Pollination involves transferring pollen from the male part of a flower (anther) to the female part (stigma), which is necessary for seed production. Understanding how bean plants achieve this is important for successful harvests and for saving seeds. Whether beans cross-pollinate depends significantly on the species grown. Some beans are reliable self-pollinators, while others rely heavily on insects, making them more susceptible to mixing varieties.
How Most Beans Pollinate Themselves
The majority of beans commonly grown by gardeners, including snap beans, kidney beans, and navy beans (Phaseolus vulgaris), are reliable self-pollinators. This dependable trait is due to a mechanism where the flower pollinates itself before it fully opens. Inside the tightly closed flower bud, pollen is shed from the anthers and lands directly onto the stigma, achieving fertilization without outside assistance.
The flower structure, with petals forming a tight enclosure, acts as a natural barrier, minimizing the chance of outside pollen reaching the stigma. Since pollination is completed while the flower is still a bud, the resulting seeds are genetically identical to the parent plant. This self-pollinating habit allows gardeners to plant different varieties of common beans close together without concern for unwanted crosses.
The Role of Insects in Cross-Pollination
While common beans self-pollinate, certain other bean species rely on insects for successful seed set, increasing the likelihood of cross-pollination. Runner Beans (Phaseolus coccineus) require a visiting insect, such as a bee or hummingbird, to trigger the release of pollen, a mechanism known as “tripping.” This tripping is necessary for effective pollen transfer and fertilization. The Runner Bean’s large, showy flowers are highly attractive to pollinators, which easily carry pollen between plants.
Lima Beans (Phaseolus lunatus) also exhibit higher crossing rates than common beans, though they remain predominantly self-pollinating. The open structure of both Lima and Runner bean flowers allows insects easy access to move pollen between nearby varieties. Insect movement carrying pollen between different varieties of the same species is the main cause of unexpected genetic mixing.
Maintaining Pure Bean Varieties
The practical consequence of cross-pollination affects the genetic makeup of the next generation of seeds, not the beans currently developing in the pod. The bean harvested this season will taste as expected, but the seed inside, if planted next year, may produce a plant with mixed traits. To preserve the purity of a specific bean variety for seed saving, especially for species prone to crossing, gardeners must take preventative action.
One method involves using isolation distance, separating different varieties of the same species by sufficient space to prevent insect-mediated pollen transfer. For home gardeners, physical barriers are often more practical, such as fine mesh netting or a row cover placed over the plants before flowering begins. Another strategy is time isolation, which means staggering planting dates so the flowering periods of different varieties do not overlap.