I have two wonderful Monkey Trees about 16 feet tall. They are spaced at about 20'. I have 8' weed free circles surrounding them, and I am looking for suggestions for a good ground cover to place at the base of the trees. I have always kept the bases free of weeds, but it is a chore I've grown tired of. I really like the crawling vine look, but don't want to harm the trees with being strangled by invasive vines. Any ideas welcome.