Planting into Cool Soils – Yes or No?

While research shows that the last 10 days of April is on average the best time to plant corn in Illinois, expectations of below-normal temperatures in most of Illinois during the last week of April has some wondering if it makes sense to plant now or to wait until temperatures warm up.

Averaged over the past 22 years, Illinois corn producers have planted 16% of the crop by April 20. NASS reported that 15% of the crop was planted by April 19 this year, so planting progress to date is right at the average. There will be some progress to report this week, though rainfall coupled with cooler temperatures will limit the rate at which fields get ready to plant in some areas. Still, we may be on track to maintain planting progress at the average rate, which would mean having close to 40% planted by April 30.

Research tells us that planting before May 1 almost always yields more than planting later, with yield loss accelerating with delays past early May. Planting date and yields over years for the whole state often give a different picture, however.  Over the past 22 years, in fact, there has been no correlation between the date by which 50% of the corn crop was planted and statewide yield, measured as departure from trendline yield in order to correct for the upward yield trend over time.

In 2014, 36% of the Illinois corn crop was planted by April 30, and the average yield was 200 bushels per acre. In contrast, 81% of the crop was planted by April 30 in 2012, but the average yield was only 105 bushels per acre. Two of the latest-planted crops in recent years – 2009 and 2013, with hardly any corn was planted by April 30 – produced yields more than 10 bushels above trendline yields. So it is clear that what happens with weather during the season can override when the crop was planted, at least over a large area.

But for each individual field we still need to try to plant as early as conditions allow. Even if planting a week or two later would have little effect on yield in that field that year, we need to “start so we can finish” – getting all fields planted by early May is a goal as we try to maximize yield potential. But might this year be an exception, with potential for harm from planting into cool soils in the last week of April, with the weather forecast indicating that temperatures may stay low for the next week?

As a principle, waiting until soil is dry enough to allow planting into good seedbed and rooting (less-compacted) conditions is more important when soils are cool than when they are warm. We never want to work soils wet and plant under wet soil conditions if we can help it, but we certainly do not want to do that in April, especially when soil temperatures are less than normal.

So our first question should be whether or not the soil is dry enough; if the answer is no, then we wait. Cool soils dry slowly, and wet soils warm slowly, so waiting might take an extra measure of patience, especially if a neighbor brings out the planter. There is some comfort in the fact that germination and emergence are slow in cool soils, so planting a few days earlier when it’s cool makes very little difference in how far along the crop will be on a given date later in the season.

According to the Illinois State Water Survey, minimum temperatures 2 inches beneath bare soil on April 21 averaged about 40 degrees in the northern half of Illinois and in the upper 40s in southern Illinois. That’s a drop of more than 10 degrees over four days. And the weather forecast indicates that soils may not warm up much by the end of April. If we go by the old standard recommendation that corn should be planted only after the minimum soil temperature 2 inches deep exceeds 50 degrees, we would have planted for perhaps half the days in April through April 19, but none since then. Maximum temperatures 2 inches deep reached the 80s on April 17, but only averaged in the low 60s on April 21.

It takes soil temperatures of 50 or above to get the germination process underway, but does this mean that we should avoid planting corn into soils where temperature at seeding depth averages less than 50? Based on a lot of planting date work, we would say that the danger from doing this in minimal. It takes about 115 or so growing degree days (GDD, based on air temperature) after planting to get corn plants to emerge, and emergence has usually been good even when it has taken 3 weeks for this number of GDD to accumulate. Planting date has had little or no effect on emergence in most of these trials.

Normal GDD accumulation in the month of April ranges from about 180 in northern Illinois to 220 in central Illinois to 300 in southern Illinois. So far in April 2015, we have had about 200 GDD accumulate at Urbana. With the slowdown this week, the total will end up somewhere around the average for this month. We made our first planting here in the planting date trial on April 1, and it emerged more or less on schedule, around April 16. We can expect corn planted on April 22 or 23 to take at least this long to emerge, and longer if temperatures don’t rebound next week.

The chances of getting good emergence when planting into cool soils are higher if here is little or no rain between planting and emergence. Cool soils bring slow germination and emergence, but they may lower the chance of emergence problems due to soil crusting or to saturated soil. Crusting problems usually develop after intense rainfall followed by warm, dry conditions that help “bake” the crust. Warm soils mean more rapid growth of seedlings, which can mean running out of oxygen sooner if soils become saturated. So while we would prefer warmer and relatively dry soils, next best is having cool and dry soils. Most stand problems occur when soils are warmer, simply because that’s when the plants are trying to grow faster. Still, warm soils help bring the crop up, and we hope that they start to warm soon.

Heavy rainfall is not predicted for this coming week or so, which is a positive. Taking the longer view, temperatures in May will inevitably start to rise at some point in time, and this will speed up emergence. Taking all the factors together, I would suggest that planting proceed as long as soil conditions are good, even if the germination process will be slow due to cool soils in the near term.

One of the concerns being mentioned is “imbibitional chilling injury” that has been reported when seeds and seedling take up water that is colder than 40 degrees. This can stiffen plant cell membranes and lead to damage, in some cases distorting growth and reducing emergence. This has usually been linked with melting snow or very cold rainfall after planting. It’s something to keep in mind, but it has been rare in Illinois (we think there was some in corn planted around April 20-25 in western Illinois in 2011) and it should probably not keep us from planting in the last week of April. Higher, drier fields are less likely to suffer from this, and should be planted first.