Bonsai T-shirts, very good idea. Why not www.bonsainut.com as a front heart logo? ...more colours, lighter for a hot sunny days and darker for working on trees... Or maybe just one colour if the idea is to be easily recognizable. You can incorporate some tree, leaf or tool sketches to design. ABT t-shirts are designed very well for example.
http://www.americanbonsai.com/Bonsai-Gear-s/1514.htm
http://www.americanbonsai.com/Bonsai-Gear-s/1514.htm