The business is holding a public celebration for the 75th anniversary on 22 July.
Meanwhile, the national collection of shrubby potentillas at Webbs has been given a new lease of life.
The collection of more than 600 plants from 200 different varieties were the first plants to go into the Riverside Gardens at Webbs in 1998.
Over the past two years, head gardener Mike Rutter has been repropagating the collection and they have now been moved to new borders in the visitors' picnic area.
The original potentilla border area at the entrance will become a shrubarium.