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The problem with white potting soil

It's a problem that practically every amateur gardener has encountered at one time or another: Until recently, your potting soil had been well-tended and brown, but its surface is now covered in a white layer. The first thing to do is keep calm! That's because the white deposits are not necessarily mould, as many initially assume.

Harmless deposits

When watering plants, particularly if you are watering them through the saucer, salt and lime from the fertiliser and the water itself may gather on the surface of the potting soil. Over time, these residues may remain on the potting soil, even when watering plants "from above" in the normal way. The residues can be recognised by their dry and crumbly texture. Deposits like these occur when the water and the nutrients it contains have been absorbed by the plant, but salt and lime from the water are left behind and build up in the potting soil. To restore the potting soil to a beautiful brown, you can simply remove the first few centimetres of soil that contains the mineral deposits and refill the pot with fresh potting soil. Make sure to only fill the pot up to two centimetres from the lip, otherwise water and potting soil may slosh out of the pot when you water the plants. On the other hand, if the white layer is fuzzy and has a musty smell, you need to act fast. Not only does the mould look unsightly, in some circumstances, it can put your plants and, more importantly, your own health at risk.

Less is more

A certain quantity of mould can be found in almost any potting soil — and that's a good thing, because moulds, together with soil bacteria and insects, naturally break down organic material in the earth into fertiliser. However, if the mould spreads excessively to the point that "fur" is clearly visible on the surface of the potting soil, this is often the result of improper care of the plants. If you are a little too generous with your plants and overwater them, then the potting soil will be constantly damp, which encourages the growth of mould and mildew. 

If you have also used potting soil with a high compost content for your plants, overwatering will provide the perfect growing conditions for mould and even "higher fungi" (mushrooms). This is because the composting substances contained within such soils are an excellent breeding ground for moulds, which develop from spores that are present almost everywhere. In order to eliminate the mould from your affected soil, the best solution is to take the entire plant outside and repot it.

Cleaning thoroughly

When outside, take the plant out of its pot and remove as much of the affected soil as possible. Break off the old soil from the edge, or shake the root ball carefully until the roots are visible. If your plants do not have a specific root pruning schedule, you can also prune the roots at this point to stimulate growth. The rule of thumb when pruning roots and repotting is to reduce the root ball to approximately one third of its original size. 

Don't forget to clean the plant pot before putting the plant back in, as spores can stick to the pot too. The best way to ensure that the spores are not transferred after repotting is to rinse the plant pot thoroughly with hot water and a vinegar-based cleaner before cleaning it with the GARDENA hand-Held scrubbing brush. Finally, thoroughly rinse the pot again with cold, clean water. The plant can then be returned to the pot together with new, mould-free, high-quality potting soil. At this point, you can prevent any waterlogging from occurring later simply by covering the drainage hole of the plant pot with a "crock" and pouring on around three to five centimetres of gravel or coarse sand. This ensures any excess water can drain directly into the saucer after watering. Make sure not to dispose of the mouldy soil in your compost — it is best to dispose of it in your organic waste, where the mould spores won't be able to cause any more damage.

Preventing mould

You can protect the new potting soil against a new case of mould by letting the soil dry slightly each time you pour water onto it. Occasional loosening the top layer of soil can also help prevent mould. This ensures the potting soil and the root ball remain properly aerated. And don't water your plants too often — only water them when necessary, and even then, only with a moderate amount of water. This prevents your potting soil becoming a constantly wet breeding ground for mould spores. You should take account of the fact that plants require less water in the darker winter months between October and March than in summer months. Add a sufficient but appropriate amount of fertiliser to the water on a regular basis — this is often overlooked. Fallen leaves and blossom on the surface of the soil should be removed, because the resultant grey mould (Botrytis) may spread to the plant. To prevent mould at the planting stage for plants that grow in pots and tubs, you should use a highly mineral-rich potting soil such as a citrus and tub potting soil. This mineral-rich soil is less prone to developing mould. 

Watering via a saucer is great way to provide water when it is needed: Pour the water into the saucer, then pour the excess water away after half an hour. If the plant "soaks up" the initial amount very quickly, add more water and then pour the excess water away.

Photo: © Reinhard Tierfoto