Advice
RHS Help & Advice
Failure of runner beans to set pods
It is normal for runner beans to set more flowers than they can carry as beans, but attention to the following points should ensure a worthwhile crop.
Good cultivation
Runner beans are deep-rooted, requiring a well-drained but moisture-retentive soil to which organic matter has been added the previous autumn. A pH of 6.5 is preferable (acid soils may need liming). Rotate the crop annually to reduce soil pest and disease problems. Don’t sow or plant outside until late May to avoid cold damage (runner beans need a minimum soil temperature of 10ºC/50ºF to germinate).
Watering
Watering the soil at the base of the plant has a beneficial effect on flowering and pod setting. Apply a mulch to conserve soil moisture. In dry weather water from when flower buds appear, giving 5-11 litres per sq m twice a week throughout the cropping period. Syringing flowers with water doesn’t help pod set, but if done in late evening may cool the flowers.
Temperature
Warm temperatures, especially at night, may inhibit bean formation by preventing pollen grains from germinating. Cropping should resume when cool nights return in September.
Pollination
Flowers need bees (mainly honey and bumble) to pollinate the flowers. Cold, windy weather deters bees, so position plants in a sheltered site. Runner bean nectar is not very rich in sugars, and bee numbers visiting runner beans often increases after July, when more attractive mid-summer nectar sources, such as brambles, lime trees and clovers, have finished flowering.
Short-tongued bumblebees cannot reach nectar from the front of the flower, and so bite a hole in the flower base to gain access. This allows bees to get at the nectar but, because they do not come into contact with the reproductive parts of the flower, such bees play no part in pollination. There is no way to prevent this but sufficient bees usually visit the flowers in the conventional way for a good crop if other factors, such as water availability and temperatures are favourable.
Birds
Birds occasionally peck flowers, leaving tattered petals on the plant and surrounding ground. If this is a recurring problem grow dwarf cultivars (e.g. ‘Pickwick’) that can be protected with netting.
Further information
Further information on mulches and watering.
Trials of flowering plants, vegetables and fruit continue throughout the year at RHS Garden Wisley. Look out for the runner bean trials when you next visit.
A list of Award of Garden Merit runner beans is available from the online Award Plants database.
Growing beans
For information on this topic visit the BBC gardening website