What is wrong with my hydrangea?

Guy Barter, Head of Horticultural Advice at the RHS, answers your gardening questions
Hydrangea
It is a good idea to spray hydrangeas with an oil-based insecticide to protect against scale insects
Question: My beautiful, 20-year-old hydrangea has got some sort of disease. Small white cocoon-type gel blobs are appearing on the stems and also under the leaves. The leaves then turn yellow and fall off. Some parts of the whole plant have also died.

Is this something that happens because I don't dead-head the flowers until after the February frost has finished? Please advise what is the best treatment as this is a lovely plant.

Answer: The white blobs sound like egg masses of scale insects, a tiny insect that sucks the sap from plants and although disfiguring, they seldom actually kills plants. The young are on the move now and vulnerable until they settle down and develop a scale before dying and leaving those white egg masses.

The remedy is to spray your hydrangea, covering every part of it, including below the leaves, with any insecticide recommended for ornamental plants. Sprays based on fatty acids or oils will kill the young scale insects but spare many helpful insects and are especially safe to pets and children.

We always suggest leaving flower heads until February - they look nice when frosted and protect young buds from winter cold. Prune out any dead bits now to smarten up your plants.

Send your questions for Guy Barter to: gardenproblems@standard.co.uk.

Only one question per fortnight can be answered. For further advice on handling problems in your garden, visit www.rhs.org.uk/advice/index.asp.

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