Emmalea Ernest, Extension Fruit & Vegetable Specialist; emmalea@udel.edu
The recommended planting rate for snap beans is 6-10 seeds per foot (2.0-1.2 inch spacing) which works out to 104,540 to 174,240 seeds per acre. In past years I have been asked about the potential for snap beans to produce economic yields at lower populations from two different perspectives. Sometimes reduced seeding rate is being considered as a means of reducing input (seed) costs and sometimes plantings experience stand loss from diseases, seedcorn maggot or environmental stress. Past research done at University of Delaware has shown that both lima beans and processing sweet corn can compensate for reduced plant populations. Processing sweet corn continues to produce similar yields with up to 40% stand reduction, i.e. same yields at 23,250-13,950 plants per acre. Lima beans can compensate for up to 50% stand reduction, i.e. same yields at 69,700-34,800 plants per acre. In 2023 I conducted a trial to determine the impact of reduced plant populations on snap bean yields.
The 2023 snap bean stand study was planted on June 2. Plants established well and grew to nearly fill the rows; however, this trial did experience heat stress during the flowering period. I tested three varieties (PV 857, Caprice, Greenback) at seeding rates of 4, 7 and 10 seeds per foot. For all three varieties, yields were statistically significantly different from one another at the three seeding rates. The highest total and marketable yields were achieved at the highest seeding rate of 10 seeds per foot (174,240 seeds/acre) and the lowest at 4 seeds per foot (69,696 seeds/acre) (Figure 1). Notably, there was a significant difference in yield between the 10 and 7 seeds per foot treatments, both of which are within the range of recommended seeding rates.
Figure 1. There were significant differences in total and marketable yield between seeding rate treatments.
Beside affecting yield, the lowest seeding rate treatment also had smaller seed size at harvest than the 7 and 10 seeds/ft treatments, indicating slower maturity at lower plant populations.
The population effects on yield were consistent across varieties and none of the varieties I tested were better able to compensate for reduced stands (Figure 2). Caprice, which is less heat tolerant than PV 857 and Greenback, had a higher percent cull at the highest seeding rate, whereas the proportion of culls remained the same across seeding rates for PV 857 and Greenback (Figure 2). This indicates that high populations might exacerbate marketable yield loss in heat-sensitive varieties.
Figure 2. Varieties responded similarly to planting rate in terms of total yield, but the proportion of cull beans increased in heat sensitive Caprice at the highest planting rate.
Overall, the results of this trial indicate that reducing snap bean seeding rate is likely to reduce yield potential, so I would not recommend lowering seeding rates to cut costs. Also, since stand establishment is especially important for maximizing snap bean yield potential, manage tillage, planting practices and irrigation for quick, even emergence. Use appropriate seed treatments to manage seed corn maggot and disease risk. Remember that rough seed handling reduces germination in beans. For early plantings into cooler soils, use high vigor (not carryover) seed.