Outdoor Classroom – Rhododendrons: common problems and solutions

1) Silver leaves. By far the most common problem is thrips sucking the chlorophyll out of the leaves, turning them silver and weakening the plant. Turn the leaf over and you may find black, thread-like insects on the back. Replace heavily infested plants – some varieties are more susceptible than others. Open up around the plant for more air movement and light. You can use a systemic insecticide – spray November, early January and, for really bad cases, late February. Neem oil is recommended by some as an alternative to insecticides.

2) Leaves with dry brown patches and edges. This is usually a sign of stress. The plant may be too dry or too hot. Move it if necessary (sun for half the day or dappled light is best) and get a blanket of mulch over the roots. Some varieties prefer a much colder winter than we have and these tend to burn and crisp on the leaves. Replace them. Some plants get touched by mildew and lichen. Open up to allow more air movement.

3) No flower buds. This is usually a sign of too much shade. Move the plant or open up around it to allow more light.

4) Leggy, bare and stretched. Again, this is usually a sign of too much shade. Some varieties have a tendency to get rangy and open, others are naturally more compact and bushy. You can rejuvenate a leggy plant by cutting back very hard but it is really too late in the season now. It is best done in the middle of winter. You are more likely to kill the plant if you cut it back to bare wood now. This plant was cut back hard two months ago and has made its new growth already.

5) Plants which make only one new growth from each stem can be encouraged to make several growths by pinching out the single shoot. Do this as early as you can or you will be pinching out next year’s flower buds. In the right hand photo, you can see a plant making several new shoots instead of only one.

6) If not deadheaded, some rhododendrons set so much seed that it can weaken and even kill them. It can also reduce flowering the next season. This plant missed being deadheaded last year. Varieties that don’t set seed are generally deadheaded for aesthetic reasons, not because it is necessary.