A great compost can help your plants to thrive, but a poor one will leave them struggling to survive.
Which? Gardening magazine has tested composts for sowing seeds and growing on seedlings and plug plants to find you ones you can rely on. We found some fantastic compost, but also some that were very poor.
Go straight to our compost results to find the Best Buys
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Buy the best compost
Which? Gardening tests composts every year and we’re always amazed at how much they vary in quality. The good news is we found some excellent composts this year, most of them peat-free and some are low price. We tested composts from Miracle-Gro, Westland, Levington, Homebase, B&Q and Wilko.
We have four new Best Buy composts for sowing seeds, and two of them are peat-free, including the highest-scoring product. There’s one peat-free and one peat-based Best Buy compost for young plants. In total four of the new Best Buys are own-brands from the big DIY stores, so you should have no difficulty in picking up a bag.
Sadly we also found two composts that were Don’t Buy composts for young plants. Neither contained any feed, so our plants struggled to grow. Neither product suggested feeding plants on their packaging, a major omission.
We concentrated on testing peat-free composts as the government has announced it intends to ban peat composts by the end of this parliament. Which? Gardening has used peat-free composts for all its testing since 2019 and we think that it can be just as good as peat, but without causing environmental damage.
For more on the issues around peat, see our guide to peat-free composts.
Which? Gardening compost testing
Which? Gardening buys all its compost for this test in garden centres and DIY stores in four areas of the UK. This is so we can find any inconsistencies in the quality of the compost. We sow veg and flower seeds, and grow in veg and flower seedlings, just as you would do at home. Unlike home trials, we can compare 25 composts for seeds, sowing a total of 15,000 seeds and for the compost for raising young plants test we grow 1000 pots of seedlings. Where we find poor composts we carry out chemical analysis to determine where the problem lies.
Learn more about how we test compost.
Which? Gardening will publish the results of our latest grow bags trial in March 2022 and the results of the composts for containers test will be ready in April. We trial a huge range of plants, veg and gardening accessories, as well as giving great advice on how to grow and what to do in your garden through the year.
How to sow seeds
Seeds are tiny parcels of new plants, just waiting to grow. They already hold a microscopic root and shoot and they also contain enough food for these to start growing before they need to start taking up nutrients from the surrounding soil or compost.
As the new roots are easily damaged, the compost needs to be fine and crumbly enough for the root to push through, while also holding the air, water and nutrients the plant needs to grow. Too much fertiliser will damage the roots, and three of our Best Buy composts for sowing seeds are specialist seed composts, formulated to give just the right conditions for seeds to thrive.
Always follow the advice on the packet for when to sow your seeds, and how deeply to plant them in the compost. Fill your pots and gently firm the compost before sowing the seeds. Give them a light water – just enough to wet the compost – and regularly check the compost isn’t drying out.
Prick out your seedlings and pot them into a Best Buy compost for young plants when they have two true leaves. These look different to the seed leaves that are the first one to open, and look more like the leaves of the adult plant.
Discover the best places to buy seeds and plants online
How to grow on seedlings and plug plants
Seedlings and plug plants are ready to get growing quickly and so will need a compost that provides all the nutrients they need for the first few weeks, has an open structure plant roots can easily push into, and holds enough water and air. Our Best Buy composts for young plants provide these ideal conditions.
As your plants grow, you’ll need to water them regularly. Check them every day, and water if the pots are getting dry. After around four weeks you may notice growth start to slow down. At this point your plants need some fertiliser.
Look at our Best Buy liquid feeds to find one that’s right for your plants.
