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August 25, 2004 Crops Vary in Their Tolerance to Frost Reports this past weekend of many below-freezing temperatures from various growing regions of the state indicate substantial crop damage occurred, according to Duane Berglund, extension agronomist at North Dakota State University. “The dry edible bean, corn and soybean crops were the hardest hit. The areas with the greatest freeze damage appear to be the northeastern, east-central, and north-central sections of the state.” A lot of the late-season row crops were vulnerable because of the earliness of the frost and the slow, late development of these crops. “It has been one of the coolest summer growing seasons in the past 11 to 12 years,” Berglund says. “Last Friday’s frost is almost equivalent to getting a hard frost the last week of July because that was the development stage a lot of the late-season crops were in.” The degree of damage will vary from crop to crop and also depends on the topography of the field, the stage of maturity and other factors. Crops such as dry edible beans, soybeans and corn can be damaged at temperatures below 30 degrees. Pinto, navy, blacks and all dry beans are very sensitive to frost (30 to 32 degree range). Bean tops are easily killed and will turn a dark green to black color. Earlier pods at the bottom of the canopy may not be as affected. The vines and stems should be examined to see if any damage has occurred. Plant tops and late green pods or flowers are easily damaged by frost. Green beans will shrivel, but should be left in the field to dry in order to separate them from mature beans. Dry beans will try to re-grow and initiate new vines, stems and flowers. Berglund notes that it is too late in the growing season for the vegetative and reproductive structures to provide any meaningful yield. The re-growth will also take away energy that is needed to fill the pods and beans on the lower sets. The re-growth also becomes a harvesting issue. Soybean leaves are easily damaged by light frost in the 31 to 32 degree range. In most instances in North Dakota and western Minnesota, the recent frost primarily damaged the upper part of the soybean canopy. The lower two-thirds of the canopy and the early pods were not damaged. Temperatures under 30 degrees will cause damage to stems and green pods, which did occur in some growing regions. At those temperatures, the upper part of the soybean plant that is still green and soft will shrivel. Upper stems and leaves will rapidly turn dark green to brown and will not recover. Soybeans planted in narrow-row spacing or drilled beans tend to have a better tolerance to freezing temperatures than wide-row planted soybeans or those soybeans in sparse populations. Soybeans, unlike dry beans, usually will not attempt to re-grow and put on more vegetative or reproduction tissue once the tops have been damaged. “Soybeans in lower pods that are filling should mature normally,” Berglund says. “Some of the uppermost pods may abort after the frost. Top yield potential has been lost in most of these damaged fields. Some delays in maturity may occur.” Flax is most susceptible during flowering and the early-boll stage. Immature seeds can be killed by temperatures from 28 to 32 degrees. After flax reaches dough stage, it is more resistant to frost and should be able to stand temperatures down to 26 degrees or lower. Sunflowers are most susceptible at flowering and during pollination. In North Dakota, more than 50 percent of the sunflower acreage was in the R5.1 to R5.9 flowering stages. “A temperature of 31 or 32 degrees can result in sterile sections or rings in the flowering head,” Berglund says. “When it warms up, the remaining portion of the head will pollinate normally. The sunflowers still in the bud stage, such as R4, are fairly tolerant to frost and can handle temperatures down to 26 degrees before injury. After pollination and petal wilting and drying (late R-6), sunflowers can withstand temperatures as low as 25 degrees with only minor damage. If temperatures dip to 25 degrees while the sunflowers are at the bud stage, stalk tissue below the bud is often damaged and seeds will not develop.” Corn is usually severely damaged when temperatures reach 29 degrees or lower. Colder temperatures will kill the entire stalk. If leaves above the ears are frosted by temperatures of 30 to 32 degrees, which is a light frost, then kernel development will continue. If the entire stalk, ear shank and leaves are frozen, kernel development will cease and soft, shriveled corn will result. Frosted, immature corn is best used for silage or fodder. Most other crops such as buckwheat, sudangrass, sorghum and proso millet are easily damaged by light frost and usually killed when temperatures fall below freezing. Buckwheat is reported by the Canadians to be very sensitive to frost prior to the mid-dough stage. Buckwheat, once frozen, will lodge and fall over within a day or two following a hard freeze. Potato tops and vines will turn black but tubers are not usually damaged by light frost. Sugar beets are also very resistant or tolerant to light frost. “We had reports of temperatures at 31 to 32 degrees at weather stations and farmsteads, but in low-lying areas, the temperature probably dipped to 28 or 29 degrees or even colder,” Berglund says. “Time of exposure to freezing temperatures will also influence the degree of damage. Usually, a two-to-four hour frost will cause damage. Soil moisture and wind velocity are also critical factors. When soil moisture is high, frost injury is somewhat reduced because the stored heat in the soil within a plant canopy isn’t released as quickly. Wind movement also helps reduce freezing to some degree by mixing the air. Cloudy nights are also better than clear nights.” ### Source: Duane
R. Berglund, (701) 231-8135, duane.berglund@ndsu.nodak.edu |
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North Dakota State University |
