Genus of shrubs or trees cultivated for their large clusters of white, blue or pink flowers (9)
I believe the answer is:
hydrangea
(Other definitions for hydrangea that I've seen before include "Shrubby plant with showy flowers" , "Flowering plant - angry head (anag)" , "Showy flower" , "Ornamental flowering shrub - handy gear (anag)" , "Garden flower - garden hay (anag)" .)