There are a variety of plants that can fall into both the desired and the weed category when it comes to lawn care and landscaping. What is enjoyed and appreciated in one flowerbed or lawn is an undesired and invasive weed in another. Stiff vervain is one of these types of plants. In some cases, it is used as a hardy ground cover becasue its sprawling stems and viny roots quickly spread. It does not need any particular type of soil conditions to grow, meaning it can thrive anywhere there is full sun. However, when undesired in a garden, flower bed, lawn, or landscape stiff vervain can quickly overrun areas.
Stiff vervain has a variety of known names. In parts of the country, it is known as verbena, stiff verbena, creeping verbena, purple verbena, slender vervain, and tuberous vervain just to name a few. However, there is one main scientific name–Verbena ridiga Spreng.
Stiff vervain is fairly easy to identify by the small, bright purple fragrant flowers. The stems are square and almost woody, and the plant can form a dense cover from 20 to 25 inches in height. Stiff, dark green leaves are several inches long and oblong in shape with serrated edges. Both leaves and stems are covered with course, stiff hairs.
This weedy ground cover is native to many parts of South America and Central America. In tropical and temperate climates, stiff vervain has become fairly common and widespread including regions of Australia, New Zealand, the Caribbean, and parts of the Southern United States as well as Europe. The hardiness of this species means it has been found as far north as Scotland in parts of Europe and Virginia in the United States.
The spread of stiff vervain occurs in two different ways. It is spread both by seed and vegetatively by rhizomes. It has spread internationally because it is desired in parts of the world as an ornamental plant in beds and gardens. It has become naturalized to certain parts of the world, and was first recorded in the United States in Texas in the early 1900s. It was said to completely cover waste places where little else would grow. Today it can be found along roadsides, riverbeds, woodlands, grasslands, waste places, and lawns. The rhizomes and seeds are able to spread out of their growing regions on their own with little to no encouragement.
There is little information regarding the toxicity of stiff vervain to animals or people. However, there are a few known parasites that feed on the weedy plant. Slugs, snails, powdery mildew, mites and certain types of nematodes are just a few types of parasites that feed on stiff vervain. Additionally, a lot of how stiff vervain spreads is not fully understood. There is little documented evidence of accidental introduction. However, certain types of this plant have been known to be spread by deer as well as by wind and gravity. It is also largely cultivated in parts of the world from seed.
If this plant has been introduced to your landscape intentionally, control is crucial to prevent stiff vervail from overrunning any flat, open ground. Seedlings and smaller plants should be pulled by hand. Rhizomous roots are a bit more difficult to eradicate, as even a small fragment of root left can result in new plant growth. In turf and pastures where growth is not desired, chemical control will be the best option for elimination. Maintaining vigorous pasture and keeping soil disturbance to a minumum in areas where stiff vervain are prevalent will help prevent new growth from taking root. Mowing plants off prior to seed head formation will reduce spread by almost 75%. Herbicides are generally only effective when plants are less than 5″ tall.
There are roughly 80 different species of vervain around the world. Most of these varieties are located in the South, and depending upon the species may be either an annual or a perennial plant. Although there are no regulations currently on growth or elimination of stiff vervain, some sources will describe the plant as weedy and invasive and can overrun pastures and turf if left unmitigated.