Furious when flower head is knocked off by branches (2,2,4)
I believe the answer is:
up in arms
'furious' is the definition.
(thesaurus)
'flower head is knocked off by branches' is the wordplay.
'flower' becomes 'lupin' (I've seen this before).
'head is knocked off' means to remove the first letter.
'by branches' becomes 'arms' (arm can mean a branch. I am not sure about the 'by' bit.).
'lupin' with its first letter taken off is 'upin'.
'upin'+'arms'='UP IN ARMS'
'when' is the link.
(Other definitions for up in arms that I've seen before include "Weapons expert should be" , "Protesting vociferously" , "being angry" , "Rioting" , "held like a small child" .)