Mould on the soil around your seedlings? Don't panic – it's actually more harmless than you'd think

One of the most common grow-your-own queries I see is this: why is there mould on the soil around my seedlings?

You can sow plenty of brassicas indoors in February, and a range of other vegetable and flower seeds, too. Starting them off early in the year has a load of benefits – stronger plants and earlier harvests and blooms, to name a few – but indoor growing also carries a greater risk of mould development.

Don’t panic, though – mould on the soil around seedlings is actually really common, and it usually isn’t a problem unless it starts forming on the seedling itself. It’s happened to lots of my seedlings in the past, so I set out to learn exactly what causes it. Here’s why mould forms on the soil around seedlings – and what you can do about it.

Why is there mould on the soil around my seedlings?

Mould on the soil around your seedlings can look scary, but it’s actually far more harmless than it seems. The white layer of fuzz is especially common indoors, where seed trays and pots are often kept in the warm – and because there are so many fruits, vegetables and flowers you can sow in February, it’s more noticeable at this time of the year.

‘Light surface mould is usually not directly harmful as it mainly feeds on organic matter in the soil, not the plant,’ says Lance Russell, brand sales lead at Thompson & Morgan.

High humidity, heat and overwatering are the most common causes, and combined, they provide the ideal conditions for mould to form. Some types of compost are more prone to mould growth than others, though.

‘Some soils can have a high amount of organic matter, which can create the ideal conditions for mould growth,’ says Richard Barker, commercial director at LBS Horticulture. ‘You cannot prevent mould forming on soil because of this, and there can sometimes be mould already present in potting soil when you open a bag.'

According to Richard, it’s actually very normal in premium quality organic soil – especially in peat-free composts. It won’t affect the quality of the soil, and actually, it can sometimes give your seedlings a boost.

‘The mould feeds on the organic matter present in the soil, and this can actually sometimes make soil nutrients more available to seedlings,’ Richard explains.

What to do if there is mould on the soil around your seedlings

So, you don’t have to discard your seedlings, or even assume that they’re doomed for the rest of the growing season, if you spot mould on the surface of the soil. It can be an eyesore, though – and there are a few things you can do to get rid of it or make it happen less in the first place.

You don’t need to throw everything out,' says Lance. 'Just try to provide the correct environment for your plants.'

‘Start by removing the worst patches, gently scraping them off, and then sprinkle a thin layer of fresh compost on top. Let the surface dry slightly and water less often. Even better, water from below if you can.’

Bottom watering is just as important for outdoor container plants. Soil moisture meters like these from Amazon can also take the guesswork out of watering plants.

It’s also important to provide plenty of ventilation if you want to keep mould growth at bay. A heated propagator is brilliant for boosting germination rates (Garland Super7 Electric Heated Windowsill Propagator, available at Amazon, is a top-rated model), but you’ll usually need to remove the lid once the seedlings emerge.

‘Improve the airflow for your seedlings by cracking the window or using a small nearby fan, making sure that you remove any covers once the seeds have sprouted, too,’ Lance advises.

It’s also best to let the compost dry out between waterings, because soggy compost can lead to other seedling problems, like damping off, too. If your seedlings become leggy, that could be down to other issues, like low light levels.

Indoor sowing essentials

So, as long as the mould is only covering the surface of the soil, and not the seedlings themselves, it’s generally completely safe. Proper ventilation, careful watering and heat control can all keep mould at bay, though.