I've Bean Had!
Dear Mrs Mace
Many thanks for your recent enquiry concerning the lack of developing runner beans from your flower-laden plants. Although directed to our pest control expert Julian Ives, I’ve taken on your query as the problem is rather more growing-based than the effects of garden pests.
That poor fruit set you’re experiencing this year with your beans can be attributed to a number of factors; a lack of bees and other pollinating insects; cool and in particular windy weather, lack of water and very warm weather at night will all play a part in reducing the amount of flowers which develop into pods. Some you can’t do much about – weather conditions especially – but planting beans in a sheltered position is certainly a help with regards to wind, and once the nights start to cool off in late August, pods will set more regularly.
Encouraging pollinating insects is something we can influence however, and a proliferation of bee-attracting plants - such as lavender and phacelia but any scented and coloured flowers will help – positioned near the beans will draw the bees in. Keeping the soil around the beans moist, especially during the hot conditions we’re currently experiencing is essential; it's easy to underestimate just how thirsty runner beans can be and my RHS book recommends between 5 and 9 litres of water are applied every 3-4 days when the plants are in flower! Installing a ground level irrigation system, such as the soaker hose can make watering a simple task.
For next season I'd certainly try sowing and growing the plants over a bean trench - fill it with well rotted compost, for example - and ensure the area you wish to grow the beans in is very fertile and rich in organic matter. The result is an improvement in soil conditions and the compost/organic matter helps keeps these hungry, thirsty plants well-fed throughout the growing season.
I hope this information proves to be of some help and your beans start to pick up very soon.