Seed Saving from Runner Beans
Runner bean flowers need to be ‘tripped’ by wind or insects before the beans set, and are much more likely to cross with other varieties grown nearby than are French beans.
Ideally, to be sure that no crossing takes place, seed crops of runner bean should be at least 1/2 a mile away from any other varieties of runner bean. In practice, buildings, trees, and other barriers will limit insect flight patterns, so if you are gardening in a built up area, you are unlikely to have problems with crossing unless your immediate neighbours are also growing runner beans.
On an open site such as an allotment, you made need to cover the crop or to persuade your neighbours to grow the same type of runner.
To collect the seeds, allow the pods to mature fully on the plant until they start to yellow and dry out. In wet weather, collect the pods individually as they get to this stage. Then spread out somewhere out of the rain with a good airflow until the pods are fully dry and brittle.
Once they are dry, shell out the beans and dry further out of the pods. The beans should be dry enough that they break when you bite on them, rather than leaving a dent. Store in an airtight container.
If they are well dried, and stored in a cool dark place, the beans will last around three years.