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Clay granules for houseplants

Updated: October 2026

A peace lily in a cachepot, a radiator on, and fungus gnats after a supermarket plant: that is the UK indoor problem, not a garden bed. Clay granules for houseplants help only after you name the water cycle. A 2–3 cm mineral cap is not a full switch. Leftover compost in the root ball can still breed gnats. The indoor bags are on the [Seramis granules comparison](/hub/fertiliser-soil-and-bark-mulch/seramis-granules). Closed-pot steps sit on [how to use Seramis granules](/hub/fertiliser-soil-and-bark-mulch/seramis-granules/sub/seramis-granules-how-to-use).

A peace lily in a cachepot, a radiator on, and fungus gnats after a supermarket plant: that is the UK indoor problem, not a garden bed. Clay granules for houseplants help only after you name the water cycle.

Our top picks

Westland Westland Hydroleca 10 L

Best overall

Westland Westland Hydroleca 10 L
9.2/10about £13.55
View offer
Invigortex Invigortex LECA grit 1 L

Alternative

Invigortex Invigortex LECA grit 1 L
8.4/10about £7.49
View offer
UK GROW UK GROW houseplant mix 9 L

Alternative

UK GROW UK GROW houseplant mix 9 L
7.0/10about £10.79
View offer
On this page
  1. 1.What goes wrong in UK indoor pots
  2. 2.Cap, mix or full mineral
  3. 3.Which houseplant bags sit in this comparison
  4. 4.Heating, humidity trays and dust

What goes wrong in UK indoor pots

UK flats often put the plant in a cachepot with no hole. Heating dries the compost surface while the base stays wet, and fungus gnats follow. Compost dries in days. A closed mineral fill does not. The same weekly drink leaves one pot wet and the other swinging from dry to soggy. A mineral cap makes wet compost harder for adults to reach. It does not kill larvae in the ball. One UK note is a dry surface and at least 0.5 cm of expanded clay, with water from below. Shop toppers are often 2–3 cm. A Phalaenopsis belongs on the orchid row, not in this refill.

Cap, mix or full mineral

A cap is 2–3 cm of Hydroleca or LECA on compost you already have. Water from below and leave the top dry. Orchid toppers in the Hydroleca reviews said tiny flies dropped. That is a review theme, not a guarantee. A mix is about one part Hydroleca to three of compost, or Invigortex at 20–50 per cent. Organic mix remains, so gnats can still breed. The 1 litre reviews used that blend on winter-overwatered plants, including a pink philodendron. A full switch keeps the soil ball, uses an indicator, and gives about a quarter of the pot volume. Leftover compost can still hold larvae. Stripping the ball also strips the feed. A mix that names Seramis is still compost.
Westland Westland Hydroleca 10 L

Testsieger

Westland Westland Hydroleca 10 L
9.2/10about £13.55
View offer

Which houseplant bags sit in this comparison

UK GROW houseplant mix is 9 L, 20 L and 40 L, about £10.79, £14.99 and £23.99. Bullets name Seramis and perlite and say feed for about six weeks. The 20 L is about 75p a litre. All three share 3.5 from 51, and the reviews include unmarked sacks. Skip the line for a labelled sack. The 40 L is 16 kg. Hydroleca 10 L is about £13.55, 4.6 from 569, for a top, a crock, a mix or a humidity tray. Invigortex 1 L is about £7.49 for a trial cap. There is no branded Seramis indoor-granule listing here. Skip rules for every pack are on best Seramis granules.

Heating, humidity trays and dust

A tray of damp Hydroleca under the pot is a listed humidity job, as is a 2 cm moist layer on a greenhouse bench. Do not stand the pot in the tray. That is a pebble job, not a closed Seramis fill. Pour in a ventilated room and do not inhale the dust. Ask a shop if roots are already black and soft, if a leak would matter on a rented floor, or if someone should not handle dry clay. Eaten granules are a GP or vet question.

Common questions

Will a top layer stop fungus gnats?
No. A dry mineral cap makes wet compost harder for adult gnats to reach. One UK note talks about at least 0.5 cm. UK shops often sell 2–3 cm. Larvae in the remaining soil ball can still emerge. Water from below and let the surface dry. A cap is not a full mineral switch.
Should I strip all soil when I switch to granules?
Not as a default. The Seramis method keeps most of the soil ball. Stripping every scrap removes feed and shocks roots. Do it only if the ball is already rotting, and have a liquid-feed plan before you pot on. Leftover soil can still hold gnat larvae.
How do I water a plant in a pot with no hole?
If you are running a Seramis-type closed fill, pour about one quarter of the pot volume and wait for a fully red indicator. If you only dropped a plastic pot into a cachepot, lift it out, water until it just appears at the holes, and empty the saucer. Do not leave the cachepot as a hidden reservoir.
Is the UK GROW houseplant mix enough on its own?
It is a compost that names Seramis and perlite, not a granule system. The bullets say feed after about six weeks. The review file is 3.5 from 51 shared across 9 L, 20 L and 40 L, with unmarked sacks in the reviews. Enough for ordinary indoor pots if you accept that file. Not enough if you wanted reusable clay.
Can Hydroleca raise humidity in a heated room?
A tray of moist Hydroleca under the pot is a listed use, and a 2 cm moist layer is the greenhouse-bench instruction. That raises local humidity. It does not water the plant from below unless you stand the pot in the tray, which you should not treat as a reservoir.
Which bag for one peace lily in a cachepot?
If you want a closed mineral fill, you need open-pore clay and the how-to method. That branded bag is not a listing here. If you want a cap or a crock, Hydroleca 10 L is the indoor pebble bag. If you want compost that names Seramis, the 9 L UK GROW mix is the small houseplant size, with the shared 3.5 from 51 in mind.
When is a nursery better than a DIY switch?
When the plant is expensive and the roots are already black and soft, when a leak would matter on a rented floor, or when someone in the home should not handle dry clay dust. A shop can pot for you. A bag of pebbles will not save a rotting specimen.

Related pages

Seramis granules or LECA

Seramis granules

Best clay granules for houseplants UK: which bag fits

Seramis granules buying guide: type, litres and peat

How to use Seramis granules: pot, ratio, first water

Sources

Public UK and maker sources from the research digest, used for this page in October 2026. Prices, stars and review counts are from the ten amazon.co.uk listings in the stored pages. A gnat, watering or CO2 line is the manufacturer's claim. Peat-free means those words on that listing. This is not a laboratory test.

  1. Seramis, granules and hydroponics (retrieved October 2026)
  2. Seramis, common questions (retrieved October 2026)
  3. Seramis 15 L granules on amazon.co.uk (retrieved October 2026)
  4. Westland Hydroleca clay granules (retrieved October 2026)
  5. Westland orchid potting mix (retrieved October 2026)
  6. RHS, growing media for houseplants (retrieved October 2026)
  7. RHS, peat-free compost (retrieved October 2026)
  8. Written answer on peat sales, 19 March 2026 (retrieved October 2026)