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Blooming Soybean and Sunflower (08/01/19)

Blooming Soybean and Sunflower Facts

Soybean

Due to later than normal planting of soybean in North Dakota, the percent of fields with blooming soybean plants has been lower than normal during July. The top line in Figure 1 represents the 5-year average (2014-2018) of the percent fields with blooming soybean plants. The second line from the top indicates the percent of fields blooming in 2019. As of July 28, 71 percent of the soybean fields were in bloom or completed flowering.

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Source: National Agricultural Statistic Service ND Crop Progress and Condition reports.

Soybean flowers typically are self-pollinated on the day when the corolla opens. The amount of natural crossing is approximately 1% for adjacent plants within a row and 0.5% between plants in adjacent rows. Natural crossing is primarily done by honeybees. The stigma is receptive to pollen about 1 day before anthesis and remains receptive for 2 days after anthesis. Shortly after pollination, the pod formation phase starts. The pod development stage, R3, is reached when the pods are 3/16 inch long at one of the four uppermost nodes on the main stem with fully expanded trifoliolate leaf. Abortion of pods and seeds can occur several weeks after pollination, but the percentage of abortion usually is low if plant stress is minimized. Moisture stress during the pod development and grain filling growth stages will result in lower yield. Description of soybean growth stages can be found in the Soybean Growth and Management Quick Guide at https://www.ag.ndsu.edu/pubs/plantsci/rowcrops/a1174.pdf.

 

Sunflower

The sunflower crop is only slightly behind normal development (Figure 1). On July 28, 22 percent of the sunflower fields had blooming sunflower. Flowering is a critical time to scout sunflower for potentially damaging insects and diseases. Sunflower requires warm temperatures for fertilization and seed development. Cool temperatures can delay or prevent the pollination and fertilization processes by affecting the activity of pollinators and the metabolism of the plant. Temperatures in excess of 86F also may prevent normal pollination and fertilization. Sunflower is a cross-pollinated plant. Sunflower is pollinated mostly by insects. Bees are frequent visitors to flowers on warm, sunny days. Little pollination takes place by wind. Sunflower pollen is rather heavy and sticky and most of it drops on the leaves or on the ground in clumps. The head of the sunflower is a compound inflorescence composed of many individual flowers in a large disc surrounded by large ray flowers. The ray flowers are normally asexual, but some may produce pollen. The disc flowers are perfect with petals and five anthers that are united in separate tubes. Early in the morning, the staminal filaments rapidly elongate and exert the anther tube from the corolla, this occurs about 7:00 AM during a warm, sunny day, but later on a cool, wet day.

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 digit in the growth stages R5.1 to R5.9 represent the percent of the sunflower head that has completed pollination. The description of sunflower growth stages can be found at https://www.ag.ndsu.edu/publications/crops/stages-of-sunflower-development.

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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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