The RHS, East Malling Research and Roger Ward of Golden Grove Nursery, Lincolnshire, have concluded a two-year HDC project that found cypress aphid (Cinara cupressivora) is partially responsible for browning in conifer hedges. Trimming hedges in autumn is another factor causing dieback, along with honey fungus, soil conditions and dense planting.
Recommendations for growers and retailers include advising gardeners to be vigilant in spring when cypress aphid populations peak and to use a suitable aphicide if the pest is detected. But nicotinoids used in aphicides could be outlawed in October when the EU votes on pesticide regulations.
Ward said: "I'm pleased we have shed new light on conifer browning and found that aphids are the main causal agent.
"We are now able to say to customers and gardeners that the best treatment would be to apply pesticides between March and April."