Gordon Johnson, Extension Ag Agent, Kent Co.; gcjohn@udel.edu
There is considerable controversy about when and how to irrigate lima beans for best yields. Past research has shown that irrigated lima beans significantly out-yield dry land lima beans and top yields generally come from irrigated fields. However, when and how much to irrigate is still a question as is the need to irrigate late in the season for fall harvested lima beans.
Research has suggested that irrigating when soil moisture drops to 50% of field capacity gives better yields than letting the soil dry out further for lima beans. It is also better to irrigate throughout the season rather than just from full flowering onward.
To complicate matters, over-irrigation, especially during pod development when there are very dense vines, increases the potential for pod diseases such as Sclerotinia white mold, lima bean pod blight (Phytophthora capsici), Pythium blight, and downy mildew and can lead to significant yield losses. A balance must be maintained between providing needed water and allowing the crop to dry out to reduce disease pressure later in crop development.
It is also important to consider temperature. The following is from Ed Kee during a heat wave in 2006: “The tremendous heat we’re experiencing makes irrigation of lima beans more critical. Blossom drop occurs when the plant is stressed, and even with adequate irrigation blossoms will abort with high temperatures. However, maintaining soil moisture is important to keep the plant cool, which alleviates stress and reduces blossom and pod drop. Lima beans will use 0.25 inches of water per day through evapotranspiration when temperatures are in the nineties. Reducing plant stress as much as possible will help in the retention of “pins” (small pods) and larger pods.”
In lima beans, first flowering generally occurs at 35 days from planting, and peak flowering at 60 days. Providing adequate water with irrigation during the entire flowering period and through pod set is critical for developing and maintaining yield.