Tumbleweed

Description for TUMBELWEED by Janwillem Van De Wetering:

Though the Secret Service had expressed mild interest in sultry Maria van Buren because of her eminent "boyfriends," no one expected to find her as detectives Grijpstra and de Gier did: felled by a knife in her back. The Belgian diplomat, American colonel, and Dutch businessman who had regularly visited the woman were the obvious suspects. But why kill a woman they had enjoyed--and shared--for years? Only when they discovered the secret of Maria's plants--weeds so powerful only a sorceress would use them--could the two detectives solve the chilling mystery behind her murder.

This is #2 in A Grijpstra and de Gier Mystery.