Some spots in the region were soaked over period from June 4-10

The rain storms that fell over the last week over much of northwest Minnesota brought rain to all, and way too much rain to others.  The North Dakota Agricultural Weather Network (NDAWN) stations tell the tale. For the week of June 4 through June 10, rainfall totals at 12 NDAWN weather stations ranged from a low of 0.85 inches in Grand Forks to a high of 5.45 inches in Kennedy

The flat topography and poorly drained soils of the Red River Valley can lead to considerable ponding even when fewer than five, or three or even two inches of rain falls. 

 

What flooded soils do to developing plants

The dangers to roots from flooded soils are many.  Flooded soils quickly become depleted of oxygen which is essential for proper root function.  If you’ll recall from your high school biology class: plant leaves use the sun’s energy to convert CO2 and water into oxygen and glucose through the process called photosynthesis.  

Respiration is sort of the opposite of photosynthesis, where oxygen and glucose are converted back into energy (and CO2 and water) that is used to run the machinery of cells. On a typical day there is a balance between respiration and photosynthesis. On sunny days more photosynthesis than respiration occurs allowing plants to make the building blocks essential for growth and development, eventually contributing to yield.

Many organisms that live in soil need to respire to live and function, including many bacteria and fungi and nematodes, insects, worms and other animals and plant roots.  Flooded soils quickly become oxygen-free (anaerobic) environments that do not support aerobic respiration.  In the absence of oxygen, respiration still continues to occur, but anaerobic respiration leads to the buildup of substances toxic to plant cells including ethanol and organic acids.  The rate of respiration increases with increasing temperatures, depleting soil oxygen more quickly under higher temperature conditions.  Additionally, while an anaerobic soil environment certainly does not favor normal cellular functions, root growth or development, prolonged oxygen deprivation can lead to cell death, death of root tissue or the whole plant.

Corn is very vulnerable to damage from flooding when plants are younger than the six-leaf (V6) growth stage and the growing point is still below-ground. Only three or four days of being submerged in floodwater can be fatal to these young plants.  Plants with above-ground growing points are better able to tolerate a week or more of standing water.  If plants survive the flooding, root growth and function can continue to be reduced even after flood waters recede.  If root growth is retarded, roots may be less able to access subsoil moisture needed to meet water and nutrient demands of plants during the reproductive growth stages.

Even should soil dry enough and quickly enough to allow replant, the expected yield of corn replanted after June 19 in southern Minnesota is 59 percent of maximum (UMN Extension, 2018). Due to our comparatively shorter growing season University of Minnesota Extension does not recommend replanting corn in northern Minnesota after June 5.  

Those in northwest Minnesota that planted corn for grain in the beginning of June 2019 would likely concur with this recommendation as many contended with elevator discounts due to low test weights or having to finish their 2019 harvest in 2020.

Flooding can also be detrimental to soybean root growth and function and nodule formation and function.  Without proper nitrogen fixation, soybean leaves can begin to turn yellow.  Research has shown that photosynthesis can be reduced by one-third with 48 hours of flooding (Oosterhuis, 1990). This reduces dry matter accumulation both during and after flooding and can reduce seed yields. One bright spot is that after water dissipates, photosynthesis and dry matter accumulation can resume.

 

So what now?

At this point in the growing season it might not be a bad idea to give your crop insurance agent a call to be reminded of what needs to be documented or done in order to file a claim should flooding result in a crop loss.