Before anyone could make any suggestions they would need to know where on the planet this garden is located. A pic or two of the area would also help.

tj

there are several flowers you can use for a Japanese style garden i recommend doing some research and looking at Japanese garden ideas on google. but while you do that i suggest these flowers the Japanese irises , Japanese tree peonies, Japanese maples, Azaleas, Ilex ,crenata. Japanese design is focused on the simplicity of things. They only add a little bit of color and look to add a main focal point another idea is to add bamboo it ads strength to the garden and gives it that Japanese feel. keep in mind that bamboo grows fairly fast and you want to keep the garden simple. Definitely, cover the ground with moss it gives it texture and adds that beautiful green that Japanese gardens have. Also, the green will make the purple flowers look more vibrant.
like the comment above if you have a pic that will be much easier to give suggestions but hopefully this helped if anything I'm from Miami where we have all year around garden designs and however i can help i will this is my site. hope all works out and let me know if you need any more suggestions landscaping Miami