Paul Peterson became concerned about the winter survival of Minnesota alfalfa fields when the ½”-deep soil temperature in St. Paul dropped to -13° on Jan. 19 and -1° the following night.

Growers and forage agronomists throughout much of the Upper Midwest have reason to worry, says the University of Minnesota Extension forage agronomist. Nighttime temperatures there dropped to zero or lower on four consecutive January nights and fields were mostly without snow.

Agronomists used to fret when soil temperatures in alfalfa’s crown region – the top 4” – dropped below 15°. Peterson believes newer varieties can handle temperatures lower than that, but the ones recorded in January may be too low.

Some alfalfa stands may have been damaged but not killed, he suspects. Winterkill usually results from a combination of stress factors, and typically happens during alternate freezing and thawing in spring.

“But there’s a potential for this cold-temperature exposure to increase susceptibility to that later on,” says Peterson.

Older stands on poorly drained soils or soils with low fertility are most susceptible to winter damage. Varieties with insufficient winterhardiness and without multiple disease resistance are at risk, too, as are fields where growers removed the insulating benefit of fall regrowth by taking late cuttings.

That benefit can be significant. In St. Paul, when the -13° and -1° temperatures were recorded under a bare soil surface, soil under a bluegrass sod didn’t drop below 17°, he reports.

Because of their added insulation, alfalfa-grass mixtures are probably more likely to survive a rough winter than pure alfalfa stands, says Peterson.

The greatest risk of winter damage may be in North Dakota, which was entirely without snow during the cold spell. Nighttime temperatures dropped to -19° in Fargo on Jan. 20, and soil-surface and 4”-deep readings in Marisol Berti’s research plots hit 8° and 15°, respectively.

“That’s pretty close to where the literature says we’re going to get the plants killed,” says the North Dakota State University forage agronomist. “But I’m not worried about the alfalfa as much as the grasses. Some of the grasses, like orchardgrass and tall fescue, aren’t as winterhardy as alfalfa. We’re hoping the alfalfa will hold, but we won’t know until spring what this weather did.”

The fall was very dry, so alfalfa plants likely entered dormancy in good shape, says Berti. But the plants may have started to break dormancy in early January, when daytime temperatures were in the 40s and 50s for a week.

“If they did break dormancy, then we got the cold spell, it’s not going to be very good news in the spring,” she says.

That happened in some southern Wisconsin and northern Illinois fields, reports Dan Undersander, University of Wisconsin Extension forage agronomist.

“We’ve had some warm-enough days for some of the less-dormant types to begin to green up and then freeze back,” says Undersander. “That generally will not kill the alfalfa if it only happens once. But it will slow down the growth next spring, so it’s a yield loss.”

Most of Wisconsin got several inches of snow the week before the cold spell, so damage was minimal.

“But we’re losing our snow cover again, and we could have colder weather,” he says. “We just have to keep praying for snow and wait and see what happens toward spring.”

He lists two management practices that can minimize winter damage in the future. The first is supplying adequate fertility.

“People have not put on near as much potassium as they’ve been taking out of the soil because of the high cost. Low potassium will increase the amount of winter injury and winterkill.”

Second, he says to choose varieties with strong winterhardiness.

“We have a good test for winterhardiness, and we have seen yield losses on anything that tests above 2 in terms of winter survival,” says Undersander.