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Crops Vary in Their Tolerance to Frost

Duane R. Berglund, NDSU Extension Agronomist

Reports this past weekend of many below-freezing temperatures from various growing regions of the state indicated substantial damage to some crops. The dry edible beans, corn and soybean crops appear to be the hardest hit, according to Duane R. Berglund, NDSU extension agronomist. 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 very vulnerable because of the earliness of the frost and the slow and late development of these crops. It has been one of the coolest summer growing seasons in the past 11 to 12 years, stated Berglund. The frost of last Friday is almost the equivalent to getting a hard frost in the last week of July in normal years since that was the plant development stage a lot of the late-season crops were in this year. 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 all can be damaged at temperatures below 30 F. 

Pinto, navy, blacks and all dry beans are very sensitive to frost (30- to 32-degree range). Tops of beans are easily killed and will turn a dark green to black color in a matter of a one-day killing frost. Lower developing, earlier pods in the bottom of the canopy may not be affected. Also, the vines and stems should be examined to see if any damage has occurred. Top of plants and late green pods or flowers are easily damaged by frost. Green beans will shrivel but should be left in the field until dry to separate them from mature beans. The major problem with the dry beans is they will attempt to re-grow and initiate new vines, stems and flowers. Berglund indicated that it is just too late in the season for the vegetative and reproductive structures to result in any yield. The re-growth also will take away energy that is needed to fill the pods and beans of the lower sets. The re-growth also becomes a harvesting issue. 

Soybean leaves are easily damaged by light frosts in the 31- to 32-degree range. In most instances in North Dakota and western Minnesota, the latest frost primarily damaged the upper part of the soybean canopy. The lower 2/3 of the canopy and the early pods were not damaged. Frost of 30 F or lower will cause damage to stems and green pods and this did occur in some growing regions. Beans in the upper part of the plant that are still green and soft will shrivel. Upper stems will rapidly turn dark green to brown and will not recover. Soybeans planted in narrow row spacings or drilled beans tended to have 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, indicated Berglund. Some of the uppermost pods may abort after the frost. Top yields, however, have 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 frost down to 26 F or colder. 

Sunflowers are most susceptible at flowering and during pollination. In North Dakota, more than 50 percent of the sunflower acreage was in the flowering stages of R5.1 to R5.9. Temperatures of 31 F or 32 F can result in sterile sections or rings in the flowering head, stated Berglund. Once 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 at down to 26 F 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 25 F temperatures occur at the bud stage, this often will damage stalk tissue below the bud and seeds will not develop. 

Corn usually is severely damaged by temperatures at 29 F or lower. Colder temperatures will kill the entire stalk. If only leaves above the ear are frosted by temperatures of 30 F to 32 F, 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 frosts and usually killed with temperatures under 32 F. 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 frosts. Sugar beets also are very resistant or tolerant to light frosts. 

Berglund also reminds us that temperatures of 32 F at weather stations or farmsteads may result in temperatures of 28 F to 29 F in low-lying areas of fields. He feels that is what happened last week. Also, time of exposure to freezing temperatures will influence the degree of damage done. Usually a two- to four-hour duration of a critical low temperature will cause damage. Two other factors that influence critical frost temperatures are soil moisture and wind velocity. When soil moisture is higher, frost injury is somewhat reduced due to heat slowly being released from the stored heat in the soil within a plant canopy. Wind movement also helps reduce freezing to some degree and is better than a still, cold night with no air movement. Cloudy nights also are better than clear nights.

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