caveats with bleach — don’t mix it with other chemicals, it can damage skin, bleach clothing, oxidize metal — but for plastic pots it does a good job at sanitizing,” Goossen said.

Then, dunk your pots, let them soak for 10 to 15 minutes, rinse them thoroughly with fresh water and dry them. If you are doing a large number of containers, you may need to swap out the bleach container after a few rounds.

“It does degrade over time with the organic matter that’s leftover,” Smart said.

Some materials are also harder to sanitize than others — namely, terracotta and ceramic containers. Goossen said that a bleach solution may be less effective for terracotta pots because they are so porous. Instead, it may be better to boil them in hot water, in a lobster pot or a similarly sized container. Smart said that ceramic pots may absorb the bleach if they are not thoroughly washed, so make sure you are extra diligent about cleaning them in the bleach-free water after you are done sanitizing.

Once your containers and trays are clean, you want to store them properly to make sure they don’t get contaminated again during the off-season.

“I wouldn’t recommend them being on the ground in a greenhouse or something like that,” Smart said. “You want to make sure they’re away from any organic matter that could have diseases associated with it.”

Some home gardeners may be resistant to sanitizing their containers if they are trying to save soil in containers from year to year. While this is possible for certain plants, it does come with some risks.

“In that situation, [you’re] usually putting in a transplant that’s beyond the most susceptible stage,” Goossen said. “I don’t think it’s usually an issue from a disease standpoint unless there was a real disease issue in the previous growing year.”

Decorative containers — namely, larger ones and those made of wood, like whiskey barrels — are also difficult to sanitize. Goossen said in that case, it might be best to just ensure all the dead plant material is removed each year, add a healthy, fresh mix of potting soil and make sure your transplants are large and healthy enough to outcompete any lingering disease pressures.

The best practice, though, is not only to sanitize between seasons, but also to switch out potting soils. The soil will not be as fertile as it was in the prior seasons.

“A lot of those potting soils have sort of slow release fertilizer in it or things like that, using up the nutrients from that soil,” Smart said. “Over time, you might see that your plants are showing deficiency symptoms in nitrogen or things like that.”