We have recently observed some alfalfa fields with frost damage and received reports of some plant heaving. Alfalfa broke dormancy with the warm weather in late February and early March (see article by Jason Hartschuh), which was followed by hard freezes over several nights in mid-March.
Observations in one field indicate that freeze damage was worse in pure alfalfa stands than alfalfa mixed with grasses. There is a wide range of plant response to the freeze from dead stems (~5% of stems in pure stands) to very little injury and normal growth (~65% of stems). The remaining stems have dead leaves with somewhat normal growing points, or growing points where the leaves are curled up and not unrolling properly, or growing points with no new leaves but that are not brown. The growing point of other stems just does not seem normal.
Time and weather conditions through April and into May will tell how much this damage will affect the first cutting. We don’t expect to see permanent damage to established, healthy alfalfa stands from this late freeze, but stands that were weak at the end of 2016 will likely recover more slowly.
Back in 2007 we had a strong spring frost that occurred in late April that caused severe damage, killing the growing stems that were 4-7 inches tall at the time. Some stems recovered, but the more severely injured stems died back and new ones had to be initiated from the crown. Our alfalfa experiments on the Western Branch Research Station near South Charleston that year showed variable injury and recovery. An experiment with soil pH of 6.4 was more severely injured than nearby experiments with soil pH of 6.8 to 7.0. The more severely damaged experiment yielded a half ton per acre less in the first cutting, but as the growing season progressed it improved and reached yields similar to the experiments that suffered less injury.
Frost injury to new spring growth puts a strong stress on plants. Alfalfa plants in early spring are at a low energy reserve status. Most of the stored energy reserves have been used to survive the winter and to initiate new growth with the early warm temperatures.
Stands that were healthy and vigorous going into the winter last fall should recover just fine, especially those with good soil pH and fertility, good surface and internal drainage, and that were not cut in the mid-fall period last year. Stands that were in a less than ideal condition going into the winter will probably suffer more injury and recover more slowly, because their energy status is lower. Keep a close eye on fields in that condition during the next two weeks.
Where injury appears severe, dig up some taproots of alfalfa and split them lengthwise. If the inner root tissue is soft, spongy, and possibly discolored, then those plants will or already have died. In contrast, healthy root tissue will be firm and white. If topgrowth is frosted, but roots are healthy, then the plant should recover in time.
If fertility levels are below optimum, make corrective applications as soon as soils are firm and dry enough to support traffic. Weakened spring growth might result in more winter annual invasion (picture below shows chickweed present that could invade), so keep an early watch for weed development. Winter annuals are difficult to control in the spring, but options exist to slow them down if applied early enough. Also, keep checking fields for alfalfa weevil feeding as spring progresses, especially in fields that were frost-damaged. You might want to consider a lower action threshold in fields with weaker alfalfa growth (click here for more info).
In fields showing more frost damage now, be sure to delay the first harvest by a week to give the plants more time to recover energy reserves for initiating regrowth. This especially includes fields that were cut last fall, or that were severely injured by potato leafhopper last summer.