They are the eighth member of the group which includes Aline Fairweather, Fromefield Nursery, New Place Nursery, Seiont Nursery, The Northern Liner Company, West End Nursery and Wyeplants.
The ALP has developed to cover the buying, marketing and training needs for all members.
The new chairman of the association, New Place Nursery's Steve Lee, said: "This is an essential time for liner producers to capitalise on the strength of the UK market. We produce some of the highest quality nursery stock in the world here in the UK and an association like the ALP enables the members to ensure that quality and reliability are maintained and improved."
Kernock Park Plants business development manager Mark Taylor said it made "perfect sense" for them to become members.
He said: "It is an honour that we have been accepted into the association. We started production of the 9cm liner range in August 2008 so it has been a steep learning curve for us. We very much look forward to cooperating with the other members in the future."
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