Lurchers are tearing through the gardens at RHS Hyde Hall and darting across surrounding Essex farmland in the hunt for hares.
The illegal chases have led to an obliterated wild flower meadow, smashed down hedges and destroyed young trees, according to the garden's estate team leader Richard Smith.
"We have blocked entrances, put up steel fencing and dug ditches to try and stop the dogs and vehicles. I've seen what happens to the hares and it's horrible," he said.
"The RHS is trying to encourage biodiversity and this destruction makes a bit of a mockery, but what we are doing is mostly for the welfare of animals."
Mounted police patrols were covering around 400ha of tenant farming land, much of it owned by the RHS, as well as the gardens themselves, which are around 140ha. The Hunting Act outlaws organising or watching hare coursing and police can seize lurchers and confiscate and crush vehicles used by the dog owners.
Richard Bowe, manager of nearby Ilgars Farm, said hares were not pests but an attractive part of the countryside.