Winter poses many challenges for gardeners. We take great measures to protect our precious plants through harsh weather; wrapping them up, moving them to sheltered areas, or even bringing them indoors. When you bring outdoor plants inside, however, it also brings the potential for a number of problems, including mold.
Mold doesn’t necessarily harm your plants, but it can be an indicator that your plants are not in ideal growing conditions. Controlling mold in a way that doesn’t harm plants requires a basic understanding of this stealthy invader, along with a common household product or two.
Why does mold grow in soil?
Mold is a natural and important part of the decomposition process. It works in your compost heap, and in your garden to break down organic matter and make it usable to plants’ roots. You don’t notice it so much outdoors, because bright sun, fluctuation in temperature and good air circulation keep it from showing up on the surface.
When you bring big pots of wet soil indoors, however, the game changes. Mold loves a stable, moist, dark environment; and large pots of wet soil at a fairly steady temperature provide just the conditions for it to thrive.
With stable temperatures, reduced light and still air, any mold that was in the soil will thrive and come to the surface to reproduce. Then, it not only looks kind of nasty, it also sends out spores that can mess with our respiratory systems.
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If this has happened already, do not panic. Your plants are probably fine, you just need to mediate your soil a bit. If you’re seeing mold, there is a good chance your soil is too wet. Plant roots need water, but they also need air. By tweaking the conditions in your soil just a bit, you can make it more hospitable for your plants and less hospitable for mold:
Removal
If you have visible mold on the surface, you will first want to remove the top layer of soil to reduce the possibility of further infection.
Sterilize the soil
There are many ways to clean up your potting soil without harming the plants it supports. Some natural household products are just as effective as heat or chemicals to destroy mold.
Vinegar
You may know that vinegar is a common household cleaner that kills mold and bacteria. A diluted solution of vinegar can also clean up your soil. Too much vinegar will harm your plants, but plants can tolerate a broader range of pH than mold. Since mold likes a steady, neutral pH, a slight alteration of your soil’s pH can quickly wipe out any mold colonies.
Using a solution of 3/4 cup distilled white vinegar per gallon of water, wet the soil thoroughly. Avoid wetting the leaves and stem of the plants, as this can cause damage. Allow the soil to drain and dry out a bit before resuming normal watering, and avoid repeating the process unless necessary — no more frequently than once a month.
Hydrogen peroxide
Hydrogen peroxide (H2O2) has a similar effect. Just as it bubbles away to kill germs on open wounds, hydrogen peroxide will react with the soil, bubbling up to form a gas that oxygenates the soil and destroys mold spores. Apply a solution of one quarter cup 3% hydrogen peroxide per quart of water to treat soil for mold. As an added bonus, it will also kill the eggs of fungus gnats or other pests, and provide healthy aeration for the roots.
Hydrogen peroxide may not be suitable for treating large pots, however, as the volatile solution acts quickly and bubbles away before it can penetrate very deep into the soil. Still, it can be used regularly to treat surface soil, and will provide helpful aeration to the amount of soil that it does penetrate.
It can also be sprayed safely on plants at a ratio of one teaspoon per cup of water. This is helpful in treating both mildews and insect infestations on the surface of leaves.
Hydrogen peroxide can be used weekly, as it does not have cumulative effects like vinegar; but do use your best judgement. Too much can cause desiccation in plants and also kill beneficial microorganisms.
Cinnamon
If you are hesitant to add more water to an already wet situation, cinnamon may be the answer to your mold problem. Cinnamaldehyde is an antifungal compound found in cinnamon, making this common spice a promising agent in the prevention and removal of mold.
In small doses, cinnamon damages mold cell membranes without harming the plants. Instead, it actually stimulates root growth and helps prevent infections. As with anything, use it in moderation to avoid upsetting the natural balance of your soil.
To treat your soil, first remove any visible mold on the surface. Then sprinkle cinnamon lightly and evenly over the soil. You can add a layer of clean potting soil on top, and the cinnamon will act as a barrier to prevent any remaining mold from maturing and setting spores.
Preventative measures
Once you have your mold under control, you can use preventative measures to inhibit the growth of mold. Reduce watering as the temperature decreases outdoors, and allow the soil surface to dry between waterings.
Improving aeration by using porous pots. Since plastic pots can’t breathe, they tend to hold moisture better. This is great at the height of summer, when it’s difficult to give enough water to potted plants, but when growing slows down the water that is absorbed by the soil just sits there. Try moving your plants into fabric pots or terracotta pots. These allow the gradual evaporation of water from the entire surface so the soil does not become waterlogged.
Provide ample sunlight by a sunny window, and provide good circulation by leaving plenty of room between plants. Using sterile potting soil will also reduce the risk of contamination. If you take these steps before moving your plants indoors, you may avoid the mold problem altogether.
