Gordon Johnson, Extension Vegetable & Fruit Specialist; email@example.com
Each year there are some lima bean fields that end up with lower stands than expected due to poor field conditions such as crusting after heavy rains, planter malfunctions, seed quality issues, or errors in setting planting rates. We also have seen stand losses due to herbicide damage.
Soil crusting can be a major concern in lima beans. As the lima bean seed germinates, the cotyledons or seed leaves are actually the two halves of the seed and provide the stored energy reserves for the young seedling. They must emerge from the soil intact for the young seedling to grow well. During emergence, if the soil crusts, the soil force may be so great that the large cotyledons are trapped in the soil and the seedling stem, the hypocotyl, breaks in half. The resulting “headless” seedlings will not recover. Similarly, if only one of the two cotyledons emerges intact, the plant will be stunted because only half the stored seed reserves are available. These stunted plants will not be productive.
In compacted surface soils, if the soils remain moist, the force required for the lima bean seed to emerge with both cotyledons intact will be reduced. However if the soil dries and forms a crust, the force required increases to a critical point.
To reduce problems with crusting, do not work soils or plant when wet (a problem this year). Limit trips across the fields with tillage equipment, especially disc harrows and use planters with furrow closers that do not compact immediately above the seed. Adjust closers to limit compaction if planting must be done in damper soils. Consider ways to reduce tillage while forming a good seed bed. From a soil health standpoint, maintaining good organic matter levels in soils will also reduce crusting in fields.
In fields that have crusted, use of a rotary hoe to break the crust may improve emergence. However this must be weighed against damage to cotyledons. Another option on soils that are crusted is to do a light irrigation to reduce the force required for emergence. If emergence is variable, as long as stands have not been reduced more than 30 %, then no action may be necessary until first cultivation which can break the crust and aerate the soil. This cultivation should take place as soon as plants are large enough to cultivate.
For baby lima beans, recommendations are for a stand of 3 to 4 plants per foot of row. However, lima beans have a great ability to compensate for lower populations by producing larger plants. In a study by Dr. Wally Pill at the University of Delaware, baby lima bean seeds were sown at the recommended rate and then some plots were thinned within 2 weeks of planting to provide 0%, 16.7%, 33.3%, and 50.0% stand reduction. The research showed that even at 50% stand reduction, overall plant biomass per area was only reduced by 14-21%. The conclusion of this research was that lima bean tolerates a considerable loss of plant stand with little or no effect on yield. Other research and field observations support this claim.
In evaluating a lima bean field with stand losses, the following guidelines are suggested:
● If stand losses are 33% or less, then replanting should not be considered. The yield potential will be close to a full stand.
● If the stand losses are between 33% and 50% and there are not a high percentage of large gaps, then replanting should not be considered. The yield potential will be 85-90% of a full stand.
● If the stand losses are between 33% and 50% and there are a high percentage of large gaps, then replanting may be considered. If fields can be reasonably divided into low stand and high stand areas, then replant only the low stand areas.
● For stands less than 50% then replanting should be considered up to July 15 (working with your processor).