By Emma Matcham
Throughout late January, much of our region experienced more snow, ice and frigid temperatures than we’re accustomed to. This column will address some common questions about winterkill of forage species.
Cold temperatures. When plants die over winter, it is typically due to either desiccation (drying out) or frost heaving. Cold, winter air holds less water than warm air, and water molecules tend to move away from wet areas and towards dry areas. This can lead to water evaporating out of plant leaves. Plants’ root systems have a hard time replacing that water when the soil is frozen.
Snow acts as an insulator, physically blocking wind from reaching plants and also keeping some moist air close to plant leaves. Pastures without snow cover have less protection from extreme cold weather and polar vortex wind events, which can lead to higher rates of winter kill.
Pastures that had low fall growth can also be more prone to damage over winter, since fall growth is important for plants to store nutrients before dormancy.
Also, areas with lower potassium fertility are more sensitive to extreme cold since potassium helps plants regulate their water content.
Frost heaving. Another common way damage to forage plants occurs over winter is through frost heaving, which happens when the upper layer of soil and plant roots lifts up and separates from lower, more frozen layers. This generally occurs when freeze-thaw cycles allow water to enter the upper layer of soil during warmer daytime temperatures before refreezing and expanding overnight.
Heaving can be particularly damaging to alfalfa, since its deep taproot can break off when the upper layer of soil moves. Well-drained soils generally heave less than poorly drained soils.
What to watch for. It’s hard to fully assess winterkill until plants break dormancy in spring, but paying attention to where snowfall is present and when can help inform where to inspect for damage during green up.
If your pastures had significant snow before the onset of cold weather, they are at lower risk of winterkill. Areas that received ice sheets instead of snow had less protection from lower temperatures, and pastures that don’t retain their snow coverage could be sensitive if we get another cold snap.
During green up, scout areas that had lower snow, poor fall growth or low potassium fertility. Plants that died from the cold weather will stay brown while the surrounding pasture starts growing.
If we get a few warm, sunny days with colder nighttime temperatures, I’ll be watching for frost heaving in areas with thin snow coverage or standing water. Frost heaving will make the soil surface look uneven and can lift plant crowns up from the surrounding areas. Damage from heaving can be deceptive since sometimes the plants will green up as normal, but if their root system is damaged, they may die once water is depleted in the upper inch or two of soil.
While we never like to see winterkill in our pastures, knowing where it could occur on your farm is helpful for predicting how much forage is available for your animals in spring.
Early identification of damaged pastures is also helpful for planning pasture management activities that can aid recovery, such as interseeding.
(Emma Matcham is an assistant professor, Horticulture and Crop Science Department, Ohio State University. She can be reached at Matcham.3@osu.edu.)
