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Sunflowers tolerate a relatively wide range of temperatures and are particularly tolerant of cool conditions during early growth. Seed can begin germinating at approximately 39°F, although soil temperatures of at least 46–50°F are preferred for satisfactory germination.
Later stages of development can be damaged by freezing temperatures, with temperatures below approximately 28°F capable of killing maturing plants. Extremely high temperatures can also reduce oil percentage, seed fill, and germination quality.
Sunflowers are adaptable to a variety of soil conditions but perform best in well-drained soils with good water-holding capacity and a nearly neutral pH of approximately 6.5–7.5. Maintaining a moist seedbed is important for uniform germination and emergence.
Field Conditions: Sunflowers can perform reasonably well under drought, salinity, or other stressful soil conditions, but proper drainage and adequate moisture remain important for successful production.
Sunflowers can be planted across a relatively broad window during the growing season. In the northern Great Plains, planting commonly occurs from early May through late June. Planting may begin as early as approximately two weeks before the last killing frost and continue as late as approximately 100 days before the first killing frost in fall.
Late Plantings: In northern growing areas, early-maturing hybrids are preferred when planting late in the season or when replanting is necessary.
Sunflowers grow successfully across a wide range of plant populations and row spacings. Row spacing can generally be selected to match available planting and harvesting equipment. 30-inch rows are common, although narrower or wider rows may also be used successfully.
Plant deeply enough to place seed into adequate moisture, but avoid excessive depth, which can reduce emergence and final stand. Shallower planting is generally preferred in soils prone to crusting or packing. Semi-dwarf varieties may perform particularly well in narrower rows at higher plant populations.
Sunflowers reach physiological maturity when the back of the flower head changes from green to yellow and the bracts begin turning brown. This generally occurs 30–45 days after bloom, when seed moisture is approximately 35%.
Harvesting at somewhat higher moisture levels can reduce field losses and limit exposure to late-season wet or cold weather, although mechanical drying may then be necessary. Commercial sunflower headers are commonly used during direct combining to help capture mature seed and reduce harvest loss.
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