I have never been to this place but I've heard good things. It's on my to do list.

http://www.winghavengardens.com/

Also:

http://gardens.uncc.edu/

Unfortunately Charlotte is kind of a garden/nursery wasteland compared to the Raleigh area. Stowe is about as good as it gets around here but the garden at UNC Charlotte is worth checking out too.

Not offended at all..Charlotte has become that 'garden/nursery wasteland' because land is so expensive here. Pocket gardens are widespread but generally within neighborhoods on private properties.
I've never seen the UNC Charlotte bot.garden at this
time of year. Winghaven is a pleasant experience under
ancient trees close to Uptown but primarily perennial until
the ladies get in there with the annuals when it gets warmer.

UNC Charlotte gardens are top of my list :) You've got several gardens outdoors, plus the greenhouses (a bonus if it is raining). Something to see at any time of the year, and May is well into prime garden season.

How far are you willing to travel from Charlotte? The NC Zoo is in Asheboro, apx 2 hours south of Charlotte. They have nice plants also, nothing like the bot gardens but still interesting.

Heavens, two hours south of Charlotte gets you most of the way to the SC beaches and you can visit the Columbia zoo and bot. gardens on the way.

The southern Charlotte area and the county of Mecklenburg abuts the South Carolina line.
From my recollection it took 90 min. from Charlotte
to Columbia , (capitol city of SC) and they have lovely gardens.
Asheboro is north east of Charlotte.

Thanks Dottie for catching my wrong directions!

Thanks to all of you for your recommendations. We are looking forward to visiting NC in a couple of weeks. -Mike

Well, mikey..you missed the azaleas and the dogwoods we have in abundance. Azaleas were very fine and long blooming this spring thanks to an oddly long spring.

You didn't say when in May for your visit but if it's soon, you'll want to bring along hats. The loopers (inchworms etc) are raining their poop on us from up in the trees.
Another week or so they'll be a non-issue.
The roses are forming buds now (inchworms love them) and the garden centers are full of customers eager to plant
before it begins to get hot. Hope you have a lovely trip.
.