The private equity firm owns brands such as Pizza Hut, Bernard Matthews and Maplin.
Chairman Jonathan Halford said: "This deal represents the best possible future for Gardman."
He added: "Over the last two years, Gardman has been through an essential but challenging period of change. Thanks to the dedication, commitment and loyalty of our employees, agents and customers, we have emerged a far stronger business culminating in today's announcement."
Halford said the investment marked the beginning of an "exciting new chapter in Gardman's future" which together with Rutland's experience in delivering business transformation "provides us with the platform to deliver our ambitious future growth plans."
This includes Gardman's focus on its recently launched brands strategy, further improvements in the product range and an "investment in operations to deliver higher service levels to customers".Gardman will launch some 300 new products at September's retail show Glee in home care, tools, core gardening and new areas.
Harper said Gardman has seen double digit growth this year.