May 29, 2009

Hoary verbena (Verbena stricta Vent.) (Jennifer Anderson @ USDA-NRCS PLANTS Database)

Hoary vervain (Verbena stricta), also known as wooly verbena or tall vervain, is a common native weed in northeast Nebraska on over-grazed rangeland, prairie, and disturbed sites in all soil types. It can be most effectively controlled when it's 3-5 inches tall, which is usually in early June.

Identification

Nebraska has several types of vervain (prostrate, white, and blue), most of which 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 feet tall, covered with soft white hairs.

Leaves are opposite and leaf blades are ovate with many teeth. The lower leaf surface is pubescent with highly visible veins. As with many other plant species, its 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 on the top of the main stem and branches producing a two-seeded fruit.

Uses

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

Control

Mechanical or chemical practices can be used to control hoary vervain.

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

Chemical. Herbicides also can provide season long control. Apply herbicide when vervain plants are 3-5 inches tall, which is usually in early June. Effective herbicides, their rates and cost per acre include: 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 Integrated Weed Management Specialist
Haskell Agricultural Laboratory, Concord