Sun is the vital element that helps plants harness the energy they need to grow, so full sun is key for vegetables like beans to thrive and produce a large harvest. Beans grow quickest and produce the most under unfiltered, direct sunlight. Outside of a natural outdoor growing environment, variations of light intensity, direction and spectrum can alter the chemical and physical traits of bean plants.
Like most warm season crops, beans need a minimum of around six hours of full sunlight each day to produce the biggest yields, but they grow best with more sun than that. As long as soil is kept moist and temperatures are not too high, beans thrive in direct sun even during the long days of summer. They can grow in partial shade as well, but this may reduce their yield. Keep soil evenly moist for vigorous growth, particularly during hot and dry periods.
For most beans, germination occurs once soil warms up to between 70 and 90 degrees Fahrenheit. If soil is under 60 degrees, bean seeds germinate much more slowly. But if air temperature rises above 90, pods may have trouble forming. Fava beans are somewhat of an exception as they can be sown in cooler weather, once soil is workable. Lima beans, on the other hand, need even warmer soil to germinate than most beans -- at least 75.
Light Direction and Intensity
Scientists have found that full, unfiltered light encourages quicker germination and taller stems in the mung bean plant when grown indoors. They discovered that mung beans grow and germinate most successfully when facing a south window with full light, as compared to a north-facing window and filtered light. Mung beans had trouble germinating and stem growth was shorter when they were grown under a light filter. The beans were more affected by light variables than the radishes grown alongside them in the Cornell University study.
Beans grown under low or no light produce less carbohydrate, protein and pigment than those grown under full, natural light conditions. The spectrum and intensity of light makes lima beans produce these chemical components differently. A natural sunlight spectrum encourages greater carbohydrate, protein and chlorophyll production. UV light decreases levels of carbohydrates and protein, but increases beta carotene and carotenoid levels.