Why is there a white crust on my pots and compost?

Every time you water a pot, some of the dissolved minerals stay behind when the water evaporates. Over a year or two in a container that is never repotted, the concentration can reach a level that pulls water back out of the roots rather than letting it in — the same mechanism as fertiliser scorch, arrived at slowly.

Step by step

1. Check that it is not alive.Rub some between your fingers. A crust is gritty or powdery and does not smear or thread. White fluffy growth that pulls apart in strands is a harmless saprophytic mould on the compost, which is a different thing entirely.
2. Work out which deposit you have.A hard white or buff scale on the outside of a terracotta pot, and often a tidemark round the rim, is largely limescale from hard water moving out through the porous clay. Cosmetic. A softer white or yellowish crust on the compost surface itself, sometimes with a crystalline look, is fertiliser salts, and that one matters.
3. Look for the symptoms that show it is affecting the plant.Brown crisp leaf tips and margins, worst on the oldest leaves; wilting even though the compost is wet; growth that has stopped; and roots that are brown at the tips. Those together with a crust mean the concentration is too high.
4. Scrape off the top layer.Remove the top two to three centimetres of compost, including the crust, and replace it with fresh. Most of the accumulation is concentrated there and in the pot rim.
5. Flush the pot properly.Outdoors or in a sink, water the pot thoroughly from the top with plain water and let it run right out of the base. Repeat three or four times over an hour. That leaches the excess salts out, and it is genuinely effective. Let it drain completely afterwards and do not leave it standing in the run-off.
6. Change the water if you can.Rainwater is free of both the lime and the accumulated minerals of tap water, and in a hard water area it makes a real difference in containers, particularly for acid-loving plants such as camellias, blueberries and azaleas.
7. Reduce the feeding.Over-feeding is far more common than under-feeding in pots. Halve the dose, feed less often, feed only in active growth, and never feed a dry plant or a dormant one.
8. Repot rather than flush if it is severe.For a heavy build-up, slide the plant out, shake off as much compost as comes away, rinse the roots gently and repot into fresh compost. Do not feed for at least a month afterwards.
9. Clean the pot itself.Scrub a terracotta pot with a stiff brush and water, or soak it in a bucket with a little vinegar to shift heavy limescale, then rinse thoroughly and let it dry before reusing. Do this before repotting rather than after.

Tips

• Flushing a pot thoroughly is also the quickest diagnostic test for salt build-up: if the leaf tips stop browning on new growth afterwards, that was the cause.
• Terracotta shows the deposit and plastic hides it. A plastic pot with the same history has just as much salt in the compost.
• Self-watering planters and pots that are always topped up from a reservoir accumulate salts faster, because water leaves by evaporation rather than draining through.
• Some plants are notably sensitive to salts and to fluoride in tap water — spider plants, dracaena, calathea and prayer plants brown at the tips first.

Common mistakes

• Increasing feed because the plant looks poor — If salts are the cause you are adding to the problem, and the symptoms of over-feeding and under-feeding overlap. Flush first, then judge.
• Leaving the pot standing in the water it drained into — The salts you just flushed out get drawn straight back up. Let it drain away completely.
• Assuming a crust on terracotta means the plant is suffering — The deposit on the outside of a clay pot is usually just evaporation and lime, and the plant may be perfectly fine. Judge by the leaves, not the pot.

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