Why Are Weeds a Problem for Sunflowers?

Growing Sunflowers

• Sunflower seeds can be sown directly into the soil following spring frosts; they'll germinate within 10 to 14 days. Stems may grow to 10 feet tall and flower heads span about 12 inches wide. The stalk shoots up, the brown flower head develops and spreads, the petals sprout, widen and then open to let the sun spur the growth of seeds. As the flower ages, wind (and birds) help the seeds loosen and fall to the ground. The flower will start to wilt and die. Seeds in the ground may sprout again the following season.

Weeds

• Weeds in a sunflower patch can reduce the number of strong, healthy blooms. As with many flower types, weeds compete for soil, water and sunlight. Although a tall sunflower stalk and developed flower can soak up sunlight, weeds can spread quickly underneath. Damaging weeds include green foxtail, thistle, kochia, marshelder and puncture vine. Chemical herbicides are recommended for successful weed control.

Diseases

• Sunflowers are susceptible to fungal diseases such as downy or powdery mildew, rust and "black stem." Downy mildew grows under leaves, causing their color to fade. Powdery mildew grows cottonlike on sunflower leaves, especially during the summer. Dead blotches or rust-colored spots can occur on the leaves before the flower grows into its heading stage. Decaying leaf veins, wilting, yellowing stems and large brown splotches can cause a sunflower plant to die. Seeds may become discolored and fall out of the flower.

Treatments

• Herbicides and fungicides help sunflowers grow healthy and strong. Chemical mixture herbicides to kill grasses and broadleaf weeds include carfentrazone-ethyl, quizalofop and pendimethalin. Some herbicides must be applied before the flowers grow, other types are added after the seeds have germinated and sprouts appear through the soil. Fungicides such as pyraclostrobin, foliar and azoxystrobin curb downy and powdery mildew, rust and leaf spot. Specific information is available at garden stores and county extension services.

Insects and Birds

• Larvae, beetles, maggots, moths, weevils and other insects which feed from sunflower's head can damage it; birds such as goldfinches, crows, sparrows, red-wing blackbirds and cardinals also destroy the flower. Although insecticides are legally approved to control bugs, chemicals are not available to keep the birds away. Netting and scarecrows are some ways to protect sunflowers from birds. Sunflower varieties in gardens depend on bees to carry pollen from one plant to another, in order to cross pollinate and create hybrids.