Men: Your best denium jeans (long or shorts) and dark color long sleeve shirts.  
         Greens, browns, black, blues.
         Khaki pants (long or shorts) dark color shirts, greens, browns, or black.

Ladies: You many also choose shirts or dress in the same color scheme. Be sure that you can sit on the ground confortably. The dress (skirt) should go over the knees when sitting.

I do not recommend red or white color shirts (tops) for a garden portrait.