Advice
RHS Help & Advice
Wisteria flower failure
Beautiful in flower, wisterias can provide colour and interest for many years. However, they sometimes fail to flower or produce poor displays.
Position
Wisterias need a sunny situation and where poor or non-flowering plants are growing in some shade, flowering may be encouraged by reducing overhead shade.
Pruning
Although wisterias, especially those with limitless space to grow in, will flower without pruning, they generally flower better if pruned twice a year.
Soil & feeding
If a plant, established for several years and regularly pruned, fails to flower, try a spring application of superphosphate at 68g per sq m (2oz per sq yd) and sulphate of potash at 34g per sq m (1oz per sq yd). This should help check any tendency to produce excessive leaf growth and encourage flowering, particularly in lighter, sandy soils where low potash levels are common.
Although showing good drought tolerance once established, it pays to ensure that plants, especially if against a south-facing wall, do not suffer from lack of water. Mulch in spring before the onset of drier weather conditions to conserve moisture, and water thoroughly and regularly during any periods of drought, particularly in spring when in flower and from late July to September when flower buds are being formed.
Poor or non-flowering plants
Graft failure: Wisterias are propagated commercially by grafting cultivars onto seedling rootstocks. If the graft fails the vigorous rootstock grows, which produces little flower. Always check when buying plants that a healthy graft union is present. A clear join should be seen, 15-30cm (6-12in) above ground level.
The RHS Advisory Service frequently receives samples of wisteria where the plant has died and the roots and lower stem have decayed completely. There is no consistent association with decay organisms, and we believe this problem is due to delayed failure of the graft, just above soil level; failures may occur as long as 20 years after planting. If this is the reason, it should be safe to replace with another wisteria. Plants raised from seed or cuttings would avoid this potential problem, but are notorious for being poor flowering.
Seed-raised plants: Plants grown from seed are generally of inferior quality, can take many years to flower and the flowers are small and wishy-washy in colour.
Frost or bird damage: Flower buds may fall in spring during fluctuating day and night temperatures, and frost can damage flowers, leaving them wilted on the plant. Birds sometimes damage buds while searching for insects, but usually a few buds escape damage to provide some display.