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How to use worm castings without treating them as compost

Updated: October 2026

The bag is in the hall, and the back of it does not match the page you read last night. One UK seller says 5 to 10 percent of the mix. Another says 10 percent. A third says 10 to 30 percent. This is how to use worm castings as a small share or a thin top-dress. The Products Compared editorial team wrote it from public UK guidance and the limits in our [worm humus comparison](/hub/diy-supplies/worm-humus). Listings were saved on 16 October 2026. A qualifying purchase can earn us a commission at no extra cost to you. We did not run a trial, so these percentages are other people's instructions. Keep neat castings out of the seed tray.

The bag is in the hall, and the back of it does not match the page you read last night. One UK seller says 5 to 10 percent of the mix. Another says 10 percent. A third says 10 to 30 percent.

Our top picks

McMillan McMillan worm humus, 10 litres

Best overall

McMillan McMillan worm humus, 10 litres
5.8/10about £26.95
View offer
Invigortex Invigortex castings bucket, 3 litres

Alternative

Invigortex Invigortex castings bucket, 3 litres
6.7/10about £10.99
View offer
B2C B2C worm castings, 1.5 litres

Rank 3

B2C B2C worm castings, 1.5 litres
6.9/10about £9.89
View offer
On this page
  1. 1.A share of a mix, not the mix
  2. 2.Top-dress in the growing season
  3. 3.Beds, seedlings and liquids
  4. 4.When the bag should not be used

A share of a mix, not the mix

Worm castings, or worm humus, are a dark soil amendment. The Royal Horticultural Society treats worm compost as a conditioner and as one part of a homemade mix, and it says a wormery does not replace ordinary composting. You still need peat-free potting compost for the bulk. Gardenware tells you to use about 5 to 10 percent by volume. Wormganix uses a 10 percent rule, and up to about 25 percent around vegetables if you add little and often. Willy Worms prints 10 to 30 percent for its own castings. If your bag says 10 percent, follow the bag. If it says nothing, stay around 5 to 10 percent. In a 10 litre bucket of peat-free compost, 10 percent is about 1 litre of castings. Stir it through. Do not layer a wet slab at the bottom of the pot.

Top-dress in the growing season

If you are not repotting, a top-dress is the smaller move. Gardenware suggests a layer about 0.5 to 1 cm deep, kept off the stem, or one to two tablespoons on a 12 cm pot. Water it in lightly, and do not bury the crown. A thick, wet cap is how you rot the base of the plant. Timing follows growth. Gardenware points at spring and mid-summer for plants that are actually growing. A British winter windowsill is often short of light. Adding castings in December does not repair that. A monthly handful is too often if the pot is already wet. A border will swallow a 1.5 litre bag without looking fed. Buy compost for the bulk. How castings compare with a compost bag is on worm castings versus compost.
McMillan McMillan worm humus, 10 litres

Testsieger

McMillan McMillan worm humus, 10 litres
5.8/10about £26.95
View offer

Beds, seedlings and liquids

Do not sow into pure castings. Wormganix says that outright, and the RHS treats worm compost as only part of a mix. Seed compost is the medium. A spoon of castings in a larger pot, after the seedlings have roots, is a different job from filling the module. Wormery liquid is not the bag of solids. The RHS says dilute the liquid that drains from a wormery at about 1:10 with water, and that the analysis varies. Use it on growing plants, not as a drench on dry seed compost. Wiggly Wigglers distinguishes that leachate from worm tea, which is castings steeped in water. A shop liquid with no volume is not that 1:10 product. A greenhouse tomato can take a modest share beside compost. A lawn or a new raised bed will not be rebuilt by one specialist bag.

When the bag should not be used

Open the bag before you commit a mix. Review texts in this set name plastic, glass, peat, slug or snail eggs and live worms. A live worm is a mild surprise. Glass, hard plastic or a mass of eggs is a stop. Do not sieve that into houseplant compost. Section 11 of the Consumer Rights Act 2015 says goods must match their description. Go back to the seller. A stated mesh, such as Willy Worms at 5 mm then 3 mm, is clearer than the word fine with no millimetre size. If you can see rubbish, the how-to ends. Castings are the share. Compost is the volume. If the listing cannot say how many litres you hold, do not invent a percentage of an unknown bag.

Common questions

How much do I mix into potting compost?
Use the rate on your bag. If it is silent, about 5 to 10 percent by volume is the Gardenware band, and about 10 percent is the Wormganix rule. In a 10 litre bucket of peat-free compost, 10 percent is about 1 litre of castings. Willy Worms allows 10 to 30 percent for its own castings. Do not average those into one official figure, and do not fill the pot with neat castings.
How do I top-dress without rotting the stem?
Keep a thin layer, about 0.5 to 1 cm, off the crown. On a 12 cm pot, Gardenware's one to two tablespoons is a practical amount. Water lightly so the layer settles, and do not pack a wet cap against the stem. If the compost is already sodden, wait until it has dried a little before you add anything that holds more water.
Can I sow seeds into pure worm castings?
No. Castings are an ingredient, not a seed compost. Wormganix says not to sow into them neat, and the RHS treats worm compost as part of a mix. Use a seed compost for the tray. Add a small share later, once the plants are in a larger pot and you can see roots.
How often should I top-dress in a UK season?
In the growing season, when the plant is making new growth. Spring and mid-summer are the Gardenware window. A monthly handful is Wormganix's houseplant rhythm, and it is too often for a pot that is wet and idle. Winter light on a British windowsill is the limit. Castings do not replace a grow light.
What do I do with wormery liquid?
Dilute it about 1:10 with water, which is the RHS instruction for liquid from a wormery, and remember the strength varies. Use it on plants that are growing. Do not pour it neat, and do not copy that dilution onto a shop liquid that prints no volume. Our Cinomania listing is in that second group.
Is worm tea the liquid from the bottom of the bin?
No. That runoff is leachate. Wiggly Wigglers treats worm tea as castings steeped in water, which is a different liquid. The RHS 1:10 figure is for wormery liquid. If you only bought a bag of solids, you do not automatically have either.

Related pages

Worm castings for houseplants

Worm humus

Best worm castings UK: the least compromised listings

Worm castings buying guide: litres, weight and pack

Worm castings vs compost: a small share, not the fill

Sources

Public UK sources for this comparison. amazon.co.uk listings were read on 16 October 2026.

  1. RHS, worm composting (retrieved 16 October 2026)
  2. RHS, homemade fertilisers (retrieved 16 October 2026)
  3. RHS, houseplant growing media (retrieved 16 October 2026)
  4. GOV.UK, waste exemption T26 (retrieved 16 October 2026)
  5. GOV.UK, compost from waste resource framework (retrieved 16 October 2026)
  6. PAS 100:2018 via NBS (retrieved 16 October 2026)
  7. Consumer Rights Act 2015, section 11 (retrieved 16 October 2026)
  8. RHS, statement on peat (retrieved 16 October 2026)
  9. Gardenware, how to apply worm castings (retrieved 16 October 2026)