Sunflower plants pass through four main development stages from planting to harvest; a vegetative phase, a reproductive phase, a period of ripening, and senescence or dieback.
It’s useful and in some cases necessary to understand and to identify when the plant is at, or has moved through different growth stages.
A standardised and easy system has been developed to help farmers, agronomist, scientists and other interested parties, clearly and accurately describe different sunflower growth stages.
When assessing the growth stage of sunflowers in the field, take representative samples to reach an average reading. In practice this means avoiding headlands where compaction will often distort 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 quickly 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 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 4cm in length. For example, if there are two leaves over 4cm 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 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 sub divided 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 form when flowering is complete (R6) through to physiological maturity (R9) and harvest.
Sunflower growth stages and description
(A. Schneiter and J. F. Miller. 1981. Description of Sunflower Growth Stages. Crop Sci.11: 635-638.)
|VE Vegetative Emergence||Seedling has emerged and the first leaf beyond the cotyledons is less than 4cm long.|
Vegetative Stages (e.g. V-1, V-2, V-3 etc.)
|Determined by counting the number of true leaves at least 4cm in length beginning as V-1, V-2, V-3 etc. If senescence of the lower leaves has occurred, count leaf scars (excluding cotyledons).|
|R1 Reproductive Stages||The terminal bud forms a miniature floral head rather than a cluster of leaves. When viewed from directly above, the immature bracts have a many-pointed star-like appearance.|
|R2||The immature bud elongates 0.5 to 2.0cm above the nearest leaf attached to the stem. Disregard leaves attached directly to the back of the bud.|
|R3||The immature bud elongates more than 2cm above the nearest leaf.|
|R4||The inflorescence begins to open. When viewed from directly above, immature ray flowers are visible.|
e.g., R-5.1, R-5.5, R-5.9, etc.
|The beginning of flowering, divided into substages depending on the percent of the flower that has completed or is in flower, eg. R-5.1 (10%), R-5.5 (50%), R5.9 (90%).|
|R6||Flowering complete, ray flowers wilting.|
|R7||Back of the head started to turn a pale yellow.|
|R8||Back of the head yellow, bracts remain green.|
|R9||Bracts yellow and brown, plant at physiological maturity.|