Gordon Johnson, Extension Ag Agent, Kent Co.; gcjohn@udel.edu
Pea plantings for later harvests in Delaware continue in April and the earliest snap bean plantings will start very soon. Early lima bean planting programs begin the third week in May. Other legume vegetables such as cowpeas and edible soybeans will also be planted starting in May.
Successful crops of legume vegetables begin with obtaining rapid emergence and adequate stands. Many issues with stands, vigor, and ultimately yield can be traced back to the seed quality and what happened with the seed in the time between seed planting and emergence.
Legume vegetables have large, relatively fragile seed. Food reserves are stored in the pair of cotyledons, sometimes called the seed leaves. Seeds can be easily broken into these two halves. In peas, during germination, the cotyledons stay below ground and the epicotyl emerges through the soil surface. In snap beans, lima beans, cowpeas, and soybeans, upon germination, the cotyledons are pulled through the soil by the extension of the hypocotyl to emerge above ground.
Legume seeds are easy to damage and damaged seeds produce lower vigor plants. Seeds may be damaged during seed conditioning, during transport and storage, and during field handling and planting. Always handle legume seed with care. Damage during handling lowers the germination percentage and increases the number of low vigor seedlings. Precise vacuum planters place individual seeds accurately with minimum damage and are preferable to older plate style planters.
Because food reserves are stored in the cotyledons, any damage that reduces these food reserves will limit the ability of the seedling to grow initially. In bean crops, where the cotyledons emerge out of the soil, there is considerable potential for damage during this process. In addition, because the cotyledons are so large, any physical impedance such as compaction or soil crusting may result in delayed emergence, reduced stand, and potential damage to the cotyledons, producing lower vigor plants. Seed corn maggot can reduce stand vigor and germination significantly by feeding on the cotyledons and is one of our major problems in April and May plantings.
Best germination for snap beans and vegetable soybeans is obtained at soil temperatures above 60°F with an optimum around 70°F. Peas will germinate at lower temperatures (40°F), but the optimum is above 50°F. Lima beans and cowpeas require higher soil temperatures (optimum germination between 70° and 80°F). Seed rots can be a major problem when soil temperature is below the range for that legume. Potential injury from soil incorporated herbicides may also be increased due to longer exposure times (slower germination).
Vegetable legume seeds are sensitive to cold water imbibition damage which can lead to reduced stands. This occurs when there is high soil moisture or rainy weather and cold soils. Overly dry seed (below 10% moisture) can increase this negative effect. You can reduce this problem by conditioning seed for several days by exposing it to humidity in the air (this allows the seed to absorb some moisture from the air). You can do this by opening bags and holding them for several days in an unheated storage area.
Planting depth for legume seeds is 0.75 to 1.5 inches. Use shallow depths with early planting dates when soils are cold and wet. Plant deeper when the soil surface is dry and soils are warm. Try not to irrigate immediately after planting to avoid soil crusting and excessive chilling in cold periods. Plant into moisture or pre-irrigate the field before planting if soil moisture levels are low.
Seedlings should emerge by 10-14 days. Delays in emergence expose seedlings over a longer period of time to herbicide effects, seed and seedling pathogens, and soil insects. Aim for uniform emergence. A delay of emergence of only 2-3 days behind neighboring plants results in the later emerging plants being barren, especially at higher seeding rates.
Ideally, use seed already treated with an approved seed treatment with a fungicide for Rhizoctonia and Fusarium control such as Maxim 4FS, a second fungicide for Pythium control such as Apron XL LS and an improved insecticide for seed corn maggot control.
If you suspect that a seed lot may have reduced germination, have the seed germination retested. Seed vigor evaluations such as the cold germination test may useful to compare or select seed lots for early planting. Adjust planting rates according to the germination percentage on the bag or the results of any seed tests performed.