Whether growing sunflowers for cut flowers, seed production or to attract game for hunting, proper weed control will help plantings grow more successfully. Grasses, broadleaf plants, vines and other weeds can quickly overwhelm sunflowers. While herbicides specifically labeled for sunflower production are limited, according to the University of Tennessee, there are a number of herbicides that can help control weeds in sunflower plantings.
Dual Magnum is used primarily in pre-planting and pre-emergence weed control. Also known as S-metolachlor, this herbicide provides good control over broadleaf weeds, sedges and annual grasses. Dual Magnum works best when incorporated into the soil. Wetting this herbicide in improves its performance and dry weather after application may reduce its effectiveness.
Another pre-planting in pre-emergent herbicide is Prowl. This herbicide is effective at controlling small-seeded broadleaf weeds and annual grasses. It is important to incorporate prowl fairly deeply within the soil within seven days of application and to provide adequate irrigation within the next seven days for optimal performance. Shallow incorporation in dry topsoil will significantly reduce its effectiveness in controlling weeds around sunflowers.
Also sold under the name Trilin and Trifluralin 4EC, Treflan HFP is used primarily on loamy soils with a high sand content. When incorporated into the soil after application, Treflan provides good control over broadleaf weeds and grasses before sunflower seeds are planted or sprout. This pre-planting and pre-emergent herbicide works well in shallow applications within the soil.
Ethalfluralin, sold under the name Sonalan, is another herbicide for controlling weeds in areas where sunflower seeds will be or have just recently been planted. This herbicide works well on annual grasses and broadleaf weeds in a variety of soil conditions and and works best in shallow applications. Sonolan is more persistent within the soil than other herbicides used on sunflowers and heavy applications may affect the next year's plantings adversely.
Select is a post-emergent herbicide used against weeds after sunflowers have begun to grow. According to the University of Tennessee, this herbicide is more effective when combined with one percent of crop oil concentrate and ammonium sulfate. 70 days should elapse from the application of Select to harvest. The University of Wyoming Cooperative Extension Service indicates that areas treated with Select should not be used for graze or forage for livestock.
Imazamox, commonly known as Beyond, is used to kill weeds after sunflower plants have from 2 to 8 leaves on the stalk. This product is used exclusively on Clearfield sunflower varieties, which are resistant to the herbicide and will kill non-resistant sunflower varieties. Apply Beyond with urea ammonium nitrate or another nitrogen-base fertilizer and a nonionic surfactant to significantly improve results.
Another post-emergent herbicide, Poast is applied to sunflower plantings to control annual grasses when they are from 2 to 4 inches tall. Poast works best when applied with a crop oil and a nitrogen-based fertilizer, such as ammonium sulphate. When appling this herbicide Do not cultivate sunflower plantings at least 5 days before or 7 days after applying Poast, states the University of Wyoming Cooperative Extension Service.