The nursery has an exclusive contract with the University of Arkansas in the US to test and commercialise the fruit, which is thought to be the world's first primocane-fruiting blackberry.
Petchell said: "Primocane blackberries will revolutionise this market sector. The improved selections we have seen already offer the possibility of dramatic season extension coupled with superb eating quality and potentially more efficient growing methods.
"We are close to releasing our first variety, but we already see much more improved material that will be fully tested before release in the medium term."
Hargreaves Plants held a presentation and field day at the end of last month to update growers, consultants, marketers and breeders on its primocane blackberry development.
Breeder Professor John Clark showed the visitors around the field to view the new selections.
Petchell said: "Standard floricane blackberries are available in short supply windows in a number of territories. The main varieties currently grown in the UK are Loch Ness and Chester."
He added: "This new material offers an improved eating experience over the industry-standard cultivars. Taking this higher-quality fruit and coupling it with season extension means that the UK market could grow significantly over the coming years.
"It is true to say that some of the standard UK varieties fail to deliver in the flavour stakes and as a result the industry is probably not getting the repeat fruit purchases that it should. We have seen a rapid expansion in blackberry fruit sales in the past five years, but this might only be the start of bigger things to come."
Hargreaves Plants has been collaborating with the University of Arkansas since 2005. Commercial director Rupert Hargreaves said: "We are privileged and proud to be working with what is considered to be the world's largest blackberry breeding programme.
"Professor Clark offers a real vision for improvements to blackberry genetics. The development of primocanes is a long-term project, but one that is already yielding excellent results."
He added: "In addition to the primocane material, Hargreaves Plants can also exclusively offer the best floricane cultivars from the same programme: Natchez, Ouachita and Navaho."
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