Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
Lima bean planting will begin soon in the region. With early lima beans, it is best to plant on a warming trend when there is good potential for elevated soil temperatures so germination and emergence is rapid. It is interesting to note that the variety Cypress was bred for good emergence under cooler planting conditions in Canada. Unfortunately, Cypress is susceptible to flower and pod loss due to heat. May-planted lima beans, both at our research station and on grower’s farms, in past years have had very poor yields due to reduced pod set.
This illustrates the problem with May and early June planted lima beans: they most often have a lower yield potential than late June and early July plantings because they flower and set pods during summer conditions when day and night temperatures are high. Research by Emmalea Ernest in Delaware has shown that our current varieties are susceptible to heat losses due to reduced pollen production when nighttime temperatures are high before and during flowering. In addition, high day temperatures will reduce photosynthesis and high night temperatures increases respiration thus limiting the number of pods lima beans can carry during July and early August. Plants will reflower when cooler conditions recur in August, but this may lead to split sets.
At the University of Delaware, one of the objectives of the lima bean breeding program headed by Emmalea Ernest is to select for greater heat tolerance and progress is being made. Unfortunately, until more heat tolerant varieties are available, growers are limited in what they can do to maintain yields in early lima bean plantings. Fields closer to water bodies where temperatures are moderated by fog, heavy dew, high humidity, and cooling breezes during summer are the best candidates for early plantings. In addition, irrigate early planted fields, paying particular attention to the flowering and early pod set period and do not plant early lima beans dryland. Daytime irrigation can also help to moderate high temperature effects during hot summer periods. It is critical to keep early planted lima bean plants from being water stressed during this period.