Can Houseplant Soil Go Bad?

You might find mold on houseplant soil after bringing your plants in for the winter, or it might grow in containers that are inside year-round. It can pop up on new additions to your urban garden, or it can appear on established members of your leafy collection. Mold occurs on soil for a number of reasons. Understanding what caused the problem and how to get rid of it will keep your garden and home happy.

“Most people don’t realize that plants have natural microorganisms that exist around them just like we have within our bodies and on our skin,” explains Shelby DeVore, founder of Garden.Farm.Thrive. “Many of these microorganisms are necessary for proper plant health.”

This symbiotic relationship is why living soil is so favorable. To create living soil, many gardeners try to attract worms, which aerate soil and supply much-needed nutrients. Adding compost is another way to get those beneficial microorganisms. However, improper plant care can disturb the delicate balance of nature.

“When you start to notice mold emerging on the surface of the potting soil,” DeVore says, “there’s an issue.”

What Causes Mold Growth on Houseplant Soil?

Mold and other fungal diseases thrive in moist, dark and stuffy environments. Unfortunately, it’s easy to create those conditions, especially when gardening indoors. Try to avoid the following:

Overwatering

Fungus will consume any extra water lingering in a potted plant, and too much water can cause roots to rot. Houseplants usually require less water than their outdoor counterparts. While direct sunlight and wide-open spaces allow outdoor plants to dry out quickly, indirect sunlight and enclosed rooms within a home let houseplant soil retain water longer.

Poor Drainage

Poor water drainage can also lead to excess moisture and moldy soil. There are a few things that cause poor drainage: incorrect pot size, lack of drainage holes and dense soil.

Andrew Levi, founder and CEO of PlantTAGG, explains that proper pot size is key. “With roots exposed in an oversized pot, you’re more likely to see root rot as plants will be unable to use the amount of water that larger pots can hold,” he says. Measure your plant before purchasing a container. Or, better yet, bring it along to the garden store to see how it fits. Some plants are easier to grow in containers.

Many decorative pots come without drainage holes, which allow excess water to run through the houseplant soil and out of the pot. When they’re missing, all that moisture stays around the roots, where mold and fungus can use it. DeVore recommends containers with several drain holes that are 1/4-in. to 1/2-in. diameter.

If your soil is too dense, water will have trouble escaping. Potting mix is specially formulated for container gardening. It has lightweight peat moss and perlite, which help drain water. You can amend dense soil with plain peat moss, or just repot your plant in better potting mix.

Poor Air Circulation

Indoor plants don’t always get adequate air circulation, especially in the winter when windows are closed. Plants that are kept in dark corners or on cramped shelves are particularly susceptible to moldy soil. Air movement helps houseplant soil dry out between waterings.

Contaminated Soil

Your potting soil itself can cause mold problems. While soil should have some microorganisms, it’s possible for it to become contaminated before it even lands in a pot.

Desiree Thompson, gardener and plant expert at Gardening Services London, says to toss “any poorly stored compost and always keep your leftover soil in a dry place with holes properly sealed.” Also, always inspect your bag before potting a plant, for even well-stored soil can absorb moisture when punctured.

Decomposing Leaves on Surface

Mold and other fungal infections feed on decomposing plant matter, so a buildup of dead leaves will encourage moldy soil. Remove dead pieces of the plant before they pile at its base. Fallen leaves can be used as mulch outside to reduce yard waste.

Will Moldy Soil Harm My Plant?

Mold on soil may or may not harm your plant. It depends on the type you’re dealing with.

Andrew Gaumond, horticulturist at Petal Republic, explains that “white mold is relatively harmless to the overall health of the plant,” while “fungal mycelia can be a real issue.” Although fuzzy, white mold on its soil may not hurt your houseplant, the conditions that allowed such mold growth will. And some molds and fungi do cause plant diseases.

Mold in the home may also be toxic to humans, or at least irritate those with asthma or allergies. So you’ll want to get rid of it right away for the sake of your plant and yourself.

How Do I Get Rid of the Mold on My Plant Soil?

Some gardeners swear by cinnamon as a natural anti-fungal. Simply wipe off the mold and sprinkle the spot with some cinnamon from your spice drawer.

If cinnamon doesn’t work, Gaumond says to try a houseplant fungicide spray or a homemade baking soda and water mixture. Test any solution on a small part of your plant to be sure it isn’t too strong. Once you’ve removed and treated the mold, it’s important to address what allowed mold growth in the first place. Find the root cause, then adjust your plant care accordingly.

How Can I Prevent New Mold from Growing?

Getting the right combination of sun, air, soil and water will keep the natural balance of your houseplant soil in check. Start by watering only when necessary. Many houseplants come with care instructions, but Levi says not to adhere to them. “Most plant tags that come with a plant provide care guidance for where the plant was grown, not necessarily your home/area,” he says.

Instead of watering on a rigid schedule, stick a finger a few inches into the soil to check its moisture level. Hydrospikes are another option for forgetful plant owners.

Avoid pots without drainage holes. If you fall in love with a decorative pot that doesn’t have holes, there are a few things you can try. One common solution is to add landscape rocks beneath the potting soil so that water has a place to pool. You can also try to drill holes into the pot, but clay or ceramic pieces may shatter. Be aware that these solutions are not guaranteed, so you may end up repotting your plant due to mold on its soil.

Finally, place your houseplant where it will get enough sunshine and air circulation. This means that tight spaces and forgotten corners might need fake plants. If an open window is out of the question, consider using a fan to mimic a natural breeze.

FAQs

Can houseplant soil go bad in other ways?

Yes, potting soil can become depleted over time as plants use the nutrients in the soil. Gnat, flies and aphids can also infest soil, rendering it unusable. In both cases, it’s best to replace existing soil with new material.

Is moldy houseplant soil dangerous?

Most mold that grows in houseplant soil is not toxic but can still be harmful to those with weakened immune systems, mold allergies and asthma.

About the Experts

• Shelby DeVore is a gardening expert and founder of Garden. Farm. Thrive., a homesteading website. She holds a master’s degree in agriculture.
• Andrew Levi is the founder and CEO of PlantTAGG, a plant identification, care and design app for homeowners.
• Desiree Thompson is an avid gardener and plant expert at Gardening Services London, a professional gardening and landscaping company in London, England.
• Andrew Gaumond is a horticulturist, botanist and director of content at Petal Republic, an online platform and flower delivery service.