Well what do you know - it's not! I suppose I should have clicked the link before replying.

This is the one I read about:

Waltham's Lyman Estate Greenhouses will host its Springtime Perennial Sale from Thursday, May 29, through Sunday, May 31. Choose from hundreds of blooms, from the familiar to the exotic, at this event. Gardening experts will be on hand to help you select the perfect plant for your home. Free. www.historicnewengland.org