May 23, 2008

Hoary vervain (Verbena stricta), also known as wooly verbena or tall vervain, is a common native weed in northeast Nebraska's over-grazed rangeland, prairies and disturbed sites in all soil types.

Plant Form

Several other types of vervain — prostrate, white, and blue — grow in Nebraska and have similar growth forms and habits as hoary vervain. Hoary vervain is a perennial forb from the vervain family (Verbenaceae) that reproduces by seeds. The taproot (perennial structure) produces individual erect plants. The stem is nearly round, simple or branched above and can be up to 5 ft tall, covered with soft white hairs. Leaves are opposite and leaf blades are ovate with many teeth; their lower surface is pubescent with highly visible veins. Like many other plant species, the overall growth and development depends on the amount and timing of rainfall. In Nebraska hoary vervain can flower from May to September, with blue or purple flowers positioned on the top of the main stem and branches. It produces a two-seeded fruit.

Hoary vervain provides forage for deer while seeds are an important food source for small mammals and upland birds. Native Americans also made a tea from the leaves to treat stomachache. It has no value to livestock because of its low palatability.

Control Options

Several methods are available for controlling this weed.

Mechanical

Mowing the plants when they are 3-5 inches tall can reduce vervain population considerably for the season. Mow once or twice during the season, depending on rainfall. One mowing in mid-June can be effective (more than 75% control) if the season is dry, due to lack of moisture needed for weed regrowth. If the season is wet, an additional mowing is needed in July-August.

Chemical

Herbicides also can be very effective in providing season- long control. Herbicide application should be conducted when vervain plants are 3-5 inches tall, which is usually in early June. The list of effective herbicides, their rates and cost per acre includes: Salvo (12 oz/ac, $4), Grazon P+D (32 oz/ac, $8), Weedmaster (32 oz/ac, $6), Ally (0.25 oz/ac, $8) and Vista (22 oz/ac, $8).

Stevan Knezevic
Extension Weeds Specialist