The trial report stated that while many subjects in a peat-free mix grew slowly and had reduced root vigour, they had reduced liverwort growth, needing less controlled-release fertiliser and less firming in at the potting stage.
The trial compared a peat-free medium, consisting of 33 per cent green compost, 67 per cent conifer bark and a slow-release nitrogen fertiliser, with a standard peat-based medium supplied by Scotts.
It was conducted at Darby Nursery Stock in Thetford and set up by Rainbow of Rainbow Wilson Associates, with assessments by Abi Rayment of Dove Associates.
Ten genera were used in the trial, chosen from a range of protected and outdoor crops (Clematis, Cytisus, Dianthus, Euonymus, Lavandula, Potentilla, Pyracantha, Salvia, Sidalcea and Vinca).
The report found the following:
- Vigour was lower in the green medium in seven out of 10 crops, especially Cytisus, Lavandula and Clematis, but the herbaceous crops and Pyracantha performed well;
- Liverwort was much more evident on the standard peat medium, especially on Clematis;
- The naturally occurring nutrients in green compost are slow-release, so controlled-release fertiliser rates could be reduced by around 20-25 per cent - saving 1p/3-litre pot.
The report concluded that the exposed conditions at Darby Nursery Stock on capillary sand beds could have contributed to the concentration of nutrients in the pots and therefore the retarding of growth of salt-sensitive subjects.
Rainbow said: "The suppression of liverwort growth, over which there is no legal chemical control for nursery stock growers, could be a huge benefit as it could save the 10p per pot that needs to be spent removing it by hand.
"The handling of the peat-free mix through the potting machine was also very successful."
- A summary of the report is available at www.wrap.org.uk/downloads/ Fact_sheet_-_Darby_Nursery_Stock.1fff47e4.5839.pdf.