The Carbon Gold products are in 90 garden centres but are now being promoted to professional growers.
Some 19 growers trialled the Soil Association-approved, peat-free products. They were found to help boost root growth, reduce the need to water and improve germination rates.
Carbon Gold executive chairman Craig Sams will be launching the results at the Soil Association's Soil Symposium on 15 November.
Delfland Nurseries co-owner John Overvoorde trialled GroChar Seed Compost on young lettuce and brassicas. "I use Klasmann modular compost and biochar did just as well. I also use Klasmann peat-free and there's not much in it."
He said he would contract grow a "reasonably-sized" order in biochar if asked and expects his first customer by early 2013.
Great Dixter head gardener Fergus Garrett said: "Excellent peat substitute, better than green waste. Would not hesitate to use as part of a John Innes mix."
Rijk Zwaan field trials manager Martin Kyte said: "I started trialling Carbon Gold's GroChar Composts against a peat-based compost but the results were so good I stopped the trials and progressed solely with GroChar."