Mould in the soil of plants, learn how to identify it

You love your plants, you look after them with care and attention. Everything is going perfectly well until one day you notice that there is a white fuzz on the substrate of one of your plants. It seems small, almost insignificant, and 24 hours later you notice that your plant is completely covered with the same whitish substance. Before you run for a bottle of bleach, vinegar and a brush, today we'll explain in detail all about mould in plant soil. Learn how to identify it, understand how and why it grows, whether it's dangerous to your plants and what to do about it.

what is mould?

"Mould is a very generic term that we apply to different types of fungi that live on organic matter made up of various microscopic organisms. They grow in places with high humidity and thrive on different surfaces. They contribute to the decomposition of dead matter and recycle nutrients. The most common types of moulds in the environment are Cladiosporum, Penicillium and Aspergillus. They reproduce through spores, which travel through the air, water and even on insects. When they reach a place with the right conditions, they begin to multiply.

As you can see, not all moulds are the same. There are some that cause breathing problems and allergies in sensitive people, but others can have positive effects on your plants. The important thing is to learn how to identify them so that you know what to do.

is mould dangerous for plants?

Humans instinctively tend to shy away from moulds, because our experience with them is linked to negative consequences: allergies, respiratory problems and surface damage are some of their effects. But when it comes to plants, the result can be completely different. There are fungi that are beneficial for plant growth.

Moulds can be of the saprophytic type, i.e. they consume decaying organic matter. As a result of this process, the mould breaks down this material into simpler minerals that are of great value to the soil. When composting, you are essentially inviting mould and saprophytic bacteria to take over the task of devouring that waste. For example, Trichoderma sp species contribute to plant growth, while Penicillium or Aspergillus are responsible for breaking down the organic matter present in the substrate, thereby releasing various mineral nutrients that can be absorbed by plants.

In places such as forests, the presence of this mould is important, as it decomposes organic matter, as well as recycling minerals to keep the forest in perfect balance, even because it can counteract the action and growth of harmful organisms. But it is not all rosy, because mould can also become a problem.

In some cases, mould on the substrate looks quite unsightly and can produce unpleasant odours, while in some extreme cases it can produce yeasts. The effects are not overly detrimental, but it can be an indicator that you should pay attention to certain conditions of your plants.

why does mould grow in the substrate?

Houseplants are the most common victims of this little invader. But for this to happen, certain conditions must be met for it to appear, because although it is a quite natural phenomenon, it is not visible on all indoor plants. This is precisely why we mentioned that mould, rather than a problem, can act as an alarm, alerting you to certain situations that could be harmful to your plants. These can be:

- The plant is not getting enough ventilation

Mould spores grow happily and multiply in anaerobic conditions, i.e. where there is not much air present. Your plants may be too close together and this does not allow air to circulate, so the fungi do their thing.

- The substrate is not well drained or you are watering too much

Fungal spores travel through the air, they float everywhere and the reason we don't live with mould is because they need a particular condition to thrive: moisture. When a substrate is poorly drained, excess water is not removed quickly, so it will remain damp for longer. The same is true if you water too often. If that's the problem, mould is the least of your worries, because plant roots will stop breathing, become waterlogged and rot.

- Not enough sunlight

If you don't allow sunlight to reach your plants (directly or indirectly, as appropriate for their species) you will not only be limiting the amount of nutrients they can synthesise to feed themselves through photosynthesis, but you will create a dark, damp environment in which mould will grow fast and happy.

- You use organic fertilisers

If you add organic fertilisers to the substrate just before planting or immediately after planting, you increase the risk of seeing white mould in the soil of your plants. This is not a bad thing, quite the contrary. Remember that this organic matter will feed the mould, which will break it down into beneficial minerals for the plants.

- There are dead leaves on the surface

Mould and other fungi feed on decaying organic matter, so if you allow dead leaves to accumulate on top of the substrate, then you are leaving the perfect food for these microorganisms.

what does mould look like in the substrate?

Every plant lover has seen small white spots on the substrate. Many rush to diagnose a mould infection, and then apply any fungicide to the runs, which could affect the plant. These white spots may just be lime or salt deposits that form naturally on the substrate. This is typical of places where the water is very hard, and may even form a hard film on the soil of the plants.

The difference is simple: check how the white spots are distributed.

In the case of mildew, they are created in very tight clusters, but as they spread, they clump together and can cover the leaves of plants. Limescale spots are less well defined and delimited and do not grow as rapidly. Mildew can also be identified by touch, because it is a wet layer, whereas mineral deposits are dry. On the other hand, there are other types of saprophytic moulds that present themselves differently.

Trichoderma, which is a blessing for your plants because it inhibits the growth of other pathogenic fungi (such as those that cause root rot), looks like a well-defined white spot on the substrate, which also looks cottony. It is distributed randomly, without any pattern. It is as if white paint had rained on the substrate and the drops remain.

Peziza ostracoderma is a saprophytic fungus that looks like a yellowish powder. You will not see it on the surface, but it lodges inside the substrate. It is completely harmless, so if you ever see a cottony yellow spot inside the substrate or a powder, don't worry.

How to get rid of mould in the substrate

- Aerate the substrate

White mould is a rather beneficial presence for plants, but for many it is unsightly and if it grows too much it can generate bad smells, so it is normal that you want to get rid of this invader. Depending on how advanced it is, there are simple solutions. If just a few white spots appear, you can mix in soil to aerate the compressed material. This interrupts the fungal growth cycle. You can also expose the plant to fresh air and loosen the overly compacted substrate to dry the substrate and dissipate odours. Remember to space the pots a little apart, so that air can circulate between them.

- Do a thorough cleaning

If there is too much mould and it has spread to the plant and pot, it is time for a deep clean. Remove the plant, root ball and all, from the pot. Shake it to remove as much of the substrate adhering to the roots as possible. Check the roots for other fungus or damage, such as rot. Smell is an excellent telltale sign, so if you find softened, foul-smelling roots or signs of fungal infection, cut out the affected part with a disinfected scissors and proceed with transplanting.

- Change the substrate and pot

Use a new substrate and a new pot. If the pot is recycled, I recommend you wash it very well, even with a little bleach to eliminate any possible source of infection, not only mould but also bacteria or any other pathogen that might be present. The substrate to be used should be well stored, in a low light and dry place, in a bag or container with drainage that does not support bacterial or fungal build-up.

- Improve pot drainage

Even if you are meeting watering and humidity requirements, the substrate may still be excessively wet due to poor drainage. This can be corrected in two ways: the first is to mix the substrate with perlite or a little sand to improve the texture. Another way is to correct the drainage holes, making sure they are not clogged and even adding a layer of pebbles or small pieces of polystyrene at the bottom. If you have a saucer to prevent irrigation water from reaching the soil, empty it a few minutes after watering, so that the soaked water does not puddle the roots.

- Use organic fertiliser in the correct proportions

If you use too much compost or organic fertiliser, even if you just leave it on the surface without mixing it well with the substrate, it is very likely to become mouldy because it is the perfect environment. Correct this by using the right ratio, which is usually 1 part fertiliser to 10 parts substrate. Mix well, the idea is to feed the beneficial bacteria in the substrate to create minerals that are usable by the plants, but don't overdo it as this can attract mould.

- Remove the mould and you're done

Yes, it's that easy to get rid of mould in plant soil. All you need to do is wet a paper towel and use it to collect all those mould particles or spots in the soil. I have used a small shovel to remove the top of the substrate. You can either throw it away or compost it. If the mould has reached the leaves of your plant, wipe them off with a damp paper towel and you're done. As mentioned, mould is not harmful, but it can look bad.

Homemade recipes against mildew

Some people don't want to see mould on their plants, either because they find it unsightly or because they suffer from severe allergies to it. You can use a commercial fungicide, using it in the proportions indicated in the instructions. Some people opt for homemade options, which can be effective. It is important to be aware that you should not use vinegar directly on the roots, because the mould may tolerate it, but your plant will not. Here are some safe homemade fungicide recipes:

- Homemade garlic fungicide. Boil 2 litres of water, add 1 clove of garlic. Boil for a few minutes, turn off and wait for it to cool. Apply with a spray in the mornings on the leaves and the underside of the substrate.

- Milk-based fungicide: mix 2 parts whole milk with 8 parts water and 2 tablespoons baking soda. Mix until dissolved and use a spray bottle to apply to plants once a week.

- Cinnamon powder: Believe it or not, the solution to mildew is in your spice cupboard. Sprinkle some cinnamon powder directly onto the substrate. Not only will this smell delicious, but it will prevent the fungus from growing.