Do bush beans cross pollinate?
Do they, or can they?
Common beans, Phaseolus vulgaris, drop their pollen and self-pollinate the evening before the flowers actually open. So the pollen is not readily available to pollinators.
Thus, crossings are very rare. In practical terms, especially if you are just saving seed for your own use, cross-pollination just doesn't happen.
Plant breeders can cross varieties in the lab. But they can do so only one flower at a time. This is why, despite what some catalog copy would have you believe, all common beans are open pollinated. There are no F1 hybrids being sold.
The federal standards for seed purity, if I recall correctly, are 12 feet of separation. Commercial seed growers prefer 25 feet. But for home gardeners growing seed for themselves, as little as two feet will do.
I agree... for bush beans only, which was the posted question. But I would not apply the same standard to pole beans, which in my experience, require more distance (or barriers between) to maintain purity.
The rule of thumb, if growing multiple varieties, is to avoid planting two similar varieties side-by-side. For instance, adjacent varieties should vary in pod or seed color, so that if a cross does occur, it will be easily noticed in the next generation (the F1 seed would still appear normal this year).
I've had a few crosses, with bush beans, over the years. I think the main culprit was the bumble bee.
What I do is plant the beans I want for seed in blocks and separate the blocks with other block plantings of other species. For instance, I may have a block planting of tomatoes and beans side by side. Also, plantings of legumes of other species (limas, cowpeas, runner beans, etc) serve very well as a barrier crop.
But I'd agree with Gardenlad, in that if you have rows of bush beans side by side, I wouldn't hesitate to save seed. Crossing is not that common.
Thanks. You all are a big help.