Which? reveals the best compost for raising young plantsGrow healthy seedlings and young plants

17 December 2015

Which? compost trials have found five of the best composts for sowing seeds or potting on seedlings and young plug plants. Unfortunately we also found several that had serious problems with the amount of fertiliser included in the compost. 

We tested 24 widely available brands and found huge variations between the best and worst composts. It is impossible to tell which compost will grow plants well by looking at the packet so reading our test results is the only way to find one that will grow great plants. Our highest scoring Best Buy compost, scored over 90% in both tests, and a Don't Buy compost for sowing seeds scored just 33%. 

Paying a premium for your compost is also no guarantee success. One of our Best Buys costs just 8p per litre, compared to other composts which cost up to 32p per litre.

Best Buy composts for all tasks 

We found five Best Buy composts. Two were Best Buys for both sowing seeds and raising young plants, while one peat-free compost was only a mark off being a Best Buy overall. Draining and clearing peat bogs causes great environmental harm, so we were very pleased to see a peat-free compost that could be relied on to grow healthy plants. 

We also found one compost that was a Best Buy for raising young plants and one for sowing seeds. So you can always find the best compost for your growing jobs, whether you prefer to grow from seed or to buy plug plants. 

To find out which compost will give your young plants the best start in life, look at our full compost test results. 

Fertiliser problems

Our Best Buy composts grew a great crop of strong seedlings, the Don't Buy compost for sowing seeds produced a tiny number of yellowing shoots. Several other composts were as almost as bad, especially when growing antirrhinum seeds, which are sensitive to imbalances of nutrients in fertilisers included in the composts. 

We sent the worst composts to a specialist lab for analysis and discovered composts with so much fertiliser that seedlings would be damaged.  Others had so little that young plants would not have enough feed to grow and some composts had huge quantities of one nutrient and almost none of another. 

Which? Gardening compost expert, Adele Dyer said: 'We were staggered at just how imbalanced some of the fertilisers are. When we saw the results, we could see just why some seeds struggled to germinate and the young plants shrivelled up or put on no growth at all. The problems seemed to have come from composts and fertilisers not being properly mixed - not a problem we would expect to see.' 

Same name, different composts

We ask compost makers what they use to make their composts and were disappointed to see that B&Q's own brand composts are still using different composts to fill bags of various sizes. Verve Seed and Cutting Compost has 75% peat in the 12L version, but just 40% in the 50L size. Verve Peat Free Multipurpose Compost is even more complicated: the 20L and 50L bags have the same compost, but the 12L and 125L bags each have their own mix of bulk ingredients. 

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