CSU – Golden Plains Extension: Corn Planting
Since soil temperature, stand establishment and uniform emergence are all key considerations when attempting to determine an optimum corn planting window, early plantings generally coincide with cold soil temperatures and delayed emergence is the result. Agronomy professionals agree that the optimum corn planting window will be related directly to a consistent soil temperature of around 50 to 55° F at the two-inch soil depth. Traditionally, these consistent soil temperatures occur between late-April through early-May. Cool soils often result in a lengthy germination period, corn emergence can often times take over three weeks from the time of planting to full stand establishment when soils are not able to warm in the spring. Table 1 outlines the approximate days to seedling emergence based on various consistent, soil temperature ranges:
Under cold weather conditions or fluctuating soil temperatures, the coleoptile (shoot) still emerges from the seed, but rather than growing upward towards the soil surface, it twists or curls around the seed. The unusual growth pattern of the coleoptile is in response to cold soil conditions; this type of growth alone is usually not cause for alarm. Once soil temperatures improve, the coleoptile will “right” itself and grow towards the surface, where it splits open when exposed to sunlight, allowing the first leaves to emerge.
Corn may leaf out underground for a number of reasons. A cloddy seedbed or improper closure of the seed furrow may allow light to penetrate below the soil surface. If light reaches the emerging coleoptile (spike) underneath the soil surface, it may rupture, causing the leaves to unfurl. Cold soils and compacted and/or crusted soils can also lead to leafing out underground. Some herbicides can occasionally magnify the seedling corkscrew problem with the soil conditions mentioned above.
Once corn is emerged, other issues can express themselves due to cold weather. Purple Corn Syndrome shows up in a handful of cornfields nearly every year. This purple leaf color results from the expression of genes for anthocyanin pigment formation. Most corn hybrids contain 5 of the necessary 8 genes required to produce this purple color, while the other 3 genes are present in only certain hybrids suggesting this attribute is a “genetically inherited” trait more prominent in certain hybrids over others.
Since several of these genes are cold sensitive, overnight temperatures in the 40s with daytime highs in the 60s are often sufficient to trigger a purpling effect on corn leaves. These temperature sensitive genes are only expressed in seedlings prior to the six-leaf stage of growth, and the early corn developmental stage often coincides with the same period most likely to have lower temperatures. Corn usually outgrows the “purple” condition by the time it is 12 inches tall. Green color recovery occurs quickly if weather remains conducive for rapid growth or slows if conditions remain cool enough to retard root and shoot growth. Technically speaking, the cold soil condition ties up phosphorus and restricts the corn plant from this nutrient’s uptake. Some corn hybrids do not respond well when this occurs.
So, will fertilizer correct this issue? Generally, the purple color is a direct result of cold soils and not an overall fertility deficiency. Even when planting into cold soils, the 10 pound rule is still in place. That is, with the seed, apply no more than 10 pounds per acre total of nitrogen and potassium. Both of these fertilizers are salts and applying more than 10 pounds per acre of these fertilizers with seed could reduce stands. Phosphorous, however, is not a salt and producers can apply as much as needed with the seed.
Realizing that cool temperatures, not the purple pigment itself, results in slow plant growth is important. Extensive research has been conducted on purple corn with no negative yield implications observed. When the weather warms, corn plants turn green and begin to outgrow this issue.
Source: Corteva
