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Best composts for containers

By Adele Dyer

You need a great compost to grow pots of bedding and bumper crops of veg. Our tests find the best composts for pots and containers.

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The best compost for containers will help you grow large, healthy vegetables and plants that are covered in flowers all through summer. 

The worst we've tested will cost a fortune and only deliver slow growth, pale leaves and tiny harvests. 

But the only way to tell the good from the bad is to grow plants in the compost and compare the plants - which is exactly what Which? Gardening has been doing for more than 30 years. 

We've pulled out our top composts for containers from our tests in the table below.  

Best peat-based compost for containers 

Only logged-in Which? members can view our recommendations in the table below. If you're not yet a member, you can get instant access by joining Which?.

Compost Our verdict Pelargoniums Potatoes Score

Compost non-member logo

Peat content: 80%

M ***** ***** 85%

Long flowering

This is often a Best Buy in our tests. It’s a high-peat blend with wood fibre and it comes with a packet of controlled-release fertiliser, which we mixed into our pots. This may seem unfair as other composts weren’t fed, but we test the composts as they’re sold. The results were comparable with composts to which we added controlled-release feed. 

The pelargoniums flowered well all summer, producing the highest number of flower stems. The potato harvest was one of the heaviest on test.

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Compost Our verdict Pelargoniums Potatoes Score

Compost non-member logo

Peat content: 90%

M **** **** 72%

Good potato harvest

This is a high-peat-content compost that also has a small amount of green compost, made from composted green waste. It also has a ‘starter’ fertiliser that lasts longer than many, promising to feed plants for around seven weeks rather than a four-week feed, which is more common. 

This mix allowed the pelargoniums to put on plenty of healthy growth early in the summer and to flower well. It also grew the largest crop of potatoes.

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Best peat-free compost for containers 

Only logged-in Which? members can view our recommendations in the table below. If you're not yet a member, you can get instant access by joining Which?.

Compost Our verdict Pelargoniums Potatoes Score

Compost non-member logo

Peat content: 0%

M *** ** 43%

Nutrient poor

This mainly ‘green compost’ is made from green-waste bin collections and this year, it was very lumpy with large chunks of poorly milled wood.

Within weeks of being planted, the pelargoniums’ leaves were pale with a red tinge. The plants hardly grew in the crucial first weeks and didn’t recover later on, remaining small and flowering poorly. The potato yield was also very low.

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Compost non-member logo

Peat content: Not stated

M ** *** 40%

Needs supplementing

This peat-based compost has controlled-release feed added to the mix.

It has scored poorly in the past and this year the pelargoniums were clearly struggling to grow a few weeks after planting. We supplemented with a liquid feed after four months, but they continued to be small with few flowers and the potato crop was also low.

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Why Which? compost reviews are better 

Which? is independent and doesn't accept advertising or freebies, so you can trust our reviews to give you the full, honest and impartial truth about a product.

Before testing we send out secret shoppers to buy all our compost based in four locations around the UK, so we know we are getting the same product as you.  We also buy two of each to see if there are any variation in the quality of each bag.

Here's a run through of how we test compost: 

  1. We grow ‘Palladian Pink’ pelargoniums, ordered as plug plants and 'Charlotte' potatoes.
  2. This year, we decided to alter our feeding regime. The potatoes were fed with controlled-release fertiliser unless the compost bag stated that it contained enough feed to last for the length of the trial. However, we decided to grow the pelargoniums without feed to see how well the compost performed on its own.
  3. The pelargoniums were assessed for flowering impact and vigour, which means leaf colour, the size and bushiness of the plant and general health. We carried out these assessments throughout the summer up until early October.
  4. The potatoes were harvested in September, when they were weighed and assessed for size and quality. In our evaluation, we only included potatoes that were of a sensible size to use in the kitchen; that is larger than 30mm in diameter.

Our scores are based on how well both trial plants did, weighting the results of both trials equally.

We test all types of compost - here's our round-ups of the best compost for raising young plants and the best compost for sowing seeds.

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