I have left a note expressing my delight at the garden and leave my number should they wish to get in touch. Sometimes one will respond, many times I am ignored. No harm, no foul.

Personally I am tickled pink when someone wants a tour of my garden and I think most gardeners would be so I'd leave a note in the person's mailbox saying how much you liked their garden and ask if you could stop sometime to have a tour adding your name and phone number to the piece of paper.I think for the most part gardeners are some of the friendliest people there are so I would be surprised if you didn't get a good response.
Janice

Thank you! I will try that. :-)

It's important that you be as forthcoming and sincere as possible so that these folks won't think you are stalking them or scoping their propery out for a robbery or something. But a friendly note is likely to bring a friendly response. Janice is right!

If you get asked over, take some homemade muffins or cookies to thank them. Or a plant from your own garden.

This subject has come up on other forums, and the consensus seems to be that a note [in the mailbox] is the best way to go both for sender and receiver. However, if you see an obvious gardener (dirty gloves, trowel or pruners in hip pocket, wheelbarrow or hose nearby, etc) working outside, then it depends on your level of shyness -- most folks seem to think it's okay to approach, especially if you have lots of admiration for what's been done :)

U.S. mailboxes are intended for U.S. mail only, and once I left a check for someone in their mailbox, and the mailman picked it up and it traveled to the dead letter office in San Francisco before making it back to me a long time later.

If you really want people to get your note, don't leave it in the mailbox!

Susan - you are right about the USMail boxes and I certainly know that -- In our neighborhood it is common practice to put communications behind the flag [tucked between flag and box], not IN the mailbox and that is what I was thinking of, and our unfortunate habit of calling this action "in the mailbox" carried over to my fingers. So let me clarify and remind all that if you put a note in a mailbox - it had better have a postage stamp on it!

Well, I did it- I stopped by today, and as I was writing the note, the gardener came outside. We had a nice talk and I left my phone number. I will get her something for a gift if she calls me. That's a great idea! I felt so adventurous.