Sunflowers are generally hardy plants which grow so vigorously that weeds don't become competitors for soil and nutrition. The plants just simply crowd out most weeds. In commercial sunflower growing, weed control is important due to successive sowing, and some herbicide injury is common. Much of the weed control done in a commercial setting is through pre-emergence herbicides and mulching or tilling in small weeds. Damage due to herbicides can be unsightly and ruin the product, so research is being done to prevent herbicide injury.
Type of Damage
Contaminant contact with herbicides can cause scarring on sunflower leaves. It can also cause parallel vein patterns and abnormal leaf shapes. Most modern herbicides do not cause significant damage. However, plant growth, seed and head size, and yield can be diminished. Post-emergence herbicides cause the most damage since they are primarily for broadweeds and are chemically potent. The amount of damage depends on the strain of seed, weather conditions, type of soil and kind of herbicide.
Sunflowers can become contaminated with herbicides by spray drift. This occurs when the herbicide droplets are carried to a non-target plant, usually by the wind. It can cause damage to sunflowers and other crops, and can end up in the soil or water table. Residue carryover is a result of herbicides ending up in the soil. If there is inadequate rain or other conditions, it saturates the soil and causes germination and growth issues to the next season's crop.
Weed Control Overview
Most sunflower growers apply a pre-emergence herbicide before weed seedlings are 10 cm tall. Sunflowers that have not yet germinated are not affected. Tilling around sunflowers that have attained two true leaves is effective, since sunflowers have deep roots. Broadleaf weeds need a stronger approach due to some resistance to pre-emergence herbicides. These generally require a post-emergence spraying. Spray drift often occurs during this treatment.
To preserve crop integrity, research has been done on several hybrids that prove to be resistant to herbicide injury--most notably a hybrid that is high yield and resistant to the chemical tribenuron methyl, the active ingredient in many broadleaf herbicides. Since broadleaf herbicides are mostly post-emergent, the benefits are enormous in crops with resistance to the toxicity. Other hybrids are being developed for resistance to the weed killers used to control grass.
The hybrid strains of sunflower have proved resistant to many of the herbicides, but location, environment and other factors may require more sensitive seeds to be used. In these cases, contamination can be avoided by drilling seed rather than broadcast sowing. This will protect the germinating seeds from over spray. Additionally, organic methods such as row mulching can prevent weed emergence. Residual carryover can be prevented by thorough watering after use to disperse the chemical.