Found a list of mystery books with a Garden theme that I had made some time ago after scrolling through the web. Most I haven't read, but they might be worth a look.
*Winter Garden Mystery by Carola Dunn
*The Garden Club Mystery by Graham Gordon Landrum
*The Hanging Garden by John Sherwood (have read most of his books and recommend them).
*Three Dirty Women and the Garden of Death by Julie Wray Herman
*Garden View by May Freeman
*Deadly Garden Tour by Kathleen Gregory Klein
*Murder in the Garden by Veronica Heley
*Samurai's Garden by Gail Tsukiyama