Managing director Mark Taylor said the land would house container beds for outdoor production as well as some glass. He said a planning application would be submitted "shortly".
Taylor added that some 4ha will be in production, to include some new glass, with pot bedding and pot perennials for 2014. This will be to supply larger customers including Verde, which supplies Waitrose, Dobbies, and the Garden Centre Group.
Taylor said plants would range from 9cm to 7.5 litre in size.
Over the past five years it has added an extra 6ha at a site near the main nursery near Hereford.
Allensmore grows 5m plants per year.