Written collaboratively by Laura Edwards and Ruth Beck.
The warm spring season this year has us all looking forward to planting our fields and home gardens. Winter wheat has greened up, and flowers are making their appearance three weeks early or more. Even though we are looking forward to getting our seeds in the ground, it’s still not too late for a spring frost to affect our plants.
In a short assessment of dry winter seasons (December through February), SDSU Climatologists considered spring frost dates. They examined the top ten driest winters in the cities of Aberdeen, Columbia, Watertown, Brookings, Sioux Falls, and Winner. In all but Sioux Falls and Watertown, seven out of ten dry winters were followed by later than normal spring frosts (28 degrees F or lower). The median dates for spring frosts are between April 26 and May 3 for these locations. In some cases, these late frosts were two to three weeks later than the median date. In Aberdeen, the average frost date fell about 2 weeks later than the median, and at Brookings, the average is about a week and a half later than median following these dry winters.
Even though the winter of 2011-2012 did not rank in the top ten driest winters at any of these locations, the median date for spring frost is still weeks away, and there is a risk of a killing frost. There is a physical explanation on how this can happen. Less soil moisture following these dry winters can lead to less water vapor in the air where the plants are growing. Dry air can cool faster than moist air, so there is an increased risk of lower temperatures during the overnight hours.
The 2012 winter wheat crop in South Dakota currently looks promising. There appears to be little crop loss from winter kill and with good subsoil moisture there is potential for good yields over much of the state. However the unusually warm spring has brought the winter wheat rapidly out of dormancy and growth is well ahead of normal for this time of year in South Dakota, raising fears that a late frost could really be damaging.
There are a number of factors that can affect the amount of crop damage in wheat from a late frost. Growth stage of the wheat is very important. Wheat is more susceptible to frost damage at later growth stages. Once the wheat plant reaches jointing and elongation, the growing point is above ground. Frost at this point can kill the growing point, greatly reducing yields.
Moisture content of the soil can affect frost damage, partially because less soil moisture increases the risk and severity of frost as explained above, but also, plants that are very healthy (good fertility and no drought stress) can be more susceptible to frost damage than stressed plants. This is related to higher water content in the plants
The duration of the sub-freezing temperatures also affects the amount of damage. If temperatures reach 24 deg. F when plants are elongating, damage is not thought to occur unless the temperature stays at that level or lower for at least two hours.
The presence of heavy amounts of light colored residue can also impact the plants’ response to frost. If the plants’ growing point is below the residue layer, it is somewhat protected. If the plants growing point is above the residue, it is damaged more severely than when residue is lower or darker in color.
And finally, cold air is heavier than warm air, so cold air will flow towards the lowest areas of fields. When assessing frost damage, producers should visit the lowest spots first, as this is where frost damage should be the worst.
For more information on frost damage on winter wheat visit “Spring Freeze Injury to Kansas Wheat”.