It happens every year. That ONE neighbor starts planting and everyone is itching to get rolling. But the real question is - how soon is too soon? What benchmarks do we need to look for? What are the CONSEQUENCES of planting too soon?
Here in Kansas, the typical last spring frost date is later in the year as we go northwest across the state (shown below on the map produced by the KState Climatologist Office). However, "average" doesn't mean ALWAYS. Each year is different and as unpredictable as the next.
Before corn is up, what impacts can cold temperatures
have on germination or emergence?
The magic number for soil temperatures when planting corn is above 50 °F (although typically recommended at 55 °F), which is the crop base temperature. When soil temperatures are reliably maintained above 50 °F, corn is able to accumulate the heat units needed to germinate (100). For an explanation of calculating heat units or "Growing Degree Days" (GDD), check out my blog post: MY KIND OF MATH.
As of April 21, 2021, according to data from Kansas Mesonet, the average weekly 4" depth soil temperatures across Kansas are shown below. Even though the average last spring frost date has passed for half of the state, temperatures remain borderline in these regions. Only the southeast corner of the state has averages at the 55 °F target mark.
The earliest stages of the germination process are perhaps most susceptible to cold temperatures. Imbibitional chilling occurs when cold water is taken in by the seed as water is initially being absorbed (1-2 days after planting). This stage is key, as the "imbibed" water is necessary to activate enzymes that complete the germination process. Among other things, imbibitional chilling can result in reactive oxygen species, which damage seedlings from the inside and result in starved, deformed, or stunted plants.
As a result of imbibitional chilling effects, uneven stands become a legitimate concern - both developmentally and spatially. Selectively stunted plants cause discrepancies in growth stages and potentially flowering dates, which can affect pollination and other management timing decisions. Starved, dying plants unable to overcome the cold stress can result in row gaps and reduced stands. These differences can reduce yield potential - more information on the impacts of uneven stands is available here: http://corn.agronomy.wisc.edu/Pubs/UWEX/NCR344.pdf
Because the growing point and a majority of the corn plant are either beneath or near the soil surface for the first part of its life cycle, soil temperatures have been suggested as more important to crop development than air temperatures! That is, until the corn plant's growing point is aboveground (~V5).
Lots of reasons to pay close attention to
what your soil is telling you before planting!
This year, we saw a warming trend in early April, which led to a significant amount of corn in the ground by the time we experienced the state-wide freeze on April 20-21, 2021. Below is a USDA crop progress update posted by my teammate, Dr. Leo Bastos, in the week of the freeze (See Post Here), as well as the low temperature map posted by Kansas Mesonet on the morning of April 21 (See Post Here).
What happens if corn is past germination and has
already emerged when cold stress happens?
Although the risk is reduced after the plant has emerged, corn is not a very cold-tolerant crop. With temperatures this low, even with snow cover, corn leaves above ground are likely significantly burned, wilting, or turning yellow. Corn leaves will show chilling symptoms even when temperatures fall into the low 40s for a period of time.
Below are images from Manhattan last spring, when corn was exposed to temperatures in the low 40s for an extended period in early May. Leaves of chilled corn plants turn yellow because of membrane damage inside the leaf, resulting in internal cell leakage. Damaged young corn leaves can result in "buggy-whipping" when the whorl becomes trapped and the developing leaves cannot expand properly.
Corn ONLY has an opportunity to recover if the growing point, still underground in these early stages, is healthy. If the growing point is dark or soggy-looking, the corn plant will not survive and a replant is necessary. Below is an image of the healthy growing point for the chilled plants shown above. Although exterior leaves were wilting and yellow, the central portion of the stem base, where the growing point is located, remains healthy. These plants recovered easily.
When leaves are killed or more severely damaged, the whorl may become irreversibly trapped and more significant problems can result - such as starvation due to lack of photosynthetic ability, or a significant delay in development. As mentioned previously, delays in development can have management impacts and reduce yield potential. Images of corn plants with symptoms of early chilling-induced development stress are shown below.
The plant on the left did not survive through flowering stages. The plant on the right produced a small ear, but the tassel was not exerted and pollen not shed properly.
If my corn was in the ground and I experienced these temperatures, what should I look for?
If you had planted recently, dig up some plants and check for disfigured, yellow seedlings. Corn commonly "corkscrews" as a result of imbibitional chilling.
If your corn had emerged, check for signs of wilting, yellowing, or water-soaked leaves. The plants have a high chance of recovering if the growing point (under soil surface) looks healthy. SYMPTOMS MAY NOT APPEAR FOR SEVERAL DAYS. Even if the leaves appear to be trapping the whorl, the corn may be able to "outgrow" these symptoms and free itself. Severely damaged plants with more leaves expanded are less likely to be able to escape the damaged leaves. Replanting economics should be considered if plants appear to be stunted or stands are reduced.
And the question we all knew was coming -
How do CORN TILLERS factor into this?
Replanting economics based on the ability of plants to recover and compensate for lost plants or time is something that we are exploring with corn plasticity traits like tillering. At what point does it make more sense to take the salvaged yield that tillers will provide rather than starting over with a replant? Stress (and cooler temperatures) encourage tiller development, and we have seen significant plant density yield compensation potential by tillers in good environments.
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