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How to use Compo potting compost once the bag is home

Updated: October 2026

The bag is already in the kitchen, and the plant is tipped on last year's compost. How to use Compo potting compost comes down to a pot that can drain, a loose fill, a feed clock taken from the sack in your hand, and a closed bag for whatever is left. This is the sequence for the Compo bags in our [Compo potting compost comparison](/hub/fertiliser-soil-and-bark-mulch/compo-potting-compost), not a star table and not a lecture on peat law. Peter F. followed Royal Horticultural Society guidance on repotting, containers and fungus gnats, plus what these listings print. This is not a personal trial.

The bag is already in the kitchen, and the plant is tipped on last year's compost.

Our top picks

Compo Compo Sana Confort universal 12 L

Best overall

Compo Compo Sana Confort universal 12 L
5.4/10about £16.92
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Compo Compo Sana potting soil 5 L

Rank 2

Compo Compo Sana potting soil 5 L
7.7/10about £20.29
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Compo Compo Sana green plants and palms 5 L

Alternative

Compo Compo Sana green plants and palms 5 L
5.8/10about £20.59
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On this page
  1. 1.Repot so water can leave
  2. 2.White specks and odd pieces
  3. 3.When the feed line runs out
  4. 4.Gnats and the open bag

Repot so water can leave

Repot when the plant is in growth, usually in spring. The Royal Horticultural Society says the new pot should be no more than about one third larger. A jump to a much larger cachepot leaves a moat of wet compost. One UK reviewer of a 5 litre Sana bag reported mould and poor drainage. The hole was the missing part. The pot needs a drainage hole. Loosen the compost, fill so the crown sits where it did before, and water until it runs out of the hole. If you are still choosing the sack, the checks are on the buying guide.

White specks and odd pieces

White granules are often perlite, expanded volcanic glass. Gardeners' World describes it as lightweight and there to open the structure. It is not polystyrene. COMPO's international Sana pages mention perlite. These amazon.co.uk bullets do not, so do not claim every sack here is a perlite mix. UK reviews of the 5 litre Sana mention a stick as well as white specks. Bark and wood are not mould. Do not bake the compost in a household oven. That is not a Compo instruction.
Compo Compo Sana Confort universal 12 L

Testsieger

Compo Compo Sana Confort universal 12 L
5.4/10about £16.92
View offer

When the feed line runs out

Follow the weeks printed on your bag, and do not borrow the number from a different Compo listing. The 5 litre and 10 litre Sana bullets say up to 6 weeks. The green plants and palms bullet says 8 weeks. The 12 litre bullets and the 7.5 litre peat-free bullets state no weeks. If your sack says 6 weeks, start a normal liquid feed after that, at the rate on the bottle. A page from another country that promises 12 weeks does not rewrite your amazon.co.uk bullet. The indoor plant match is on Compo potting compost for houseplants.

Gnats and the open bag

Fungus gnats are small flies, adults about 3 to 4 mm, whose larvae live in damp organic potting media. UK reviews of the 5 litre Sana mention flies and larvae, and they also mention bags with no bugs, so a single anecdote is not a batch failure. Let the top centimetre dry, and keep the rest of the bag closed and cool. If the tape fails, fold the plastic and use a clip. Stop when a collection is still dying after one careful repot, or when dust from the bag makes you ill. Peat wording is on the peat-free page.

Common questions

What are the white bits?
They are often perlite, a white volcanic glass used to open the mix and hold a little water. They are not packing foam. COMPO's international pages mention perlite. These amazon.co.uk bullets do not, and Spanish comments about perlite in the 12 litre bag are not a UK specification. A woody stick in a UK 5 litre review is debris, not mould. Fuzzy growth on compost that never dries is a wet-pot problem.
When do I start feeding if the bag said 6 or 8 weeks?
After the window on your bag, not after a number from a different listing. Six weeks is the 5 litre and 10 litre Sana bullet. Eight weeks is the foliage bullet. Then use a normal container feed at the rate on that bottle. Do not empty another bag of compost into the pot just because the weeks have passed. If growth is weak in the first fortnight, check the saucer before you add more food.
The 12 litre listing has no weeks. What then?
Treat it as a container compost with no printed head start. The UK bullets do not state a number. Feed once the plant is growing, on the schedule you already use for pots, rather than waiting for a figure the page never gave. A Spanish review that mentions 12 weeks on a page and 6 weeks on a bag is unresolved here. Read the sack you received, and follow that if it prints a number.
I have gnats after repotting. Did the bag fail?
Not by itself. Fungus gnats breed in damp compost, especially around houseplants. UK reviews of the 5 litre Sana include both flies and "no bugs". Dry the surface if the plant can cope, empty the saucer, and keep the rest of the bag closed and cool. If seedlings are in the same room, they are the more vulnerable crop. The Royal Horticultural Society's advice is less water and fresh mix at the next repot, not a brand recall.
Where do I keep the open bag?
Fold it, clip it, and keep it cool and out of a warm room. An open sack by a radiator stays damp and invites gnats. Use it up on the next pot rather than storing it for a year. If the closure peels, the Spanish notes on the 12 litre bag already describe that, so a clip is the practical fix. Gloves keep the dust down when you next open it.

Related pages

Compo potting compost comparison

Compo potting compost for houseplants

Compo potting compost buying guide

Compo peat-free compost

Best Compo potting compost for the job in front of you

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 five amazon.co.uk listings captured on 19 October 2026. A UK peat ban is not in force. German and Spanish review samples are not British customer experience. Bio on the 7.5 litre title is not a Soil Association number in the saved listing.

  1. Defra, sale of horticultural peat to be banned (retrieved 19 October 2026)
  2. RHS, retail peat-free from 2026 (retrieved 19 October 2026)
  3. Baroness Hayman letter, 29 January 2026 (retrieved 19 October 2026)
  4. Horticultural Peat (Prohibition of Sale) Bill (retrieved 19 October 2026)
  5. RHS, fungus gnats (retrieved 19 October 2026)
  6. RHS, growing plants in containers (retrieved 19 October 2026)
  7. COMPO SANA universal potting soil (retrieved 19 October 2026)
  8. COMPO BIO universal potting soil (retrieved 19 October 2026)