We are exploring Tuscany in April and recently read about Venzano's Gardens. What is not clear is whether it is open for visitors. Has anyone experienced this unique spot?

Nope, but do you mean Il Vivaio Giardino di Venzano? It's a nursery and presumably open to anyone (pretending to be) interested in buying plants.
http://www.telegraph.co.uk/gardening...e-of-fete.html

Owned by the same person is Giardini di Venzano, which seems to be relatively upscale accommodation. It may or may not be(probably is) the same as Il Vivaio. http://venzano.net/id1.html
There's an email form.

I attempted to visit the Venzano gdn on 24 June only to find it all very closed,so left a note to see if one could visit later. I was emailed by the Caretaker/gardner advising it was sold 3 yrs ago and is now private !