Which? reveals the best composts for containersFor superb pots and hanging baskets this summer
18 March 2016
Peat-free composts rarely outperform peat-based composts, but this year in the Which? compost trials a peat-free compost is the only Best Buy compost for containers, beating both peat-based and peat-free composts.
It's the second year running that this compost is a Best Buy for containers, as well as being a Best Buy for raising young plants and one of the best composts for sowing seeds in this year's tests. The great performance of this peat-free compost is a major step forward as harvesting peat can cause enormous environmental harm. All composts sold to home gardeners should be peat-free by 2020, according to a voluntary code that all compost producers have signed up to, so they have a vested interest in producing peat-free composts that gardeners are happy with.
However, we were disappointed to see that not all peat-free composts can be relied on to grow good plants. Our highest-scoring Best Buy compost scored 46% more than our lowest-scoring compost, which is also peat-free but contained too little fertiliser.
We tested 24 widely available composts by growing begonias and potatoes over four months last summer. We were staggered by the differences between them, with masses of flowers and big potato crops in good performers and sickly, weak plants in poor performers. There is no way of knowing which compost will be a winner from looking at them - you need our expert testing to know which are the stars and which are the duds.
Perfecting peat-free compost
There are many materials that can be used for peat-free compost.
- Composted bark
- Wood fibre
- Composted green waste made from council collections
Of these, green waste has a bad reputation as a variable and unreliable material as it is made from grass clippings in summer that rot down too quickly and woody prunings in winter, which make the compost lumpy. As a result some manufacturers have now removed green waste completely from their composts. Others are changing sources to include materials that do not change through the season, such as by-products from farming and brewing. For more on this, see our guide on how to the choose the right compost.
We found many of our lowest-scoring composts for containers stated they contained enough fertiliser for four months, the length of our test, but all would need to be topped up with liquid feed if the plants started to look pale. We followed the instructions, but still found these produced the smallest plants and very few potatoes. We had a very wet summer in 2015 when we carried out the testing, and so our liquid feed would have quickly washed through the pot.
Most composts have enough fertiliser to see them through the first six weeks, and so to feed them for the rest of the season we add a six-month controlled-release fertiliser at planting time. We find this gives plants a steady boost through the season and is easier to use than a liquid feed.
See our results to find the best controlled-release fertilisers to feed your pots.
Best Buy compost for all tasks
We found two composts this year that we could recommend for containers, sowing seeds and raising your plants such as plug plants or seedlings. One is a peat-free compost and the other is made largely from peat. To discover the ideal composts to start off your plants, look at our Best Buy composts for raising young plants results.