The research from Gardening Which? tested 12 peat-based and four peat-free composts on how they performed in sowing seeds and growing young plants.
Gardening Which? editor Ceri Thomas said manufacturers would have to up their game in order to meet government targets. She said: "We are still astounded by the variable quality of peat-free products."
New Horizon Organic & Peat Free Growbag was the only compost deemed to be a "best buy" for growing-on young plants.
None of the peat-free composts tested for sowing seeds were awarded "best buy" status due to variable results in quality between bags of the same compost bought from different parts of the UK.
But the results on the peat-free composts have been criticised as being misleading. Growing Media Association chair Catherine Dawson said many of the products gave good results, despite not being used as recommended by the manufacturers on the label.
She said: "Peat-free products typically require feeding sooner than peat-based media. If left unfed, as was the case in this trial, it is not surprising that some of the results were not as good. Even so, five of the top-performing composts had less than 51 per cent peat."
- See special report, p37.