This is one of a new series of posts based on real questions I have been asked. Feel free to add to the answers or include your ideas and experiences.
One of the most charming of our native wildflowers is the wild honeysuckle (Lonicera periclymenum). It scrambles over hedges everywhere here including those that bound my garden and for many months in summer I enjoy their delicate beauty and delightful evening perfume. It is no wonder that gardeners have been enjoying them in their own plots for centuries. It is hardy and there are several cultivars in addition to similar species and hybrids that are popular in gardens. They are a popular choice for pergolas, arbours and trellis.
The problem with them arises when people forget the nature of the plant. People see photos and want a climber to clothe a piece of 2m square trellis, clothed with leaves and flowers from bottom to top. Unfortunately this is not how honeysuckles grow and it is not possible, except with very skillful pruning and training, to have flowers anywhere but at the top and with an ugly, bare lower half.
Think of how the plant grows in the wild. It twines up other shrubs to reach the light, where its flowers open, so that pollinators can get to them. It does not have enough strength in its stems to remain upright, and it hitches a lift with other plants, though it is not in any way a parasite. In some cases, however, its foliage may smother and weaken its host and the twining stems may be restrictive. But you can see that the lower part of the honeysuckle will be bare of leaves and all the flowers will be at the top. The flowering stems will be in the sun but the lower parts will be in shade and constantly moist.
And that is where the problems start because, all too often, honeysuckles are planted to climb up trellis against a sunny wall, often planted in a narrow bed that is baked dry in summer. And that leads to a common question:
‘I treated aphids on my honeysuckle with diluted washing-up liquid. However two weeks later the sprayed leaves have gone brown. The others are okay. What do I do now?’
This question brings up several issues. Firstly, the aphids. When honeysuckles are stressed by hot, dry conditions they are very prone to mildew and to attack by aphids, most commonly grey, mealy aphids. These breed and feed so rapidly that they completely cover the developing flowers and suck the buds dry which then fail to develop. If the infestation is light then ladybirds usually mop up the worst of them. But the problem can be largely avoided by not planting a honeysuckle in a hot, dry place. This will suit a passionflower or a ceanothus but not a honeysuckle. So, avoid the problem in the first place by choosing the right plant in the right place.
And then we come to the vexed issue of washing up liquid. Washing up liquid is designed to clean dishes and it is not an insecticide. I find it rather strange that people try to use all sorts of strange products to kill pests and diseases when there are products that have been tested for efficacy and safety. It is a fact that the range of chemicals available to gardeners is far fewer than in the last but there are chemical and non-chemical products to help you keep your plants free of pests, if you wish to use them. Washing up liquid is not tested for use as an insecticide and, apart from possibly being ineffective, it may harm your plants. If it does have an effect it is probably because the detergent destroys the waxy coating of the aphids, along with the larvae of ladybirds and hoverflies, but it may also destroy the waxy coating of the leaves, causing them to desiccate. In this case, where the foliage has been damaged by the washing up liquid, the plant will probably recover.
And then there is the legal issue: in the EU (and presumably still in the UK) it is illegal to use a substance as an insecticide unless it has been tested and approved. So don’t go spraying your roses with bleach, shampoo or oven cleaner either.