Winter’s freeze-thaw cycles can cause frost heaving in alfalfa and other winter crop fields. The colder than usual winter in Ohio this year could increase frost heave risk. This article covers the basics of identifying frost heave and planning for spring management if needed.  

What is frost heave? Frost heave, also called heaving, is a lifting of the upper layers of soil and plants due to the expansion of water as it freezes, similar to how ice formation during freeze-thaw cycles expands potholes on roads. Frost heave can happen on soils regardless of whether they have crop or bare dirt, but it is generally only problematic when it damages the roots of cash crops.  

Figure 1: Frost heaving in alfalfa. This photo in April shows unheaved plants with early spring growth, and heaved plants showing broken taproots (photo credit: Dave Barker) 

Figure 2: Close-up of frost heaved alfalfa plants, showing i) exposed root and crown of damaged plants, and ii) partial recovery of less affected plants (photo credit: Mark Sulc) 

How does frost heave impact alfalfa? Physical disturbance of the soil can move alfalfa crowns above the soil surface increasing the chance of winter injury and possibly breaking plant roots. If frost heaving is shallow, affecting only the topmost inch or so of soil, the impacts are typically minimal. Crown damage is more likely when very cold temperatures occur after heaving, since crowns and upper roots are less protected from extreme temperatures when they’re raised above the soil surface. Taproots are typically too deep to move with the crown, so they will break instead of moving above the soil surface. In many cases, the plants with broken taproots can be pulled out by hand, often revealing the tap root broken at deep as 10-12” below the surface. Taproots contain nutrients and starches that help alfalfa jumpstart spring growth, and spring growth will be sluggish when taproots are broken. In most cases, the crucial fine roots are broken from the taproot, and the plant is unable to recover. Alfalfa can typically only survive a few weeks with a broken taproot; plants will initially green up in the spring as normal but begin to die once water is depleted in the upper two inches of the soil profile and summer stresses take hold, since superficial roots cannot access deeper moisture.  

What weather is associated with frost heave? Frost heave is more common when lightly snow covered or frozen soils thaw partially and then refreeze due to liquid water entering gaps in the soil profile and expanding as the water and soil refreeze. The weather patterns that contribute to expansion of potholes on the road are the same weather patterns that lead to frost heave in soils.  

Can frost heave be prevented? Well-drained fields typically have less frost heaving damage, since snow melt drains and reduces freezing damage. Managing fall cutting to preserve stubble over winter can also reduce heaving, since more stubble helps retain snow cover and reduces standing water. Other management strategies like choosing well-adapted varieties and maintaining adequate soil fertility can improve root system health, which can reduce root damage if heaving does occur. In cases where interseeding with a grass is an option it can lower the risk of heaving but is not going to remove all instances. 

How can frost heave be managed?  Identifying fields affected by frost heave is important so that you can determine whether root damage was extensive enough to warrant replanting. Spring rains can cause some soil and plant settling, which helps plants recover but can also camouflage taproot damage. Be sure to assess root health in addition to stand density. If the damage was extensive (more than 10-20% of stems affected), terminating the stand and planting annuals may be necessary. In less severe cases, delaying first cutting and raising your cutting height is a management option that can minimize the risk of cutting the crown of heaved plants and promote root system recovery. Attempting to push heaved plants back into the ground with a roller or cultipacker will likely cause more damage than it fixes.