By Mike Stamm
The question of whether or not the canola crop is going to survive the current stretch of bitterly cold temperatures is one worth asking. Considering temperatures haven’t been this cold across the state since the late 1980s, we have entered uncharted territory for the crop. Winter survival of canola in Kansas is a complicated issue. Winter stand losses can be caused by one or more abiotic and biotic stresses including poor plant establishment, low temperatures, the duration of cold temperatures, wind desiccation, dry soils, soil heaving, and damage by diseases and pests. A cultivar’s freezing tolerance, its ability to cold harden, and its interaction with the environment ultimately determine whether a crop will survive cold temperatures.
Selecting a winter hardy variety and using good farming practices are the best ways to support improved survival. Canola that possesses 6 to 10 true leaves, 6 to 18 inches of fall growth, a ¼ to ½ inch root diameter, and an extensive root system has reached adequate growth for overwintering. The dormancy period is when winter canola is most tolerant to cold temperatures. During this stage, the rosette should be at the soil surface with no visible stem elongation.
The winter hardening process
In order to survive the winter, canola must go through a hardening process. This begins in the rosette stage in the late fall after several days of near-freezing temperatures (about 35ºF). At these temperatures, plant growth is slowed, resulting in smaller cells with a higher concentration of soluble substances more resistant to frost damage. A few hard freezes (about 26ºF) are beneficial to halt leaf growth and promote hardening. Longer acclimation periods with fewer dramatic swings in temperature are beneficial to hardening and increased freezing tolerance in plants.
Hardened winter canola can endure a certain amount of time with temperatures at or below 0° F. However, extended periods of temperatures at or below 0° F, especially without snow cover, can be detrimental to survival. Frequent diurnal temperature fluctuations above and below freezing were common with temperatures trending warmer than normal in January. Despite having more green leaf tissue than what we typically see at this point of the growing season, the majority of the crop was healthy and hardened off heading into February.
Factors involved in the “un-hardening” of canola
Ultimately, it may not be the cold temperatures per se that cause winter kill but the rapid fluctuations in temperature, which can be a common occurrence in Kansas during the winter. “Un-hardening” of canola is accelerated when temperatures increase to 60ºF or above for an extended period of time (approximately 2 weeks). Un-hardening is a loss of freezing tolerance. However, the effect of fluctuating temperatures and un-hardening during the winter is complicated.
Research conducted by K-State indicates winter warming trends can actually have a positive effect on winter survival in some ways. Green leaf tissue may have increased metabolic activity, rejuvenating the overwintering plants. This partly explains why plants growing in the field can survive colder temperatures than plants acclimated at continuous cold temperatures in a controlled environment. If the warming trend is followed by a gradual cool down and no stem elongation occurs, then plants can re-harden. In addition, as long as low nighttime temperatures accompany warmer daytime temperatures, the rate of un-hardening should be slowed.
Assessing winter canola stands
Depending on how temperatures fare the rest of February, it will be a few weeks before we can fully assess the effects of the recent cold temperatures. After temperatures begin to improve, producers can evaluate their stands for winter kill. When evaluating winter survival, look for green leaf tissue at the center of the rosette or crown. If green leaf tissue is present and the crown is firm when squeezed, it is likely the crop will resume active growth as temperatures rise and day length increases. The root may be examined as well for firmness and vigor. If temperatures warm for several days and the crowns remain limp and fleshy, this could be indication that cold temperature damage has occurred. Remember that the crop can sustain some winter stand loss and still produce an acceptable yield as long as the losses are evenly distributed across the field. Normally, a final winter survival assessment can be made after the danger of further stand loss has passed, which is usually mid-March to early-April in Kansas.
In the picture below, you can see winter canola in a plot near Manhattan, KS. Note the presence of green leaf tissue (Figure 1). If winter temperatures would have been colder up to this point, most of this leaf area would have been lost by this time. We expect this green leaf tissue to be lost after the current cold period and that is acceptable. As long as the center crown and root remain green and firm, the crop has the potential to recover. Some fields will have the benefit of snow cover, which will help to insulate the plants from the bitter cold.
Figure 1. Winter canola nursery near Manhattan, KS on the morning of February 15, 2021. At the time of this picture, overnight low temperatures had been below 0°F for four consecutive nights. A low temperature of -18°F was recorded on the night of February 16. Source : ksu.edu
Winter hardiness traits in canola cultivars
Winter hardiness is an important trait to consider when selecting a cultivar for any canola cropping system. Differences exist, however, so decisions should be based on results from multiple years and locations.
To increase canola’s consistency in the southern Great Plains region, the canola breeding program at K-State continues to select and incorporate winter hardiness traits. Breeding accessions possessing longer vernalization periods are being crossed into the germplasm pool. One theory on improving winter hardiness is that canola can harden more easily after a winter warming trend prior to the vernalization requirement being reached. Therefore, extending the vernalization requirement may allow plants to withstand more variations in temperature during the winter months. The record breaking cold temperatures are some of the coldest to ever impact the breeding program’s selection for winter hardiness. The impact on crop development over the next several months will be important to characterize.
New research insights on winter survival
A recent review conducted by a team of researchers from K-State (M. Secchi, PhD student; M. Stamm; and Dr. Ciampitti) in collaboration with other relevant industry partners provided new insights on the role of environmental variables on winter canola survival. The main objective of this study was to improve our understanding of the impact of meteorological factors on survival of winter canola, in addition to providing an assessment of the risks for winter kill. Research data was obtained from the National Winter Canola Variety Trial from 2003 until 2018 (190 site-years) and auxiliary meteorological data over the last 40 years.
Environment was the main factor explaining the variation in winter survival, accounting for 71% of the variation on this variable. Overall winter survival averaged 84%, but a large range of variation across all site-years was present. The main meteorological variables explaining mean winter survival were the number of days with temperatures between 14°F and 5°F, the number of cycles when temperatures fluctuated above or below 32°F, and wind chill temperature during the cold period (winter). Lastly, variety selection is a key factor for improving the probabilities of obtaining better winter survival. This information will be valuable in assessing new growing environments for winter canola and will aid breeding programs in evaluating the impact of environment on selection for this trait.
Winter survival will depend on the ultimate cold temperature, the duration of those temperatures, and the variety selected. Improving our understanding on the main factors affecting winter survival is critical for canola production. When scouting your canola after this bitterly cold period, check for a green, healthy crown. If you find that, then expect the crop to survive. Re-growth will begin when average daily temperature is 40° F or greater and day length increases in the spring.