Why Is There Mildew On The Potting Soil

Last updated on October 23rd, 2023 at 08:28 pm

Why does the potting soil mold?

Every hobby gardener has experienced it – the potting soil of a freshly repotted houseplant suddenly becomes moldy.

First, you should clarify whether it is actually mold on the potting soil.

Why Is There Mildew On The Potting Soil

This is because if you live in a region with very lime-rich tap water, the deposits on the potting soil may also be lime or other minerals. This often occurs if the plant is near a heater.

Due to the heat near a heater, or at windows with direct sunlight, the irrigation water rises through the root bulb and evaporates on the surface, leaving dissolved minerals behind.

How do I tell the difference?

Just take a wooden skewer and scrape off some of the possible mold. If it is hard and brittle, it is a mineral deposit. These are then purely an aesthetic problem and can be easily scraped off the surface with a spoon or spatula.

After that, fresh potting soil can be added to the pot if necessary, and the problem is solved for the time being. It becomes more problematic if there is a fluffy soft white coating, as this is most likely mold.

Is mold on potting soil dangerous?

Mold usually not only adheres to the surface of the root ball, but also penetrates into the interior with its mycelium. In addition, mold usually smells a bit musty.

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Not all molds are problematic, but some are considered hazardous to health when their spores accumulate in high concentrations in indoor air… Mold spores can also trigger asthma attacks in people with chronic or allergic respiratory conditions.

Why does mold form on potting soil?

Basically, any potting soil with a high humus content will develop mold. Molds have an important function in nature as decomposers – a biological term for organisms that survive by breaking down dead organic matter.

However, the excessive proliferation of individual species indicates that the potting soil has lost its biological balance. This happens primarily when you overuse your water supply, because mold spreads especially quickly in permanently moist environments. Experience has also shown that low-quality potting soils and black peat with a high compost content are particularly susceptible to mold.

One reason is that the structure of cheap soil is often unstable and deteriorates rapidly with age. Susceptibility to mold growth also increases with reduced aeration.

How to get rid of mold in flower pots?

First you should take the pots with moldy potting soil outside and then thoroughly ventilate the room or apartment.

Now, using a hand spatula, carefully scrape the moldy soil from the surface of the potting soil. After that, you can then remove all the loose soil as much as possible, leaving only the deep-rooted remains. After this process, one should thoroughly clean the pot with a brush and warm vinegar solution.

Now you can replant the plants with new high-quality houseplant soil to put them back in their original place afterwards.

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Now make sure that the soil is as rich as possible in minerals. Here it is often helpful to mix in a few “ingredients”, such as sand or lava rubble.

It is also important to make sure that the planter has enough drainage holes.

Small tip – the drainage holes won’t clog as easily if you place a few rocks or clay chips in the bottom of the pot before filling it with plant soil.

Before potting the houseplant, you can sprinkle a thin layer of net sulfur over the surface of the old pot ball with a teaspoon. The biological agent is used in organic farming against fungal diseases, such as powdery mildew, and also shows a good effect against molds.

With good drainage and high-quality potting soil, you have already created very good conditions for preventing the mold from spreading again.

Now the main thing is to properly dose the amount of water. One should water his beloved houseplant dear something scarcer than to keep the pot ball permanently moist. To check whether the potting soil still has enough moisture, a simple finger test often helps.

More professionally, you can check the soil moisture and even the PH value by using a moisture meter from a specialty store.


  • James Jones

    Meet James Jones, a passionate gardening writer whose words bloom with the wisdom of an experienced horticulturist. With a deep-rooted love for all things green, James has dedicated his life to sharing the art and science of gardening with the world. James's words have found their way into countless publications, and his gardening insights have inspired a new generation of green thumbs. His commitment to sustainability and environmental stewardship shines through in every article he crafts.

    https://www.linkedin.com/in/james-jones-436784297/ gardeninguru@outlook.com Jones James
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