Composted green waste lacking phosphate has caused the abandonment of a tomato trial at RHS Wisley.
RHS trials development manager Mark Heath said two RHS tomato trials failed this year because of issues with growing media. Rosemoor trials failed because gardeners used manure contaminated with herbicide on border soil. Both will rerun in 2013, possibly without using peat-free compost at all.
Wisley tomato trials failed because of phosphate deficiency in bagged consumer compost used for growing 30 varieties sent in by breeders and suppliers.
Heath refused to name the supplier of the commercially available peat-free compost. He said tomatoes at Wisley had gone blue and had poor leaf growth.
Newly appointed Heath, who used to work for Sakata in Holland, said: "We will be wary of using the product again. Everyone is saying peat-free composts aren't very good for tomatoes. There's a huge debate concerning peat, but it's only in the UK, which is farcical." He added: "This was a peat-free compost for tomatoes that we put our faith in, but it hasn't come off."
Suttons horticultural manager Tom Sharples said this is the second year in succession that RHS tomato trials have failed because of problems caused by peat-frees.
Sharples added that last year Wisley, Harlow Carr and Rosemoor trials failed: "The trouble is the RHS has the policy to use peat-free wherever possible." Tozer and Thompson & Morgan were also among those supplying tomatoes to the trial.
Steve Harper, managing director at green waste compost maker Vital Earth, which is not accused of supplying the compost, said: "All PAS100-certified composters have to go through a large number of hoops to ensure that their product is fit for purpose, including growth trials."
He suggested waiting for results from the Sustainable Growing Media Task Force, "which among other things is investigating which substrates are sustainable - peat will be judged fairly against all other substrates - and looking for methodology to ensure that all growing media sold in the retail and professional markets do what they are supposed to do."
Clopyralid pollution - Chemicals Regulation Directorate view
The Chemicals Regulation Directorate (CRD) said of clopyralid pollution: "We do not hold a specific register or record of these sorts of cases.
"We are aware of the potential problem but not of any specific evidence, other than perhaps anecdotal evidence, for example from information received from organisations such as the RHS."
WRAP has been looking into the potential for compost contamination as part of Defra's policy to reduce peat usage.
The CRD meets twice a year with the RHS and other groups where queries from consumers that could be attributable to herbicide contamination of compost have been mentioned.