It must get pretty hot on summer afternoons there, right?

Verbena Homestead Purple (perennial here) will do well in afternoon sun, as will lantana (sometimes perennial). Both will get big enough to cover up your rose's bare legs. No care required - just plant it and get out of the way :)

I like geranium Rozanne under just about anything - it has blue-purple blooms all summer for me. Very easy care. Not sure if it could take the blazing afternoon heat though.

Under the Japanese maple, a low carpet of sedum would be nice. Try Ogon - a brilliant yellow/green for great contrast with the maple's fall color.

For the mini gardenia, plant something really low in front, like mazus reptans or blue star creeper. Or, try something taller behind the gardenia. I like salvia - it seems to go with everything. They are all easy-care, take your pick.