As with all plants, beans grown in the home garden have a preferred temperature range for optimum growth. Plants growing in temperatures that are either too high or too low exhibit a range of symptoms, including stunted or stalled growth and disease. Beyond ensuring that beans are planted in a timely manner, effects of unseasonable weather on bean growth can be completely beyond the gardener's control. Knowing what to look for can aid in identifying and remedying problems with bean plant growth related to temperature.
Effects of Low Temperatures
Cool temperatures slow bean plant growth, and greatly reduce the rate at which plants grow and develop. Beans are a warm-season vegetable crop, and planting too early in soils which have not adequately warmed is a recipe for failure. Bean seeds planted in cool, moist soils tend to rot before germination, and those seeds which do manage to sprout successfully often succumb to mildew and other bacterial ailments which kill them off before they can become established. In particular, beans should be planted once nighttime temperatures reliably stay above 60 degrees F.
Effects of High Temperatures
High temperatures retard bean plant growth mainly due to the reduced availability of water. Water in soils evaporates more rapidly during hot weather, reducing the volume of water available to bean plants' roots. Plants also slow or shut down transpiration, or breathing through pores on the undersides of leaves, to reduce water loss during especially hot weather. In effect, hot temperatures retard plant metabolism, and growth slows or completely halts. Long stretches of hot weather may also cause flowers to drop, pollen to die, and pods from forming. Normal bean plant development resumes once temperatures reach a more sustainable range, usually between 75 and 85 degrees F.
Temperatures and Insect Pests
Few insect pests bother beans to the extent that they cause crop death or significantly reduced yields. However, moth and bean beetles larvae, which both feed on bean foliage and occasionally flowers and young bean pods, grow at a faster rate during warm weather. Extant pest populations can therefore cause more damage during a stretch of warm weather than in cooler temperatures. Root knot nematodes, a worm-like pest which forms damaging galls on bean roots, can attack at any time during the growing season, regardless of temperature.
Temperatures and Disease
In general, cooler temperatures or long stretches of warm, humid and overcast weather are associated with many of the diseases prevalent to most types of bean plants. White mold and powdery mildew cause leaf and pod damage and can restrict growth and crop yields, and usually occur early and late in the growing season. Lesion-causing leaf spot bacteria favor cool, moist weather, as does the anthracnose bacterium, which attacks and destroys ripening pods. Warm, humid weather contributes to attacks by blight and bacterial wilt, which can stunt or completely kill maturing bean plants. Root rots caused by separate species of fungus can occur in any temperature range.
Especially for stubborn soil-borne pathogens, prevention is the best disease remedy. Rotating crops by planting beans in a different spot each year on a four- or five-year cycle reduces incidence of disease; crop rotation can also help break insect pest cycles, as larvae and adult pests alike frequently overwinter on-site by burrowing into the ground. Choosing bean strains which have been bred for resistance to rust, blight, wilt and fungus helps reduce attacks by disease, as does carefully inspecting seed before planting. Throw out seed which appears to be moldy, cracked, excessively shriveled or has a strange odor.
Fungicidal sprays formulated with copper and sulfur are available for treating crops which already show symptoms of disease. Systemic fungicides may be of some assistance for treating diseases of the stem and roots.