Mold on potting soil

Mold in potting soil

Mold spores are everywhere. They are in the air, stick to plants and animals and can even be found on the human skin. Like many other microbes, mold spores are always in our proximity. In soil, mold plays an important role: Together with other microorganisms, it keeps things in balance, helps keeping nutritients in circulation, regulates humidity. Dead tissues decompose and can be used by plants once again. So essentially, that helps plants.

It gets bad when a single mildew takes over and dominates the environment in a plant pot. Often, plant soils which contain a high amount of humus are prone to mold formation. That is we recommend organic earth without humus.

Not every white film covering the soil is mold

When the potting earth is covered in a thin white coating, it's not necessarily mold. In most cases it's just chalk from using water with a high content of calcium carbonate. When so-called hard water is used to water the plants, a part of it evaporates on the surface, leaving a layer of chalk behind. When poked with a toothpick or a stick, it crumbles.

Those mineral residues are neither bad for plants nor for humans. If you don't like the look of it, you can just remove the top layer and replace it with new potting earth. Watering the plant with "soft water" will prevent the white chalk layer from emerging.

If, however, you poke the white coating with a toothpick or a stick and it feels soft and cottony, then its most probably mold, and you should get rid of it quickly. On the surface, only a part of the mildew is visible. The actual mildew exists as mycelium – a network of fungal threads – in the soil. It smells musty and it can be a health hazard, not just for the plant but for humans and animals too.

Chalk on potting soil

Mildew in the flower pot: A white coating

Mold in the flower pot usually looks like white cotton. Terracotta pots can get discolored blueish or blackish when infested by mold. It doesn't matter what kind of mildew exactly is affecting your plant pots: It emits toxins which are a health hazard and should be eliminated.

The mold spores can trigger allergic reactions and can lead to asthma attacks in people with chronic airway diseases. If left untreated, the mold can further spread in the apartment and cause many other diseases too. A high level of toxins in the air can weaken the immune system and make people exposed to it susceptible to various other germs.

Changing soil and cleaning pots

Mold can't just be removed off the top of the soil. It goes deeper than most people think. The visible part is actually the smallest part of the mildew. Thus, it's essential to replace the potting soil completely.

  1. First, carefully pick up the plant by its stem. Put the contaminated soil into the trash. Clean the roots as good as you can from any remaining earth. The less residue, the better the chances the mold doesn't regrow. However, try not to lacerate the fine roots of the plant.
  2. Most of the soil can be removed by shaking. Any remains can be carefully stripped off by hand.
  3. In the autumn and winter, the roots can be carefully cut with a kitchen knife. Slice off thin layers of earth and roots from all sides. Shortly before winter rest, plants usually don't take it as bad. As long as you don't cut off too much.
  4. Put the plant aside, maybe on an old newspaper. Thoroughly clean the empty pot from any soil and mold spots to make sure any residues are gone. If there are any stones or terracotta shards on the bottom, clean those as well. Use a brush and warm water infused with vinegar.
  5. Dry the pot.
  6. Pick a suitable potting soil. We recommend turf free organic earth.
  7. When refilling the pot with earth, place stones or ceramic/terracotta shards in the bottom, and then put foamed clay on top of it. Then add the potting soil. This will make sure the water can drain, preventing mold growth in the future.
Moving plants into a new pot

It's all about the mix

To make sure the mold doesn't come back, avoid dammed-up water. Water your plants moderately, and make sure any excess water can drain quickly. This is how you best do it:

  1. Use a pot with holes on the bottom.
  2. Use terracotta shards or rough stones right as the bottom layer.
  3. Put a layer of foamed clay on top of it.
  4. Put potting earth on top of it, preferrably with a high mineral content. We recommend turf free organic earth.

Routinely loosen up the earth. Do not use cheap potting soil with a high compost content. They are structurally unstable, tend to clump and aren't permeable to air.

Pot plants usually take kind of a winter break. In autumn and winter, they require far less water than in the spring and in the summer. A good rule of thumb is to only water the plants when the surface of the soil starts to feel dry. That means, it hasn't yet dried all the way to the bottom, but at least some of the water has been evaporated or absorbed by the plant, so the water doesn't build up in the pot.

Sometimes, not the soil is at fault, but the plant

Often enough, the mildow isn't brought in with the soil but with the plant. There's nothing you can really do about that. High quality soils can somewhat keep the mold in check, because mold prefers fine structured turf. Also, you can mix your own soil from regular potting earth, sand, compost and coconut humus.

Terra-cotta pot

Never handle mold without protection

You should not come in direct contact with mold. Use gloves and breathing masks to prevent inhaling spores.

When the soils is rather dry, pay even more attention to spores: The dust contains mold too and can spread this way. If you have allergies or a respiratory disease, make sure to wear a breathing mask and protection goggles.

After repotting, clean the workspace and make sure to remove all of the old soil from the apartment, if you did the repotting inside. Air the room to lower the concentration of mold in the air.

Protective gloves and mouthpiece

Household remidies against mold in flower pots

If repotting is too cumbersome for you, there's still some things you can try:

  • Soak some rags in tea-tree oil and use them to cover the surface of the soil.
  • Sprinkle some active carbon across the soil to keep mold in check and help prevent new mold formation.
  • Sprinkle cinnamon across the soil, the essentail oils can help prevent new mold formation.
  • Regularly air the room to make sure the air humidity stays low.
  • Temperatures around 18° C / 64° F and a relative air humidity below 60 % help prevent mold.

You can check air humidity by using a hygrometer. To reliably lower the air humidity, you can use a dehumidifier.

We recommend turf free organic earth. It is less prone to mold and can be ordered over the internet so no dragging it home from a store is required.

Hand holding a plant