The horticulture industry says it is being undercut by prisons that grow plants using cheap inmate labour.
The industry says some prisons are running large-scale planting schemes, which have driven some growers out of business.
The Nursery and Garden Industry Assocation has about 400 members including nurseries operated by prisons.
Inmates are reportedly paid between 20 cents and 60 cents an hour for their garden work. Private-sector workers earn up $20 an hour.
The association says its members can't compete and the situation is unfair.
But the Department of Corrections denies it is cornering the native plant growing market, disadvantaging commercial nurseries.
Spokesperson Ruth Turner says it has many overheads and charges only market prices. Ms Turner says Corrections has to pay for custodial supervision, training costs and a high turnover of prisoners.
The Nursery and Garden Industry Association intends to meet Corrections Department officials next month in Hamilton.