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Sunflower Development at Latitudes Ranging from 31 to 49 Degrees¹

Sunflower (Helianthus annuus, L.) varieties were planted May 14, 1965, at nine locations in central North America ranging from 31 to 49° north latitude. Second plantings were made at four locations to study the effect of planting date.

Seed was soaked before planting and irrigation and fertilizer were used where needed to minimize location differences in water imbibition by seed, soll moisture, and soil fertility.

In plantings made May 14, the growth periods – planting to emergence, emergence to ray flowers, and planting to ray flowers – increased greatly from south to north each degree of latitude northward, days from planting to ray flowers increased on the average nearly 2 days. Whether growing degree-days from planting to ray flowers decreased, varied about an average, or increased from south to north was dependent upon the temperature base chosen. Heights and leaves per plant showed no trend associated with latitude. The lengthening of the three periods of growth from south to north may have been due to temperature gradient rather than photoperiod, since leaves per plant showed no trend associated with latitude. However, daylight durations for the northern and southern locations were only about 16 and 14 hours, respectively, when photoperiod effects occurred.

When practical dates of planting for the locations were compared, i.e., April 5 at 31° latitude and May 14 at 49° latitude, daylight duration when photoperiod effects occurred was about 13 hours in the south and 16 in the north. Under this range of photoperiod, three closely related inbreds – ‘HA6,’ ‘HA7,’ and ‘HA43’ – responded morphologically like short-day plants. Therefore, when such inbreds are used for hybrid seed production, latitude and planting date are important considerations.

Key Words: sunflowers • photoperiod • growing degree-days • heat units

² Associate Professor and Research Assistant, Department of Agronomy and Plant Genetics, University of Minnesota; Assistant Agronomist, South Dakota State University; Research Agronomist, Vegetable Oil Division, Cargill Inc. (formerly Assistant Professor, University of Minnesota); Research Agronomist, ARS, USDA; Associate Professor, Kansas State University; Assistant Professor, Oklahoma State University; Director, Experimental Farm, Morden, Canada; Assistant Professor, North Dakota State University; and Associate Professor, University of Nebraska.

Received for publication September 2, 1966.

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