Sunflower Growth Stages 101
May 7, 2018
Assessing growth stages accurately can help with in-season control.
When assessing the growth stage of sunflowers in the field, take representative samples to reach an average reading. Avoid headlands where compaction often distorts plant development and avoid obvious patches of uneven growth caused by diseases or changes in the soil type. Sunflowers, particularly hybrids, tend to grow uniformly across the field, so you should be able to determine the growth stage reasonably quick in most situations.
The growth stage key is divided into either vegetative (V) or reproductive (R) stages of plant development. Vegetative development is further divided into two phases, vegetative emergence (VE) and true leaf development.
Vegetative emergence (VE) covers the period from seedling emergence to when the first true leaf is less than 4 cm (~1.5”) long. As a general principle, this is when pesticides are best avoided because plants at this stage are more susceptible to the phytotoxic effects of agrochemicals.
The next stage is the vegetative (V) period with the growth stage given as “V” plus the number of true leaves over 4 cm in length. For example, if there are two leaves over 4 cm the growth stage would be V2, if there are four leaves it would be V4 and so on. If lower leaves have died and fallen off, count the leaf scar and include it in the assessment.
Growth stage V2 to V8 are particularly significant as this is when many post emergence herbicides are applied. The next stage is reproductive (R) and is separated into nine stages based on flower development.
R1 to R4 describe the stages from when the flower bud first emerges through to just before the start of flowering.
R5 describes the beginning of flowering and is subdivided to describe the percent of the flower that has completed or is in flower such as R5.1 (10%), R5.5 (50%), R5.9 (90%).
R6 to R9 cover the period from when flowering is complete (R6) through to physiological maturity (R9) and harvest.
(A. Schneiter and J. F. Miller. 1981. Description of Sunflower Growth Stages. Crop Sci.11: 635-638.)