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Martyn Davey, Head of Horticulture and Design, Easton College
Monday, July 9, 2012
Question: My hydrangea seem to have a mixture of bigger leaves and some other branches with smaller leaves at its ends. Is it due to the short period of drought earlier in the year? They do have flower buds on though. Should I prune them off to make room for the bigger ones? Some even lately appear with brownish tips. (J Davey, via email)
Hydrangeas will tolerate most soils and nearly all are very hardy. They will tolerate full sun or full shade but, as a general rule of thumb, some shade is particularly helpful especially for white hydrangeas and also for the clear blue varieties. A good garden loam is best for hydrangeas – neither too heavy nor too light.
Where you have a light sandy soil, improve it by adding peat and leaf mould and/or well-rotted manure. Very heavy clay soils can also be improved by adding sand and leaf mould. An alkaline pH of 6.5 is best for hydrangeas that are white or pink. For blue flowers you MUST have a much more acid soil and a pH of around 5 is ideal.
Fertiliser in spring is helpful but do not overdo this as you will encourage too much lush growth and too few flowers. Hydrangeas genuinely enjoy being cut back hard. If they get tall and straggly with increasingly small flowers cut them back to around 30cm above ground level in autumn or very early spring. Do not, however, expect much in the way of flowers immediately. When the flowers come they may well be rather larger than the previous ones but it does take time for them to rejuvenate. You will, however, end up with a much more vigorous plant.
People often get confused about when and how to prune hydrangeas. You should always prune out the old dead flowers – but do this is in March when the sap is only just starting to rise. For best results cut back the stem only to the first pair of good buds. This may mean removing one or two buds closer to the old flower. Where you have too many too-thick stems by all means prune some of them out completely. But avoid pruning too hard and removing the flower buds for the forthcoming season.