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4 February 2021

Best composts for raising young plants

Before planting your seedlings and small plug plants, look at our round-up to find out which compost will give them the best chance of growing big and healthy
Adele Dyer

Young plants, such as seedlings and plug plants, need a compost with enough fertiliser to let them grow well, but not so much that it burns their roots. It also needs to retain enough water, while not drowning young plants in soggy compost. 

However, as there is no way to know how well a compost will work by simply reading the ingredients or paying through the roof, we decided to test them for you. 

We selected 15 different composts and grew tomatoes and antirrhinums over a six-week period. The best produced large, flowering plants, while the worst made tiny, pale-leaved tomato plants. 

Three composts scored well enough to be a Best Buy this year. Find out the results in the table below.  

Best Buy composts for raising young plants

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Compost Our verdict Young tomato plants Young antirrhinum plants Score
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Great peat-free compost

Peat content: 0%

Price: £7.99 for 50L

Price per litre: 16p

It’s heartening to see a peat-free compost that grows better plants than peat-based composts. The basic mix of composted bark and wood fibre is free-draining yet retains enough moisture for young roots, and the base fertiliser has been carefully adjusted to suit the compost mix. The tomato plants were robust and the antirrhinums flowered prolifically.

Where to buy: Garden centres.

Compost Our verdict Young tomato plants Young antirrhinum plants Score
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Member content
Highest peat content

Peat content: 80%

Price: £14.99 for 70L

Price per litre: 21p

This mix of peat and wood fibre comes with a sachet of controlled-release feed as there is no base fertiliser added to the compost. We incorporated the feed into the compost before we filled our pots so the plants were in no danger of running short of nutrients through our six-week trial. The antirrhinums were good plants with plenty of flowers, while the tomatoes were the largest in the trial.

Where to buy: Online.

Compost Our verdict Young tomato plants Young antirrhinum plants Score
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Member content
Large plants

Peat content: 45%

Price: £5.99 for 50L

Price per litre: 12p

Westland doesn’t share details of its formulations, but we know this contains 45% peat and BIO3+, and doesn’t have any green waste. ‘John Innes’ usually denotes some added sand. A base fertiliser has been added that Westland claims will feed plants for five weeks. The tomatoes were both tall and broad, while the antirrhinums were covered in flowers.

Where to buy:

Not found the right product for you? Browse all our compost reviews

Things to look for when buying compost for raising young plants 

Here are two useful features to look for when buying your compost:

  • Good structure and aeration - it encourages strong root development. 
  • A medium-crumbly texture - fine compost can be prone to water logging 

For more advice on choosing the best compost for you, head to our compost buying guide

Getting the best from compost

It’s best to buy only the amount of compost you need, and use it up as soon as possible. This is particularly true of peat, which ideally should be used sparingly as it's bad for the environment. If you need to store any compost for more than a month, put it in the shed where it will be cool and dry, or undercover, especially through wet weather. If you can’t use up peat-free compost within three months of buying it, use it as a mulch on your borders.

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 so we know we are getting the same product as you. 

We then grow cabbage 'Caraflex' seedlings from the two true-leaf stage, and Antirrhinum 'White Appeal' in 25 different composts over six weeks. 

After six weeks, we judged both the young plants on how vigorous they were, which includes looking at size, leaf colour, and whether the plants are stocky and strong or long and straggly. We also noted how well the antirrhinums were flowering. 

To find out more, head to how we test compost. Alternatively, browse all our compost reviews

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