Mid-April approaches and days slowly grow longer. But, irrespective of the calendar, soil temperatures at 4 inches hovered around 40⁰F early this week and seven-day lows stayed near freezing. Our hopes for an early start to the corn planting season may be futile. Nevertheless, warmer temperatures in the forecast will quickly alleviate our concerns and provide encouragement for a good start to the planting season. We’ll address here our thinking on planting corn in cold soils as well as the question in our title: Is planting early always best?

What if it Does Stay Cool and Damp? 

What do we need to keep in mind as we consider planting corn? This question was summarized last year in CropWatch. Let’s review some of the points we made then.

Before planting, check the weather forecast and soil temperatures for your area. It’s wise to check the soil temperature of each field early in the morning you intend to plant. Wet soils or fields with more crop residue tend to have lower soil temperatures relative to drier soils or fields with less crop residue. Check the seed tag or check with your seed dealer regarding the cold tolerance of your corn hybrids. Be aware that hybrids vary in cold tolerance and company rating scales differ. Cold tests can help ascertain seed viability. Let’s review ideas on cold stress.

Cold Stress in Corn — Imbibitional Chilling

Certainly plant hybrids that are more cold tolerant first. Be aware though that “imbibitional chilling” is a physical phenomenon that can override genetics. Imbibitional water uptake occurs within the first 48 hours after a seed is planted. Once planted, corn seeds need a two-day (48-hour) window when the soil temperature at planting depth does not drop much below 50°F. When soil temperatures drop much lower than 50°F within that two-day time frame, chilling injury may affect seed germination and subsequent seedling growth (Figure 1). During the osmotic phase of water uptake, which starts about 48 hours after the initial imbibitional phase of water uptake ends, the risk of chilling injury approaches zero. Thereafter, temperatures below 50°F can slow germination and delay seedling emergence. This can result in a longer exposure to soil-borne pathogens, particularly in soggy wet soils. If you expect the latter, use fungicide seed treatment for at least your earliest planted corn fields.  

When corn seeds imbibe (take up) water, cell membranes stretch and cells expand. When a damaged cell membrane rehydrates, it may not return to its normal shape and size. This can create a "leaky" cell. Water is at its densest at about 39°F so when cold water is imbibed, it may result in additional membrane damage. These ruptured membranes may occur in the cell walls and in the mitochondria. In the plant this action may disrupt the embryo/endosperm enzymatic conversion to energy, but mostly results in leakage of cell solutes and sugars. This, in turn, is likely to reduce growth rate and interfere with growth of the emerging seedling.

• Debate exists about what specific temperature and timing causes imbibitional chilling. However, corn seeds that imbibe cold water (in the low 40s) in the first 48 hours after planting undoubtedly are affected.
• Planting when soil temperatures are above 50°F — and are not anticipated to drop much below this the following 48 hours — alleviates concerns of imbibitional chilling affecting corn emergence. Some scientists suggest that corn will not be injured at soil temperatures as low as 41°F; however, there is certainly some risk of injury from imbibitional chilling at those low temperatures.

Corn Planting Windows: Is Early Planting Always Best?

We’ve learned a lot about corn planting dates in the last 5 -10 decades. Early on planting dates were restricted by the necessity for tillage for weed control and to improve seed-soil contact. In addition, later planting helped avoid specific, destructive corn insects — e.g., corn rootworms — and probably helped us avoid pathogens associated with cold, wet soils. Larger farms, innovations in planting equipment, more cold-tolerant hybrids, and improved herbicides, fungicides, and insecticides, have allowed us to plant seemingly earlier and earlier every year. In addition, climate variability manifests itself in unprecedented swings in temperature and precipitation forcing us to change our assumptions. One of these assumptions is that planting early is always best.