Frost anytime before the sunflower crop reaches physiological maturity (R9) can cause damage. Once sunflowers reaches the R7 stage (ray petals have dropped, back of head starting to turn yellow), sunflower can withstand temperatures as low as -4° C, but temperature, duration and crop stage will influence the type and amount of damage.
A killing frost in sunflowers is considered to be -4 to -5° C for 6 or more hours, as this low temperature for the extended period is required to penetrate the thick layer in the back of the sunflower head and start the dry down process. See attached bulletin for more details
Visit www.canadasunflower.com for more updates on all sunflower issues
Visit MAFRD http://www.gov.mb.ca/agriculture/crops/ for more frost information for other crop types
submitted by Anastasia Kubinec, MAFRD Oilseed Crop Specialist