The threat of frost on a canola crop is always a possibility late in the growing season and can impact both the potential yield loss and quality of the crop. Canola is susceptible to late season frost damage between bud formation and pod fill growth stages and should be inspected for frost damage if the overnight low temperature falls below 35°F (2°C). Visible frost damage on a maturing canola crop will vary within a field due to many factors including:
- crop development stage
- degree and duration of frost
- moisture content of canola seed
- soil type
- soil moisture
- cloud cover
- wind speed
- position in the landscape (slope of the field)
- crop nutrition
- crop density1
Frost damage can also make the canola crop more susceptible to disease and insect attack, which can complicate diagnosis.
One of the most important variables when assessing the amount of fall frost damage to a canola crop depends on the crop’s stage of maturity. Canola is most susceptible to frost damage late in the season from flowering to the clear watery stage, around 60 percent seed moisture.2 Frost during flowering usually causes flower abortion (Figure 1).3 Plants can recover from frost at the early flowering growth stage by producing more flowers. Frost at flowering can delay maturity but typically results in only minor reductions in yield potential. A frost when flowering is finished can cause more yield loss because it is too late in the season to produce more pods. Frost after flowering can result in significant yield reductions and grade loss. The most economically damaging frosts on canola are severe events that kills the developing seed. A 27° F (-3°C) frost is enough to kill immature seeds containing 50 to 60 percent moisture, while those ready to swath at about 35 percent moisture normally will escape damage (Figure 2).3
Figure 1. Aborted buds; unopened buds may abort but should be more tolerant of cold temperatures than opened flowers or new pods. Photo © Queen's Printer for Ontario, 2021. Reproduced with Permission. 2021. Courtesy of Meghan Moran, Canola & Edible Bean Specialist, OMAFRA
Figure 2. Frost damaged pods will have a scarred or blistered appearance and contain mushy or shriveled seeds.
Photo courtesy of the Canola Council of Canada
What to look for after a late season frost.
- Frost damage can occur randomly, resulting in high variability within the field and even on individual plants. Plants along the field edge may have less frost injury.
- Look first in low lying areas. The coldest air tends to move to low lying areas (cold air drainage).
- Areas in the field with light-coloured soil types, dry soil areas and areas with more retained stubble are more prone to frost damage.
- Frost damaged plants can lose flowers, abort young pods, have twisted inflorescence or have blistered pods with damaged seed, depending on the stage of growth when the frost occurred.
- More severe frost causes developing buds to turn yellow, die and fall off, leaving gaps in the flowering spike between more developed pods and new flowers that form afterwards. There is often a twisted flower stem in this section.
- Check stems for splitting, discolouration and/or bending
- It will take five to seven days after a late season frost to assess damage to newly formed pods
- Severe frosts can damage developing seed, which turns into a mushy green, brown mass that dries to a small black or brown speck.
- The pod surface can blister, turn yellow green or develop a paler blistered surface. Partial seed death leads to unevenly filled watery ripe pods.
- A late frost event, during early seed fill, can cause significant losses with shriveled seed that may retain its green colour and reduce oil quality.3
- Frost events can also lead to substantial desiccation and pod splitting, often within a day after a severe frost, and damage may resemble a hail event.
Green seed issues caused by a late frost
Many of the green seed problems in canola are usually the result of frost. Even a light frost can fix the green colour by damaging the enzymes that clear the chlorophyll in higher moisture seed, preventing additional clearing even if favourable weather conditions return after the frost. A killing frost also causes rapid dehydration of seed and plant tissue, which may be just as important for reducing the ability of these enzymes to function properly. Seed at less than 20 percent moisture should typically be safe from frost damage.4 Distinctly green cut-offs for grades No. 1, 2, and 3 Canada canola are two, six, and 20 percent respectively.
Managing a late season frost on canola.
Stand uniformity and early ripening can help limit the amount of frost damage to a maturing canola crop. Uneven and thin stands can have late maturing seeds more susceptible to a late frost. Swathing one to three days prior to freezing temperatures may reduce seed chlorophyll levels by allowing for a more rapid seed dry down when compared to standing crops at the same stage of maturity. The optimum stage to swath is up to an average of 60 percent seed colour change (SCC) on the main stem.4 If most or all seed is mature, start swathing the canola as quickly as possible after a frost. Swathing an immature canola crop following a frost will not reduce the number of green seeds at harvest. The time required to dry down the swathed crop to a safe harvest moisture level is always dependent on the weather conditions experienced after swathing.
Other injuries that can resemble frost injury to a maturing canola crop.
- Group 1 (Lipid Synthesis Inhibitors) herbicides injury can cause similar flower petal retention problems and unopened flowers.
- Hail damage, particularly from small hail stones, can be misdiagnosed as frost damage because it often results in the typical “pod blistering” symptom often seen after a late season frost.
- Chemical desiccation can cause similar small and crimped seeds.1