Alfalfa Stands: Watch and Wait

By Jane Fyksen, Crops Editor
Thursday, April 12, 2007 11:07 AM CDT

"Here we go again," say Paul Peterson and Dan Undersander, "neighbor" university forage agronomists in Minnesota and Wisconsin, respectively. A lot of alfalfa broke dormancy courtesy of unusually warm weather followed by the more recent freeze.

In between it rained, resulting in no snow cover. Of most concern, of course, is the deep-freeze (with soil in many portions of the state also water-saturated).

Alfalfa crowns and roots in frozen water-saturated soil are prone to heaving, particularly on heavier clay soils, they note of temperatures that have plummeted into the teens.

At the University of Minnesota's St. Paul Campus, under bare soil, the temperature down a couple inches hit 22 degrees. Under sod, temperature down two inches was a warmer 34 degrees. The critical low temperature for an alfalfa crown that's broken dormancy isn't known; a lot of things affect it. However, the two Upper Midwest forage experts think it's in the range of 20 to 25 degrees. Alfalfa leaves and growing points can be nipped when air temperature gets into the low 20s. Growers can expect any shoots that developed to die back.

However, alfalfa crowns under a lot of residue or in sod-like conditions are better insulated from cold air temperatures, they assure.

Mike Rankin, Fond du Lac County crops agent, thinks it's a "hair early" to make a final determination on how the recent cold impacted the alfalfa. If new growth from crown buds at or above the soil surface froze, then new growth will have to come from below ground. Then growers can "hope there's enough root carbohydrate to sustain new growth," he remarks.

"Alfalfa until the cold snap looked good," he says. "In the southern part of the state, there was a fair amount f growth on it, too."

He, too, says temperatures in the mid-20s can freeze alfalfa tissue. "Been there and done that," he wryly states of the recent thermometer plunge. But Rankin, too, notes that the "micro-climate at the soil surface" is another thing.

"Wind helps, too," as it doesn't frost quite as easily, he adds.

"By the end of this week, we should know where we stand," he told Agri-View at the paper's Monday press deadline.

The forage experts advise growers to examine their alfalfa as soon as possible and dig a few plants. Look carefully at shoots that may have already developed, the crowns, and crown buds. If soil temperatures are still below freezing, crown tissue and the upper root area will probably feel solid initially, so growers should bring some plants inside and warm them up. Crown tissue that's been damaged will turn soft and water-soaked, while crowns tissue that's still healthy should remain firm. It's too early, they say, to see any rot resulting from this freeze.

Bruce Anderson, University of Nebraska forage specialist, is also worried about the alfalfa in his state, as are agronomists in many others to Wisconsin's south. "To make management decisions, though, ignore the thermometer and watch the plants," he stresses.

"Focus attention on the 'growing point.' This growing point, also called the apical meristem, is the initial development source of all new leaves, stems, and branches on the aboveground alfalfa. It is located inside the dense cluster of unfolded leaves at the top of the main stem. Because it is inside a cluster of leaves, the growing point is somewhat protected from cold injury. Exposed leaves and stems all around it can be frozen, wilted, and dying while the growing point cluster survives, waiting for warm weather before continuing to grow," says Anderson.

Depending on how advanced the alfalfa was, if the growing point froze and there's wilt below that cluster, a significant delay in recovery is likely.

During forthcoming warmer days, watch for new growth emerging from the tip. This means the plant is recovering nicely. If new growth emerges as branches below the tip, it means the growing point was killed, but the plant is recovering. Plant development will be significantly slower.

If new shoots emerge from crown buds, Anderson says it means the growing point was killed and little new growth can be expected from existing shoots. Let new shoots develop and expect to take first cutting much later than normal.

The other scenario is if nothing happens. According to Anderson, this is the most likely reaction. If the growing point cluster froze and has wilted severely, additional growth from the existing plant is highly unlikely. New shoots will come eventually, but much slower and less dense than if existing injured growth is removed.

As for last year's planting, recovery, especially from crown buds, is likely to be much slower due to small root and crown containing low level of nutrient reserves.

Anderson says any plants damaged beyond singed leaf margins can be expected to become ready for first harvest later than if a freeze had not occurred (assuming one did; as noted, earlier this week, that wasn't necessarily a given). Be aware, though, that these plants may not bloom as they normally would. Don't rely strictly on bloom for harvest decisions, he reports. Instead, use your usual calendar date or appearance of new crown shoots as a harvest guide. Plan accordingly.

Say Undersander and Dennis Cosgrove, UW-Madison forage specialist at UW-River Falls, slow green-up is one of the most evident results of winter injury. If other fields in the area are starting to grow and yours is still brown, you'd best start wondering.

Asymmetrical growth is another sign. Buds for spring growth are formed last fall. If parts of the root are killed and others not, only the living portion of the crown will give rise to new shoots resulting in a crown with shoots on only one side or asymmetrical growth.

Look for uneven growth too. During the winter, some buds on the crown may be killed, others not. The uninjured buds start to grow early, while the killed buds need to be replaced by new buds formed in spring. This results in shoots of different heights on the same plant; shoots from buds formed in spring are several inches shorter than ones arising from fall buds.

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