Heavy rains and overflowing creeks have caused some fields, particularly those in low-lying areas, to flood. In other fields, water may pond for a period after the rains, and then soak in, leaving producers to ask: How long can plants be underwater before they die?

In a recent newsletter article, Bob Nielsen, Purdue University agronomist, described how early season flooding affects the crop:

• The longer an area remains ponded, the higher the risk of plant death.
• Corn that is completely submerged is at higher risk than corn that is partially submerged.
• Plants that are only partially submerged may continue to photosynthesize, albeit at limited rates.
• While most agronomists believe that young corn can survive up to about 4 days of outright ponding, in a related article Paul Hay relates his experience with corn dying after one day. Corn will survive longer when temperatures are relatively cool (mid-60s or cooler) than when it's warm (mid-70s or warmer).
• Soil oxygen is depleted within about 48 hours of soil saturation. Without oxygen, the plants cannot perform critical life sustaining functions. For example, nutrient and water uptake is impaired and root growth is inhibited.
• Even if surface water subsides quickly, the likelihood of dense surface crust forming as the soil dries increases the risk of emergence failure for recently planted crops.
• The greater the deposition of mud on plants as the water subsides, the greater the stress on the plants due to reduced photosynthesis. Ironically, such situations would benefit from another rainfall to wash off the mud.
• Corn younger than about V6 (six fully exposed leaf collars) is more susceptible to ponding damage than corn older than V6. This is partly because young plants are more easily submerged than older, taller plants and partly because the corn plant's growing point remains belowground until about V6. The health of the growing point can be assessed initially by splitting stalks and visually examining the lower portion of the stem. Within 3 to 5 days after water drains from the ponded area, look for fresh leaves appearing from the whorls of the plants.
• Extended periods of saturated soils after the surface water subsides will take their toll on the overall vigor of the crop.
• Some root death will occur and new root growth will be stunted until the soil dries to acceptable moisture contents. As a result, plants may be subject to greater injury during a subsequently dry summer due to their restricted root systems.
• Associated with the direct stress of saturated soils on a corn crop, flooding and ponding can cause significant losses of soil nitrogen due to denitrification and leaching of nitrate nitrogen.
• Significant loss of soil nitrogen will cause nitrogen deficiencies and possibly additional yield loss.