Environmental causes and control
Growing conditions play a greatly influence how a plant flowers. Most of the problems and causes below are to do with the site and conditions and, of course, the weather.
Shrubs often fail to flower after drought, including drought the previous year. Rhododendrons, camellias and other spring-flowering plants initiate flower buds in that late summer period and drought stress at this time can lead to poor flowering. Mulch damp ground in mid- to late spring and water if necessary during periods of drought.
Plants grown in a frost pocket or in an exposed, windy position could have their flowers damaged; while sun-loving plants grown in shade (or shade-lovers in sun) will be reluctant to flower. Move plants to a more sheltered site, or give them their favoured growing conditions; sometimes shade can be remedied by pruning back surrounding plants to improve light levels.
A wet season, especially in spring and summer, can result in lush growth, but few flowers. This is usually temporary and the plant should flower normally in drier years. In containers, excess moisture can kill plant roots due to a lack of oxygen and subsequent root rot. Ensure pots have adequate drainage and raise plants up on feet in winter.
Impoverished soil will affect flowering. Feeding should help. Apply sulphate of potash around the shrub in spring and if plant vigour is generally low, apply a balanced fertiliser over the soil as well. Composts used in pots have sufficient nutrients to provide food for six to eight weeks after potting on. After this, plants need additional feeding to maintain healthy growth, apply controlled-release fertilisers once a year in spring, or liquid feeds more frequently (weekly to fortnightly).
Insufficient ripening of the wood
Some plants will not set flower buds without a warm period to ripen the wood. Wisteria and Campsis flower best on a south-facing wall. Hibiscus needs a long, hot summer to flower well. In cool summers, or on north- and east-facing sites, these plants may struggle to flower.
Excess nitrogen encourages leafy growth at the expense of flowers and is usually a problem where gardeners have applied high-nitrogen feeds or mulches, such as sulphate of ammonia, poultry manure or farmyard manure.
Root-bound plants will rapidly suffer from drought and lack of food. If this happens, pot on to the next pot size. If plants are in the largest size pot that can be managed, top dress or remove excess compost before replacing it in the same pot. Ensure plants are kept well fed and watered.
Old age, virus or disease may weaken plants, reducing flowering. Renovation, replacement or control may be necessary to improve the display.
Gardening on wet soils
Mulches and mulching
Waterlogging and flooding