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Cultivated sunflower (Helianthus annuus) is grown in many temperate, semi-dry regions of the world, often in rotation with small grain cereals such as wheat. The largest areas of sunflower cultivation in the US are in the northern plains (North and South Dakota) and southern, high plains (western Nebraska and Kansas, plus areas of Colorado and Texas) where the growing season is often too dry and/or too short for profitable soybean and corn production. Most commercial sunflower is the oilseed type; in addition, the crop is grown for confectionery seed and is common as an ornamental in home gardens throughout the US.
The US is the center of diversity of the ancestral species of cultivated sunflower (Heiser 1954). The crop is capable of hybridizing with its wild progenitor, wild H. annuus, but most crosses with other Helianthus species such as H. petiolaris are unsuccessful or yield infertile F1 progeny (Rieseberg et al. 1999). Cultivated sunflower also occurs as a volunteer weed. Although volunteer domesticated plants can represent a significant portion of the weeds infesting subsequent crops (Auwarter and Nalewaja 1976; Gillespie and Miller 1984), they do not persist for more than one or two years under most cropping systems and are not known to spread. For these reasons, the working group focused on the consequences of gene flow to wild H. annuus.
Wild H. annuus is an outcrossing annual that occurs in disturbed sites and is widespread throughout much of the US, reaching its greatest abundance in midwestern states (Heiser 1954). Wild sunflower occurs at elevations ranging from sea level to 3,000 meters and in a variety of habitats that include roadsides, agricultural fields, abandoned fields, construction sites, and rangeland. Populations are typically patchy and ephemeral, relying on the soil seed bank and long-distance dispersal for opportunities to become established in available clearings. This species occurs as a common but manageable weed of wheat, cultivated sunflower, corn, soybean, sugarbeet, sorghum, safflower, and other crops (Al-Khatib et al. 1998; Geir et al. 1996; Irons and Burnside 1982; Schweitzer and Bridge 1982; Teo-Sherrell 1996).
Pollen from cultivated sunflower is certain to spread to adjacent wild populations by the movements of foraging insects, especially bees. Commercial sunflower seed companies are required to have 1.6-2.4 km of isolation between hybrid seed production fields and wild sunflower and/or other cultivated sunflower to prevent contamination by “foreign” pollen (e.g., Smith 1978; Schneiter 1997). The extent of pollen movement from the crop to wild sunflowers is greatest at the crop edge, where up to 42% of seeds can be crop-wild hybrids, diminishing to nearly zero at distances of 800-1,000 m (Arias and Rieseberg 1995; Whitton et al. 1997). F1 crop-wild hybrids are fertile and capable of backcrossing with nearby wild plants, but they typically produce fewer flower heads per plant than purely wild genotypes (Snow et al. 1998). Once crop genes enter wild populations, they can spread farther by both pollen and seed dispersal. Seeds can be transported inadvertently by farm equipment and as contaminants of hay, manure, topsoil, and seed lots. Whitton et al. (1997) and Linder et al. (1998) have documented long-term persistence of crop genes in populations of wild sunflower.