Take a look at some of the common problems you might encounter on wisteria.

Question: After many years of healthy growth, the shoots of my wisteria are suddenly wilting and dying. What has gone wrong?

There are a number of possible causes for this symptom.

• Wisteria can suffer from root diseases such as honey fungus and Phytophthora root rot.
• Waterlogging can also cause root decay, in both soil- and container-grown plants
• Roots of container plants in particular can suffer damage from vine weevil grubs.

All of these problems prevent adequate water uptake through the roots, leading to wilting and die-back.

• Wisteria also appears prone to graft failure, sometimes after many years of satisfactory growth. When this happens the graft union (usually close to soil level) often decays, although this is a secondary symptom. A clue to graft failure is the production of new shoots from below ground level, while the top part has died back – these are being produced by the rootstock.
• If the dead branches have numerous pinhead-sized, raised, coral-pink pustules on the surface then coral spot is involved. However, this disease often attacks plants already weakened or dying back due to other problems.
• Finally, a severe attack by wisteria scale (see below) can weaken the plant enough to cause some dieback.

Question: The bark of my plant has numerous brown, limpet-like structures on it. What are they?

These are scale insects, of which a number of different species can affect wisteria. Found most commonly is brown scale. In south-east England a much larger blackish-brown scale, wisteria scale, may sometimes be found. Heavy infestations of the latter scale can lead to branch dieback.

Question: What is causing the brown blotches on the leaves of my plant?

Irregular dark brown marks and blotches, usually with a yellow margin, are a symptom of infection by the fungal disease powdery mildew. On most other types of plant this disease produces conspicuous white fungal growth on the leaves, but on wisteria it can be very difficult to see any growth at all on the blotches (without using a microscope).

The disease produces another atypical symptom when it first develops on the leaves in early summer – a pale green or yellowish mottling, which can be mistaken for virus infection. Wisteria can also be affected by viruses though, so if the problem does not respond to mildew controls and persists from year to year, virus may be involved.

Question: Why won’t my wisteria flower?

There are a number of possibilities here.

• If your plant has been grown from seed, it can take up to twenty years to flower, and even then the results may be disappointing! A named cultivar should be more successful, particularly if it is already flowering when it is bought. However, do not be alarmed if such a plant is reluctant to flower in the first year or two after planting. This is perfectly normal and the plant will settle back into flowering once the roots are established.
• The flower buds on wisteria, like those of many spring-flowering plants, start to develop in late summer of the previous year. Adverse conditions, particularly dry soil, occurring between July and September can cause the buds to abort. Ensure that your plant has an adequate supply of water during this critical period.
• Sharp spring frosts can cause flower buds to drop before opening, or result in distorted flowers. Other possible causes for poor flowering are too much shade, or inadequate levels of potassium. On poor soils it may be worth applying sulphate of potash in spring at 20g per square metre.

Regular and timely pruning can help to increase the flowering potential of the plant by producing a framework of flower spurs. In contrast, heavy pruning or pruning in early summer will disrupt successful flowering.