The common bean, Phaseolus vulgaris, is a highly diverse crop. Whether a plant climbs depends entirely on its genetic makeup and growth habit. Some varieties possess the physical ability to climb, while others do not. Understanding this fundamental difference is the first step in successfully cultivating this versatile vegetable.
Understanding Pole and Bush Varieties
The two main classifications for common bean growth are determinate and indeterminate, corresponding to “bush” and “pole” beans. Determinate, or bush, varieties grow to a compact, self-supporting height, typically reaching only one to two feet tall. Their growth is limited because the main stem terminates in a reproductive bud, stopping new vegetative growth once flowering begins. This growth pattern leads to a concentrated harvest period.
Indeterminate varieties, known as pole or climbing beans, have a vegetative terminal bud. This allows the plant to continue growing vertically throughout the entire season, producing long, vining stems that can reach heights between six and ten feet. Because of this continuous growth, indeterminate beans are not self-supporting and require an external structure to climb. The Type IV classification specifically describes these indeterminate beans with twining stems and a strong climbing ability.
The Twining Mechanism
Climbing beans achieve vertical ascent through twining, where the entire stem wraps around a support structure. This differs from plants like peas, which use specialized, modified leaves called tendrils for grasping. The young, growing tip of the bean vine actively searches for support using a sweeping, helical movement called circumnutation. This movement involves the stem tip waving in a circular or elliptical path as it grows upward.
When the growing stem contacts a vertical object, circumnutation allows the plant to assess the support’s mechanical properties. Once a suitable support is found, the stem exhibits a thigmotropic response—growing in response to touch—and begins to coil tightly. The stem itself becomes the attachment structure, generating a squeezing force as it matures and stiffens. This provides the stability needed for the plant to climb and support its weight.
Essential Support Structures
Because pole beans cannot support their own weight, a robust structure is necessary for a successful harvest. These climbing varieties require a structure that is both tall and strong enough to bear the weight of a mature, heavily-podded plant. Support systems, such as trellises, teepees, or single vertical poles, should be at least six to eight feet high to accommodate the vines’ growth potential.
The support materials must be sized and textured so the twining stem can easily grip them. Stems wrap best around supports between the thickness of a pencil and a broomstick; anything thicker than about three inches can be difficult to coil around. Common materials include bamboo poles, rough-surfaced wooden stakes, strong netting, or durable twine strung between posts. The base of any structure must be firmly anchored in the ground, often by burying the poles a foot or more deep, to prevent collapse under the combined weight of the plants and wind.