Soil in the oven? Some houseplant enthusiasts “bake” their soil—an expert explains why it’s not worth it
On social media, you may see people heating soil in the oven to keep pests at bay. We asked botanist Visa Lipponen whether there’s any point to this practice.
Visa Lipponen answers:
You really shouldn’t heat houseplant soil or any other growing medium in the oven before use. Depending on how long it’s heated, you’ll end up destroying most or all of the beneficial microbes in the soil. Heating also reduces the fertilizer effect of the soil by causing nutrients to evaporate.
The importance of microbes in the growing medium is underscored by the fact that studies show they filter unwanted compounds from the air even more effectively than the plants themselves. They also support plant health by releasing nutrients, especially nitrogen, for the plants to use.
“For instance, coconut fiber ignites extremely easily with a matchstick and burns almost instantly.”
Store-bought soil can, of course, contain fungus gnats or other critters. However, fungus gnats are fairly harmless and can be kept in check by allowing the soil to dry out between waterings. If they do multiply, it’s usually a sign of overwatering or overly compact soil. Other bugs typically aren’t an issue—the only ones that can be truly troublesome are root aphids, which I have personally found in store-bought soil. Heating soil would certainly kill them, but they can also hitchhike in with a purchased or gifted plant, and obviously no one bakes their plants to avoid pests.
Most of soil, and all coconut fiber, is organic matter. Growing media also often contain partially decomposed materials—like small twigs, bark, or coconut fiber—that present a fire risk in the oven and can also produce carbon monoxide if they don’t burn properly. For instance, coconut fiber ignites extremely easily with a matchstick and burns almost instantly. The same goes for peat and coconut husk chips: they spark quickly, and I doubt that pre-moistening them reduces the fire hazard. An oven should be reserved for food—nothing else!
If you’re determined to heat your growing medium, I recommend placing only the necessary amount in a heatproof container and pouring boiling water over it. After the water has cooled, spread the medium out to let the excess water evaporate until it’s still moist enough for potting. Of course, this method destroys beneficial microbes and reduces nutrient levels as well.
