Farmers utilize a method to track down the total time required for a sunflower to develop in order to determine the growth stage of a sunflower. The same method applies for classifying either a single head or a branched flower. The total time required for a sunflower to develop in all stages depends on the genetic background of the plant and the environmental growing conditions.
Vegetative Emergence (VE) is a stage when the sunflower seedling emerges. The first leaf beyond the cotyledon, a significant part of the embryo within the seed of a plant, measures 4 centimeters.
Determining the number of true leaves that measure at least 4 centimeters in length, coded with letter V and a number next to it--for example, V1, V2, V3, V4, and so on--will determine the vegetative stages. Counting the leaf scars if senescence (biological aging) of the lower leaves has occurred will determine proper stage within the vegetative stage.
Coding reproductive stages involves observing the appearance of buds and measuring their sizes. The reproductive stages follow the growth and ability of the sunflower to bloom from bud form to a full-grown flower, until the time it dies. Representing each reproductive stage with the letter R and a corresponding number will help identify the specific stage during this period.
R1 stage is when a terminal bud forms a miniature floral head instead of just a cluster of leaves. This is identifiable when viewed from the top, where immature bracts appear pointed and star-like.
R2 stage is when the bud starts to elongate 0.5 to 2.0 centimeters above the nearest leaf attached to the stem. In this stage, you will omit the leaves attached directly to the back of the bud.
R3 stage is when the bud further elongates, exceeding 2.0 centimeters above the nearest leaf.
R4 stage is when the inflorescence (a group or cluster of flowers arranged on a stem) begins to open. Immature flowers appear when viewed from above.
R5 stage is when the flower starts to bloom. There are many sub-stages within R5, coding each stage with an R5 and a decimal point next to it and a corresponding number of sub-stage; for example, R5.1 (reproductive stage 5, sub-stage 1). The sub-stage depends on the percentage of the head area that is flowering. For example, R5.1 means 10 percent of the head area appears while R5.3 means 30 percent of the head area appears.
R6 is the stage when flowering is complete and the ray flowers start to wilt.
R7 is the stage when the back of the head starts to turn pale yellow.
R8 is the stage when the back of the head is yellow, but the bracts remain colored green.
R9 is the stage when bracts turn yellow and brown. A sunflower reaching this stage is physiologically mature.