Evaluating Early Season Frost Damage

Frost symptoms, tolerance varies by crop and conditions

Temperatures below 32 degrees will cause water in plant cells to freeze, but how different crops react to freezing temperatures depends on the crop, stage of plant development, duration of freezing temperatures, and intangibles such as cover and humidity levels.

Wet soils can provide a buffering effect to the cold, as seen in cases last year.  As well, environmental conditions before or immediately after low temperatures can greatly influence the extent of the damage. If a temperature drop is gradual, plants are in better condition to resist injury and can stand surprisingly low temperatures. Slowly rising temperatures after a frost and satisfactory soil moisture conditions are desirable to aid recovery. Drought, wind and high evaporation are likely to aggravate frost injury and decrease the chances of recovery.

Here are early frost notes and symptoms to look for by crop, from Kent McKay, area agronomy specialist, North Dakota State University Research Extension Center, Minot and NDSU extension agronomist Duane Berglund.

Small Grains: Are very tolerant to temperatures as low as 22° F; the growing point is below the soil surface until the 5½ leaf stage to jointing. After the 6-leaf stage and at jointing, the growing point moves above the soil line and the cereal crop becomes more susceptible to freezing temperatures. 

The only significant damage that could occur would be that if a frost was hard enough to collapse the stem, which would cause the plant/stem to wilt and most likely die. In most instances, wilted, dark green and discolored leaves will be the normal appearance. Recovery usually is quick with a new leaf emerging from the stem within 2 to 3 days with a normal green color.

Cereal grains will lose leaf tissue that freezes. New growth will follow at the growing point which is protected below ground, prior to jointing. In some cases, the eventual maturity date may be delayed. Several hard frosts and destroyed leaf tissue can and will greatly weaken plants and may result in yield reductions.

Winter wheat/jointed (5½ leaf): The extent of time that the plants were exposed to cold temperatures will be the issue. The worst case scenario would be that the developing spikelets were injured to where no kernels will develop in those spikes. This usually will be the top of the head and that area will be white in color along with the awns.

Canola, Flax, Mustard: These crops can handle a frost of 24° for a short time with canola the most tolerant to cold temperatures. Frosted leaves/plants will have a dark green or black in color. If discolored or injured, wait at least 2-3 days before any decisions are made to see if the growing point is alive. Within 3 days, there should be a new leaf emerging from the growing point located in the center of the plant. The best indication that the plant has been killed by a frost is the stem. If the stem below the cotyledons is wilted and doesn’t straighten out within 48 hours of the frost, the plant is most likely dead.

Pulse crops ( field peas, lentils and chickpeas): Similar to small grains, the growing point is below the soil surface to the fifth node stage or about the 4” height stage. If the stem is collapsed, wilted on the soil surface for 48 hours and regrowth has not occurred, that plant has most likely been killed. Recovery is usually quick with the newest leaf emerging from the stem within 3 days.

Sunflower: When in the cotyledon stages, can withstand temperatures in the 25-26° range for short periods of time if they are just emerging from the soil.  Sunflower true leaves in the 2, 4 or 6 leaf stages become more sensitive with each development stage and the terminal bud can be permanently damaged. It’s believed that when
sunflower are in the V2 stage, then the lower limit would be 26-27°,  whereas if sunflower are in the V4 or V6 stages then the lower limit would be 28-29°. If
sunflowers become brown or blackened and the terminal bud is damaged then the plants will not recover.

Corn: Historically very few northern corn fields have been destroyed by spring freezes, although research is limited on this subject. Plants less than six inches tall (V-5) will usually recover from frost, because the growing point is still below the soil surface and usually not damaged. Lethal cold temperatures (28° or less) can penetrate the upper inch or two of soil, especially if the soil is dry, and kill plant tissue including coleoptiles and growing points.

Non-lethal injury by cold temperatures may cause deformed elongation of the mesocotyl or physical damage to the coleoptile in non-emerged seedlings, resulting in the “cork-screw”symptom and subsequent leafing out underground.

Soybeans: Broad leaf crops that have the growing point at the top of the plant are more susceptible to frost damage than grass species. Thus, soybeans are quite sensitive to frost, easily damaged by frost in the 28 to 32° range. Temperatures of 28° for any extended period of time can completely kill soybean plants (buds, stems and leaves).

During the early seedling stage (VE to VC), soybeans have some tolerance to temperatures of 29-30° for short periods of time. If the seedlings have been somewhat hardened off by cool temperatures for several days, then temperatures as cool as 28° can be tolerated. Once true leaves emerge (V1 and V2), soybeans become more susceptible to freezing temperatures below 32° for any extended period of time. Soybeans in the unifoliolate leaf stage are slightly more frost tolerant than soybeans in the first or second trifoliolate stages.  

Average Date of Last Spring Frost

	28°	32°
Fargo	May 4	May 12
Williston	May 4	May 13
Oakes	May 5	May 13
Bowman	May 6	May 14
Mandan	May 7	May 12
Grand Forks	May 6	May 16
Fessenden	May 8	May 17
Dickinson Exp. Station	May 12	May 19
Minot Exp. Station	May 12	May 20
Langdon Exp. Station	May 17	May 28