Rhododendron flower buds are initiated during late summer and early autumn. A short period of very dry conditions at the roots at this time may cause a check in this process. The plants may subsequently fail to produce flower buds, or buds may only partially form, eventually drying up and dropping unopened, or being easily detached.
Prevent this by keeping plants well mulched, watering thoroughly and regularly during dry periods, particularly from July onwards.
Greyish-black flower buds
The disease rhododendron bud blast (Pycnostyanus azaleae) is believed to be spread by the rhododendron leafhopper (Graphocephala fennahi) which is a sap-feeding pest that causes little direct damage. It lays overwintering eggs in slits cut in the flower buds, providing a means of entry for the fungus. Affected buds do not rot and can remain on the bush for two to three years.
Control by picking off and destroying affected buds, which can be recognised by the fringe of black fungal spores produced along the edges of the bud scales. Adult insects are found on the foliage from mid-July to October. Spraying with a suitable insecticide such as bifenthrin in early August and repeating as necessary, may be beneficial.
Recently planted rhododendrons
Young plants are usually in bud or flower when planted but may then take two to three years to establish before resuming flowering. Potash encourages flower production so check that the soil is not deficient in this fertiliser.
Further information
The RHS Rhododendron, Camellia and Magnolia Group
Founded in 1945, this group is for all RHS
members who share an interest in rhododendrons, camellias,
magnolias and associated plants.
Further
information