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Sunflower Reproductive Stage (07/16/20)

The sunflower growth stage for a specific planting date and location can be estimated by selecting “Sunflower GDD,” under the applications section of the NDAWN web site (https://ndawn.ndsu.nodak.edu/sunflower-growing-degree-days.htm).

The sunflower growth stage for a specific planting date and location can be estimated by selecting “Sunflower GDD,” under the applications section of the NDAWN web site (https://ndawn.ndsu.nodak.edu/sunflower-growing-degree-days.htm). Assuming a May 20 sunflower planting date estimated growth stages in eastern North Dakota are between R1 and R3, and a small number of earlier planted fields has reached the flowering stage (R5).    

Cultivated sunflower normally has only one head per plant. In the hybrid seed production process, inbred lines with a branching character are available which have numerous heads enabling pollen production over a longer period than the single-head type. Branching can be induced in normal sunflower plants (with a single head) by removing the apical floral bud, which will stimulate new floral buds in the leaf axils. Alternatively, branching can be a result of a hybrid’s genetics, environmental influences, hail, animal feeding, or herbicide injury.

Sunflower requires warm temperatures forplsc.1 fertilization and seed development. However, temperatures greater than 86 F may prevent optimal pollination and fertilization. Although sunflower hybrids are developed to be self-pollinating, a degree of cross-pollination also takes place. Sunflower is pollinated mostly by insects. Bees are frequent visitors to flowers on warm, sunny days. Little pollination is accomplished by wind. Sunflower pollen are rather heavy and sticky and most of the pollen drop on the leaves (Photo 1) or on the ground. The head of the sunflower is a compound inflorescence composed of many individual flowers in a large disc surrounded by large yellow ray flowers. The ray flowers are normally asexual and do not produce a seed. The disc flowers are perfect with petals and five anthers that are united in separate tubes. The disc flowers are arranged in concentric circles radiating from the center of the head. The ray flowers open first and flowering then proceeds from the periphery to the center of the head at the rate of one to four rows per day. The R5 growth stage in sunflower is the flowering stage. The stage can be divided into sub-stages (e.g., R5.1, R5.2, R5.3, etc.) dependent upon the percent of the head area (disk flowers) that has completed (R5.3 30%, R5.8 80%, etc.).

Seed set can be detected by the plumpness of the seed and the uniform dense color of the pigmented layer of the seed coat. These characteristics are usually visible at about 20 days after fertilization. Sunflower heads should not be harvested before the onset of yellow color in the head and browning of the bracts. This stage usually occurs at about 45 days after fertilization. Flowering is a critical stage for insect management and producers should scout sunflower fields regularly during this critical period in order to establish if economic insect thresholds are reached.

 

Hans Kandel

Extension Agronomist Broadleaf Crops

This site is supported in part by the Crop Protection and Pest Management Program [grant no. 2017-70006-27144/accession 1013592] from the USDA National Institute of Food and Agriculture. Any opinions, findings, conclusions, or recommendations expressed are those of the website author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.

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