When Fall Frost Can Damage Sunflowers

Written by Mark Jeschke

Key Points

  • Frost prior to sunflowers’ physiological maturity can cause damage, reducing seed yield and quality.
  • Temperature, duration, and growth stage affect the type and severity of damage.
  • Once plants have reached the R7 growth stage, they are less susceptible to frost damage.

Sunflower Flowering Stage (R5)

  • Sunflower is most susceptible to frost injury at the bud (R4) and flowering stages (R-5.0 to R-5.9) of development.
  • Temperatures of 30 °F (-1 °C) or less can cause damage to the anthers and stigmas of the pollinating disk flowers.
  • Disk flowers mature from the outside in, with the flowers at the center of the head maturing last.
  • Injury caused by frost during flowering appears as a ring of underdeveloped seed. 

Photo - Sunflower heads in flowering (R5) stages.

Figure 1. Sunflower heads in flowering (R5) stages. Flowers that are open and pollinating are most susceptible to frost injury.

Back of Heads Begin Yellowing (R7)

  • Once the backs of the sunflower heads start to turn yellow, the potential for frost damage decreases.
  • Sunflowers that have reached the R7 stage can withstand temperatures down to 25 °F (-4 °C) with only minor damage.
  • Only a frost duration of 6 hours or more below 25 °F would penetrate the thick layer on the back of the sunflower head and cause damage.
  • Frost injury at this stage can cause losses in yield and test weight, reduced oil content, and discolored seeds.

Back of Heads Completely Yellow (R8)

  • Stage R8 arrives when the back of the flower heads are completely yellow but the bracts are still green. The heads of the plants will begin to turn down.
  • Frost damage at his stage will result in some reduction in yield, test weight, and oil content, but most seeds will be marketable.

Photo - Sunflower heads at R7 and R8 growth stages.

Figure 2. Sunflower heads at R7 and R8 growth stages. Sunflowers are less susceptible to frost injury once they have reached the R7 stage.

Physiological Maturity (R9)

  • At the R9 stage, the plant has reached physiological maturity and final maximum seed weight has been reached.
  • The bracts on the sunflower head will appear yellow to brown.
  • Frost at this stage will not impact seed yield or quality.

Photo - Sunflower head and plant at physiological maturity.

Figure 3. Sunflower head and plant at physiological maturity.

 


See:
Berglund, D.R. 2007. Other Pests and Damage, in D.R. Berglund (Ed.), Sunflower Production. North Dakota State University Extension Service A-1331.

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The foregoing is provided for informational use only. Please contact your Pioneer sales professional for information and suggestions specific to your operation. Product performance is variable and depends on many factors such as moisture and heat stress, soil type, management practices and environmental stress as well as disease and pest pressures. Individual results may vary.