Maui IS a garden! I recommend www.aliikulalavendar.com

Here's a list of ten of the most popular :

http://www.hawaii.edu/sciref/mauigrdns.html

I enjoy Kahanu Garden the most because its in the wet Hana side of the island. Its looks more like a tropical garden that you would expect to see on a place like Maui. Some of the other gardens are located in either high elevation areas or on the dry side of the island.

The Ke'anae Arboretum is also worth visiting.

Here's the link

Here is a link that might be useful: Maui gardens