"Throughout August, children under 16 get in free to all four RHS gardens when accompanied by an adult and a calendar of family events," said the RHS.
Gardens include Wisley in Surrey, Harlow Carr in Yorkshire, Hyde Hall in Essex and Rosemoor in Devon, and events include plants and wildlife learning.
Harlow Carr is due to host a forest party on 1 August with music and dance, and Hyde Hall will host bug hunts on three days.
Jim Gardiner, RHS director of horticulture, said: "In the long summer holidays parents can have a tough time trying to keep the kids entertained without breaking the bank.
"This is why we have again decided to make entry free for children throughout August. We hope families will enjoy repeat visits be encouraged to involve their children in gardening back home."
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