With the recent temperatures experienced across the state in the last week, many growers may be wondering about the damage to their canola crop. Prior to talking about the recent freezes, we need to look at the status of the crop as a whole.
This last winter was one that had several unique challenges. While this had not been the coldest winter we have had in recent years, periods of severe cold weather definitely hurt the crop. Part of this was due to the overly warm fall that most of the state experienced. These warm conditions lead to excessive growth of the crop, leading to crop stem reaching nearly 2 feet in isolated conditions. While much of that state did receive periods of cold and chilling conditions prior to any major freezes, these were mixed with multiple day stretches of over 80°F. This resulted in the crop not being able to properly harden-off and the crop was still somewhat sensitive to freezing temperatures. This meant that freezing temperatures around Thanksgiving and severe freeze (-5 to -20°F) around the 14th of December resulted in fields or areas of the field seeing substantial winter kill. Most of these fields have been seen north of the I-40 corridor; however, area agronomist, Heath Sanders, has seen a number of fields in SW Oklahoma having moderate to severe winter kill. While the outlook may sound bleak, one of the most positive aspects of winter canola is its ability to overcome thinner stands by flexing and increasing branch production during reproductive growth. Therefore, a couple of plants per square foot can still produce adequate yields. Additionally, many of the canola plants may look like they have been completely winter killed but are showing “tillers” out of the base of the plant. While these will not have the same yields as the primary stem, they can help overcome some of the yield losses that can be expected.
This brings us to recent freezing events. The damage that spring freezes can cause to winter canola greatly depend on temperature, time at specific temperatures, growth stage, and moisture. Soil moisture at time of freezing can drastically help save a crop that might otherwise see much higher freeze damage. Another positive about the freezes in the last 7-10 days is that the canola crop has been in the rosette to bolting stages. During these stages, canola can recover from mid- to low-20s temperatures for 1 to 7 hours with relatively little yield loss. Longer consecutive hours, however, can result in substantially increased plant damage, which could state to add up to noticeable yield losses. Therefore, the positively aspect out the crop is that most of the crop should still be in a relatively good conditions (as it pertains to freeze damage). The major loss over the last week might have come at the expense of the “tillers” from plants that were partially killed during the winter. Many of these lateral branches have seen significant damage and many will probably not successfully produce.
As we look forward to the remainder of the season, it is important to be able to identify cold damage to the developing crop and know when the most sensitive periods occur. Typical freeze damage and canola response, at each growth stage, that can be expected are below.
Rosette– Leaf burning can be expected; however, this is typically solely cosmetic. As temperatures warm, new leaves should start to emerge from the rosette. If no new growth is seen, severe enough damage could have been experienced that growth and yield may be limited.
Bolting– Similar to the symptoms that can occur during the bolting stages as they did during rosette but these symptoms can occur not only on the leaves but the stem. These symptoms include a discoloration of the tissue, resulting in the leaves and stems turning yellow, purple, or white. Even with some damage, these plants should progress to flowering with little or no issue. In more severe conditions, the still will be split or crack. While this should have little effect on the plants ability to continue growth, these split stems may increase lodging and provide a port secondary diseases.
Flowering and pod development– Cold temperatures at flowering can be far more damaging. Unopened buds should still produce viable flowers (depending on temperature and time at that temperature). Open flowers or pods can be more damaged by colder temperatures. Flowers can abort or not produce viable seed following a major freeze event. Early pods can become malformed and potentially terminate. These will produce a permanent twisted and curled appearance.
One thing is certain, these cooler temperatures helped slow maturation, if only for a short time. This may help the crop in the coming weeks as freezes later in the season could potentially be more damaging the further the crop matures. One thing must be noted, even with areas of field being killed over winter, we still must treat the field in the appropriate manner to ensure the crop following canola will still receive the benefits. This includes making herbicide applications (as the canola crops still allows for it) to help control problematic winter weeds.