Frost heaving can significantly damage alfalfa plants, leaving stands with much less or even zero yield potential. Plants may initially appear undamaged, but taproots are typically broken and unable to pick up enough nutrients or moisture, and stands eventually die.
Frost heaving of alfalfa happens when repeated freezing and thawing pushes the tap root and crown out of the soil. Heaving occurs through-out the winter season heave cycle (similar to fence posts), as well as resulting from the “jacking” action of the spring freeze-thaw cycles.
Factors that increase the risk of frost heaving in alfalfa include:
- heavier soils (clays),
- soils that were saturated going into winter,
- fields that lack insulating and snow-holding residue because a fall harvest was taken,
- older stands with larger tap roots and crowns, and
- straight alfalfa as opposed to alfalfa-grass mixtures.
Late seeded summer seedings of alfalfa that did not form sufficient crown going into winter are also higher risk of heaving.
Plant survival will depend mainly on whether the tap root is broken or not. If the crowns are heaved less than 2.5 cm (1 inch), the taproot is probably not broken and the stand is salvageable. These stands will likely “reseat” themselves over the season by natural settling and secondary root growth. If crowns are still elevated by the following winter, exposed crowns will be more susceptible to dessication (drying out), freezing, mechanical damage and disease, and plants are more likely to winterkill.
Do not cultipack or roll heaved alfalfa fields, as this does more damage than good by damaging and breaking crowns. Raise the cutterbar at harvest to avoid crown damage.
Moderate To Severe Heaving
If crowns are heaved more than 4 cm (1.5 inch), the taproot is likely broken. Heaving can sometimes elevate crowns 15 – 20 cm (6 – 8 inches) or more out of the soil. Obviously, broken taproots cannot heal. Broken plants will green up, but then die, depending on how deep the break is, so digging some plants will help in assessment. Plants with taproots broken 8-10 cm (3-4 in) below ground will likely die soon. Part of a first-cut might be salvageable if roots are broken below 15-20 cm (6-8 in). Depending on how elevated the crowns are, these fields can be difficult to harvest without further damaging crowns. If possible, rotate these fields to corn and take advantage of the nitrogen credit and rotational benefit.