Assessing Climate Trends, Soil Temperatures, And Planting Risks

By Tyler Williams

Early Warming of Soil

Planting in proper conditions is a perennial concern for Nebraska farmers.  With planting merely weeks away, analyzing soil temperatures can help determine when conditions are conducive for proper plant germination.  Soil temperatures in Nebraska are highly variable from year to year and location to location, and this year is no different.

Figure 1. Median date of last 28°F freeze (1981-2010) in Nebraska.

Current soil temperatures in Nebraska are 5-10 degrees above normal and the seven-day average temperature at some locations is already above 50⁰F, causing some to ask: Is there a way to take advantage of this early season warming? Unfortunately, there is no simple answer due to extreme variability from field to field and the risk from Nebraska weather.  It is not that uncommon for a snowstorm to move through Nebraska in May, leaving a long cold spell in its wake.

While current soil temperatures may have warmed to adequate temperatures for planting in some areas, it is imperative that growers consider all their risks and current climate trends and information before planting.  There may be some benefit to early planting in order to capture a longer growing season, but you should consider whether the reward warrants the risk.

Last Frost Dates

The average last frost date is one way to analyze the risk of frost damage to your crops.  Figure 1 shows the median last 28⁰F freeze dates for Nebraska, averaged from 1981 to 2010, and ranging from the first week in April in southeast Nebraska to the second week in May in northwest Nebraska. NebGuide G2122, "Soil Temperature: A Guide for Planting Agronomic and Horticulture Crops in Nebraska," describes the recent shift in soil temperatures. From 2000-2009 soil temperatures of 55⁰F to 70⁰F occurred one to two weeks earlier than they had in the previous decade.  While soil temperatures may have shifted, allowing for earlier planting, the potential for a late spring freeze still exists. (See NOAA NCDC information on freeze/frost occurences for Nebraska.)

Planting Time Weather Conditions

 The temperature outlook for April from the Climate Prediction Center (Figure 2)  shows Nebraska as having equal chances for above or below normal precipitation. It indicates that the trough pattern that has prevailed over the northeast US for the last couple months will continue to dominate in April.  This means Nebraska will again be in the transition area between warm temperatures out west and cool temperatures in the east.  In March cold temperatures shifted east, but that line may shift back west.  We could see temperature swings and an east to west temperature gradient across the state, which could stall soil warming.  This will make it difficult to forecast more than a week in advance.

Figure 2. National Oceanic and Atmospheric Administration (NOAA) forecast for April 2015, made mid-March. Links to larger version.

Warming Trend Can Still Take a Dip

If we look at current trends, there are a lot of reasons to believe soil temperatures will be adequate for planting over the next couple weeks; however, this trend may not continue.

We have experienced above normal temperatures and below normal precipitation through March, which has allowed our soils to dry out and warm up. While southeast Nebraska has had recent precipitation, most of the warmer, drier soils in southwest, central and northern Nebraska did not receive much precipitation.  Dry soils will warm up faster than wet soils and warm soil temperatures will help to warm the air, compounding the warm-up and creating a situation similar to 2012. If this trend continues, we may have an opportunity to capture a longer growing season; however, forecasts aren't showing much confidence in this trend continuing in Nebraska. Waiting to plant may be the best insurance plan.  As we get into the first couple weeks of April, we may be able to use weekly forecasts to better assess planting risk.

Source:unl.edu