March into early April has been warm enough and dry enough to the point that soil conditions are good for planting. The calendar date is a bit early; after all the crop insurance, replanting is April 11. Regardless, many are pondering if they should be planting corn already, and some maybe already have.
Now’s the time to exercise caution and patience. The 5 to 10-day weather forecast is moving to the cooler side of things with many daytime temperatures forecasted to be below 40oF and nighttime temperatures below freezing. Additionally, date of planting trials in Iowa and surrounding states suggest that maximum yields come from planting dates after mid-April. Here are a couple things to consider that might halt the desire to plant ahead of this cold spell that is forecasted.
Seed germination is a two-step process. The first step is imbibing water; a corn seed absorbs about 30% of its weight in water. Absorption of water is not dependent on the temperature of the water or soil. The second step of germination, which does depend on soil temperature, is the growth of the radicle root and coleoptile shoot. If temperatures are below 50oF, initiation and growth of the radical and coleoptile will not occur or will be very slow. This delay results in higher risk of insect and disease pathogen pressure, resulting in lower seedling emergence or seedling vigor.
Even though soil temperatures are above 50oF in the days ahead of this article they will quickly plummet. The odds for more cold weather and possibly snow are high in the coming week. With that in mind, to minimize risk, begin planting when soils are 50oF and rising (Guide to Iowa Corn Planting).
Problems associated with corn planted into cold soils
Cool soil temperatures early in the season increase variability in final stands. It is important to give every seed the chance to emerge and uniform emergence is important for maximizing corn yields.
Cool soil conditions early in the season lead to uneven emergence, growth, and development from one plant to another. Once the seed begins to germinate, a significant change in soil temperature can cause problems for mesocotyl growth such as reduced vigor, leafing out underground, corkscrewing, or in severe cases seed and seedling death. To maximize yield, manage corn to reduce plant-to-plant variability.
In addition to the effects of early planting on seed development and growth, early planting also exposes seeds and seedlings to increased potential for frost. Yes, frost on emerged crops is still possible into early May. We know that since a corn seedling's growing point is below ground until V6 — the sixth leaf stage — it can withstand short bursts of freezing temperatures when plants have emerged until the V6 stage.
What we don't always say — or for that matter, understand — is that frost often affects individual plants differently resulting in more variability from one plant to another. That variability can result in unequal interplant competition and lower yield potential. Depending on the potential date of replant though, keeping the surviving stand — albeit of variable plant heights and development — may still be the best option.
In addition to the impact on seedlings, extreme cold snaps can refreeze soils down to seeding depths. This can and does kill seeds and growing points, reducing stands and forcing a complete replant.