a bit of research and i found the answer on this page
http://www.rambridge.com/info/tropf2/blumatqa.aspx
NEED HOW MANY UNITS?
Outdoors: 1 Tropf Blumat can water a circular area of 30 cm (12"). FOR FLOWER BOXES WITH A WIDTH UP TO 20 cm (8") AND A LENGTH OF 100 cm (40"); you need at least 3...