The addition of the centre, located near Chester, makes the Guernsey-based Blue Diamond group the third biggest in the country, with a turnover of £68m.
Blue Diamond MD Alan Roper said:
"The negotiations have been going on for 18 months and there were three garden centre groups in the running. It's a centre with a £6m turnover and it's catchment has the percentage percentage of higher demographic we like to trade to."
Blue Diamond is now the third largest garden centre chain, behind Garden Centre Group and Dobbies.
Roper said he can increase Grosvenor's annual turnover from £6m to £8m and up profits to at least £1.2m.
Blue Diamond now has centres at Chatsworth, Wilton and Grosvenor estates, leased from two dukes and a earl.
The leasehold for Grosvenor is 35 years. Acting for Grosvenor were Gilbert Evans and Brabners Chaffe Street.