Landlord First Franchise director Ian Silverton said: "He was on a rolling 28-day contract and his contract was terminated because the garden centre needed the space.
"He was offered alternative space at Harleston Heath, but turned that offer down. We have offered assistance to find other centres."
Champney said: "They've offered me somewhere else but I think it would be too far away from where my customers are now."
Plans that were submitted to Northampton Borough Council by Waitrose owner John Lewis in 2011 showed a concessions building belonging to the centre being demolished, with the supermarket being built to the rear of the garden centre.