I hate to do a post without a photo but will have to make an exception today because I can’t find a photo of this problem. But I am sure we have all seen it even if you don’t have it yourself: a neatly clipped conifer hedge, that glows with health and is as smooth as a billiard table suddenly develops random brown patches. These spread each year and eventually large patches develop.
I am often asked what causes this and what can be done about it. The culprit is easy to identify but, unfortunately the solution is not easy. It is caused by the sap-sucking habits of the cypress aphid (Cinara cupressivora) – vora means ‘to eat or devour‘ so the name is very appropriate. It affects Cupressus macrocarpa (which is very popular as golden-foliaged hedges) and the much-maligned ‘leylandii’ as well as Lawson’s cypress and Thuja occidentalis (Northern White Cedar).
The aphid sucks the sap and causes the green branches to turn brown. In some cases the aphids drop lots of honeydew and this leads to black sooty mould on the lower foliage. The problem is that, these conifers do not respond well to hard pruning, cutting back into leafless stems. Practically, this means that these dead areas are going to stay dead. If you cut them out you can sometimes weave in surrounding green branches but this is not always possible.
What makes this pest especially cruel is that it hates a windy hedge and likes cosy, closely clipped hedges. So those people that plant huge conifers and let them block out everyone’s light never have to deal with this problem. While those gardeners who neatly clip their hedge every year and manicure it carefully are the most likely to suffer from a hedge with large brown patches.
You can, if you feel inclined, spray the hedge with an insecticide, in late spring, when you spot the problem, but because birds love hedges for nesting, and they should, or might, feed on the aphids, I don’t like the idea of this. The aphids are present all year, and most active from late spring to late autumn so perhaps spraying in mid spring or into autumn might be safer. If using organic or contact sprays you will have to spray with a coarse spray to get the insecticide deep into the hedge.
In theory, you could hang peanut feeders near the hedge to try to tempt the tits to feed on the aphids but I don’t really think this is going to be very effective.
In some cases the only answer is to replace the hedge – a big job but often anything else is putting off the inevitable.
To make up for the lack of photos, here are some of the garden this morning. We had the coldest night of the winter so far. It was -1c by 9pm last night and was -4.5c by dawn. It lingered well beyond midday in the shade. But at least it was dry.
The anemones are trying to bloom but won’t today.