Evaluating Frosted Corn!
April 12, 2012 by Mike Kavanaugh, CCA
Evaluating Frosted Corn!
As the first significant cold front of the 2012 growing season passes through, many questions are arising concerning the fate of planted corn in the case of near or below freezing temperatures. With such a warm, early spring, many acres have been planted throughout the central and southern corn belt. In this newsletter, we have identified some key factors involving frosted corn evaluation and how to determine a plant's fate. History and experience has taught us that the key in this evaluation is to be patient. It is always advantageous to race out to these fields and want to replant immediately which is what NOT TO DO. There are several stages of corn plant size that has been planted throughout late March and early April of 2012. The key through this process is to understand the growth pattern of these small corn plants and give it at least a week for proper evaluation. We have had a lot of experience with frosted corn over the years. Below are some different stages that the majority of the crop is in right now throughout the danger zone of cold weather. The following photos were taken from past years.
Obviously, the bigger the corn is, the more susceptible it is to freeze injury. This is true because the depth of the growing point becomes shallower within the soil surface as the plant continues to grow. Keep in mind that the growing point or crown is located at approximately ¾ inch below the soil surface, providing that planting depth itself was below 3/4 inch. The growing point does not actually reach the soil surface until the plant reaches the V5-V6 growth stage. This should mean that the only way the growing point could freeze is if the soil temperature at ¾ inch deep would be 32oF or less. Since soil temperatures have ranged from 65oF- 75oF for the previous 10-12 days before the cold snap, one would think that the time to bring these soil temperatures down to 32oF would be an extended period of time, more than 3-4 days. Unfortunately, history and experience has showed us that corn may withstand one night of below freezing temperatures but may struggle through the 2nd and 3rd and even 4th nights, even though the soil temperature at 3/4 inch may not be as low as 32o or below. Small, spiked corn plants all the way up to 3 leaf (4 inches in height) corn plants can have a tendency to "wick" the freeze into the crown; also known as the "popsicle" affect.
Corn Seedling Anatomy
The first most important step throughout the evaluation is understanding the anatomy of a young plant and understanding how it operates. As the seed germinates, the heart of the seed (the embryo) pushes the first vegetative structure out that is known as the plumule. The plumule eventually grows (within seven days) toward the soil surface and develops into three major organs of a corn seedling, the mesocotyl, the crown and the coleoptile. The mesocotyl is the connecting link between the seed and the crown. The mesocotyl is sometimes referred to as the "umbilical cord", because the small seedling is dependent on the seed and the seed roots for approximately three to four weeks, until the first set of nodal roots are established and can take up necessary nutrients for plant growth. The crown is the area that is referred to as the growing point and is where the first set of permanent nodal roots are developed. Lastly, the coleoptile is the spike that pushes its way thru the soil surface. As said above, the crown, or growing point, will always form approximately ¾ inch below the soil surface. If planting depth is shallow, ¾ inch or less, the mesocotyl will have no elongation and nodal root development will take place at or near the soil surface. If planting depth exceeds ¾ inch then the mesocotyl will lengthen and the growing point will remain at ¾ inch and have plenty of room to establish the small plant's first set of nodal roots. That is why 1.5 to 2 inches is a desirable planting depth.
Stage 1-Kernels that have just germinated or have coleoptiles still below the soil surface. Everything in this stage should be protected and uninjured. The growing point is well below the soil surface and chances of "wicking the freeze" into the crown is virtually impossible. See healthy seedling in photo at right.
Stage 2-Coleoptiles or Spikes that have emerged above the soil surface. Spiked corn should be safe. When temperatures get below freezing the tops of the spike will most likely be brown but the area below the soil surface should still show a nice white color with a yellow glow from within. This yellow glow represents the first 4-5 embryonic leaves that emerge first and are wrapped tightly within the spike itself. These leaves are naturally yellow until they make contact with sunlight, then they turn green. To left are photos of healthy corn seedlings. The far left shows a nice, white, healthy coleoptile with no "freeze line" or "wicking line" present within the growing point. The photo to its right shows the brown frozen tissue at the top with the 4-5 embryonic leaves with a yellow cast or color below.
Stage 3- Corn plants that range from V1 to V4 growth stage and are 2 to 4 inches in height. Obviously vulnerability has increased dramatically at this stage. Depending on how low temperatures are for extended periods of time, will determine the severity of frosting or burning the green tissue. It is common to see small corn yellow up a bit with some brown edges or even discolored leaves a day or two after a frost. The plant to the right represents an injured crown. The freeze was able to "wick" down into the growing point and into the mesocotyl, causing it to turn brown and die. The picture to the far right is the same plant that had the outside peeled back with a fingernail. Notice the yellow beneath the growing point and into the mesocotyl. This is not good, this area should be nice and white. It will continue to turn brown, get mushy and die. The photos to the lower left represent a 3 inch tall plant that appears to be very questionable. After digging this plant up it was found to be healthy and growing. This is why one must be patient when evaluating the crop. When digging into the crown area with a fingernail which must be done most times, the crown was nice and white with a glowing yellow-green growing point that we like to see. Some correlations have shown that corn planted shallow (less than 1.3 inches) and corn that was planted on sand or very light soil seems to be more vulnerable than the opposite. As stated before, the first night of below freezing temperatures can turn the plant brown or black. The 2, 3 and 4 nights following will generally injure the growing point, if temperatures get into the mid to upper 20's. Wait at least 7 days after the cool snap to give these plants time to recuperate and regrow.