Alfalfa Heaving Occurs Throughout Winter

Mike Rankin
Crops and Soils Agent
University of Wisconsin - Extension

       According to a research from Cornell University, heaving of alfalfa plants occurs throughout the winter; not just during the spring freeze-thaw cycles as is usually thought.  In fact, mid-winter heaving exerts a far greater pressure on alfalfa plants than the early spring type.  Researchers monitored the movement of 100 2-year old and 108 3-year old plants through two winters at weekly intervals using a surveyor's level and attaching tags to the alfalfa plants at the soil surface.  Five types of heaving were characterized as follows:

1.  Essentially a "no-heave" situation:  Only a minimal amount of uplift occurs because of the small amount of soil surface heave over the winter.  In this situation, the forces anchoring the plant (i.e. branched taproot) are greater than the heaving pressure.

2.   The soil heaves with little plant uplift:   Evidently, the taproot provides enough anchorage to prevent the uplift of the plant, but was not branched enough to retard soil heaving.

3.   Soil and plant heave with elastic recoil of the tap root:   Crown uplift is less than soil heave due to the presence of lateral roots anchored deeply in the soil profile.  As the soil thaws in the spring, both the soil and crown return to their normal position with little, if any, plant damage.

4.   Irreversible winter uplift, but no "jacking" action in the fall or spring:  The final plant uplift reflects the maximum amplitude of the major winter frost heave cycle.  Over the winter, the upper part of the root system is lifted irreversibly above its original position and is likely accompanied by the breakage of lateral and/ or taproot.  In the spring, the plant crown remains above the soil surface as the soil thaws.

5.   Same as #4, with additional "jacking" during the spring freeze-thaw cycle:  Plants with weakened, shallow, or broken roots are especially susceptible.  No examples of jacking of mature alfalfa plants were found associated with freeze-thaw cycles. This means that the degree of heaving is determined primarily by the winter-long heave cycle rather than the spring freeze-thaw cycle.