Gordon Johnson, Extension Vegetable & Fruit Specialist; firstname.lastname@example.org
Lima bean planting has begun in the region. With the expected warming trend, there is good potential for rapid germination and emergence this year due to higher soil temperatures. It is interesting to note that the variety Cypress was bred for good emergence under cooler planting conditions in Canada. We planted a trial using Cypress the first week in May this year and had excellent germination and emergence. We will harvest this trial the last week in July and then allow it to regrow and harvest a second time from the regrowth in October. Unfortunately, Cypress is very susceptible to pod drop due to heat. May planted lima beans, both at our research station and on growers farms in 2011 had very poor yields in the summer due to severe pod drop, even though some fields were well irrigated.
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. Day temperatures greater than 90°F cause stomates to close early during the day to limit water loss, reducing lima bean photosynthesis. This results in fewer pods being carried by the plant. Night temperatures in the 70s or higher will also adversely affect yields because higher levels of carbohydrates are consumed in night respiration, limiting the plants ability to set and retain pods. Plants will reflower when cooler conditions recur, but this may lead to split sets.
Unfortunately, until more heat tolerant varieties are available (at the University of Delaware, one of our lima bean breeding objectives is to select for greater heat tolerance) , growers are limited in what they can do to maintain yields in early lima bean plantings. Fields closer to water bodies were 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.