Introduction:
     After a wet and relative warm winter the South was hit with severe cold weather on March 11, 1998. Most of the wheat already was out of dormancy and starting to joint. Most producers were able to put out fertilizer in February and this resulted in green, lush wheat when the freeze hit. Unfortunately the lush growth made the wheat more prone to cold injury.

Winter wheat needs low temperatures:
     For proper growth winter wheat needs a period of cold temperatures and short day length. During this period the transition from vegetative to reproductive growth takes place. This transition takes place in stage 3 of the Feekes scale. The wheat plants usually have 3 to 4 tillers before stem elongation takes place. In this stage wheat plant are the least vulnerable to low temperatures. The gradually decrease in temperature in the fall hardens the plants even more. Temperatures of minus ten will be tolerated by the wheat at this stage. Planting at the recommended planting dates will usually give the wheat ample time to reach this stage.

Why is wheat damaged by a spring freeze?
     In February, as soon as temperatures rise the wheat begins growth and becomes more susceptible to low temperatures. The application of nitrogen enhances damage by low temperatures. Nitrogen application results in a lush growth of the wheat with ample water in the plants. Crystals formed in the cells at low temperatures disrupt the cell structure and damage the leaves.

Temperatures that cause spring freeze injury:
     The degree of injury depends on the temperature and the duration at that temperature. During tillering in spring time a temperature of 12°F for two hours will damage the plants. At jointing a temperature of 24°F for two hours will damage plants. Low spots in the field or fields surrounded by hills might hold the cold air longer than other parts and show more damage to the wheat plants.

Symptoms of spring freeze injury:
     In the fourth leaf stage wheat still can resist low temperatures. The growing point is still under the soil surface and reasonably protected. However above ground plant parts might freeze. Symptoms at this stage are: leaf chlorosis; burning of leaf tips or silage odor. In some cases, instant cold damage will occur and the fields will have a blue color. Leaves will be dark green/blue and look water soaked. At jointing the growing point is now above ground and very vulnerable. Cold damage symptoms at jointing can include leaf chlorosis or burning and silage odor. In addition stems may split or develop lesions and form additonal tillers.
     In case of lesions the growing point might still be alive while the stem below is dying off. In this case the growing point is not killed directly by the cold but by the limited or stopped supply of assimilates to the growing point. Symptoms like chlorosis of the leaf or burning of the leaf tips can be delayed as much as 14 days in cool weather. These symptoms might only occur on the main tiller, while the growing point of some or all of the side tillers are killed directly by the cold.

     A description of freeze injury symptoms at different growth stages is found in table 1.

How can I tell if growing point is still alive?
     A wheat field with the above ground parts frozen can look devastated. However before deciding to replant with other crops check the growing point of some of the wheat plants. The growing point will be just above the uppermost joint. Symptoms will be easier to detect several days (7 to 10) after the freeze. Cut a stem longitudinally with a knife or razor blade. A healthy growing point has a crisp, whitish-green and turgid appearance. A damaged growing point will turn yellowish to brown or gray and look water soaked. A hand lens will help to detect subtle freeze damage symptoms. At jointing, a yellow or necrotic leaf tip that appears from the whorl indicates that the growing point is damaged. However, this symptom is not always obvious. The best way to check for freeze injury is to examen several whole plants and checking the growing point of all tillers, 7 to 10 days after the freeze.

Table 1: Temperatures that cause freeze injury, symptoms and yield effect of spring freeze injury to winter wheat at spring growth stages.

Adapted from Shroyer, James P., Merrel E. Mikesell and Gary M. Paulsen, 1995. Spring freeze injury to Kansas wheat, Agricultural experiment station and cooperative extension service, Kansas university, Manhattan.

Fertilize or replant another crop?
     Before putting out fertilizer or deciding to replant another crop after a cold spell, wait for a week. Then check the wheat fields. Regrowth might have taken place. Winter wheat can recover from cold damage when it occurs prior to the fourth or fifth leaf stage. The yield is effected only slightly to moderately. At later growth stages check the growing points as described above before making any decisions. Yield reductions will be moderate to severe when wheat is damaged at growth stages later than the fifth leaf stage (past tillering; See Table 1).

     More information can be found in the excellent online publication C646 'Spring Freeze Injury To Kansas Wheat' from Kansas University's Extension Service or download a local copy of the same publication C646.