Virginia knotweed, or jumpseed, is an herbaceous perennial growing from rhizomes. It might be most eye-catching in spring, when its new, oval leaves are marked with brownish V-shaped splotches. In summer, it blooms with tiny, teardrop-shaped white flowers on wandlike stems. Look for it in woodlands and other shady places.
The stems are usually single and unbranched and may be smooth or hairy. The leaves are alternate, simple, pinnately veined, with the blades up to 7 inches long and 4 inches wide, generally oval, with a sharply pointed tip, and angled to rounded at the base. Upper leaves are smaller and may lack leaf stems; lower leaves are larger and have longer leaf stems lower down the plant. Leaves may be smooth or hairy.
The stem nodes where the leaves attach are sheathed by a fairly cylindrical, ¾ inch long, brownish, papery structure called an ocrea (AW-kree-uh). (These structures are a key identifier for the smartweeds and knotweeds as a group.) A close look at the ocreae (plural, AW-kree-ee) shows the margins to have bristles and the surface to be densely hairy with appressed, often woolly hairs. The leaves, especially when the plants are beginning to grow in springtime, may have a reddish or purplish mark shaped like a chevron (V), crescent, or triangle. By summer, these markings have often disappeared.
Flower stalks arise from the tip of the plant stem or from the upper leaf axils and can be solitary or a few per main stem. The flower stalks are usually 16–50 inches long, green, wandlike, upright or more commonly held at an angle or arching. Flowers are about ⅛ inch long, are in groups of 1–3, and are well spaced along the flower stalk. The flower bases have their own sheaths (ocreolae; “AW-kree-OWE-lee”), which conceal the flowers’ short pedicels (flower stalks). When the flowers open, they have 4 white (sometimes pink), pointed, petal-like sepals, 4 stamens, and 1 pistil whose style is 2-branched. Blooms June–October.
As the flowers mature, the flower stalks (pedicels) lengthen, and the dry, spindle-shaped fruits bend downward, with a hooked beak prominent at the outer tip. Thanks to tension that builds at the joint of the flower stalk as the fruit matures, the fruits can be catapulted several feet from the stem at the slightest touch, hence the common name “jumpseed.”
Similar species: Sixteen species of smartweeds (Persicaria spp.) are recorded as growing wild in Missouri. Virginia knotweed can be distinguished from the others by its long, wandlike flowering stalks whose flowers are well separated (the others have the flowers clustered together in rather dense spikes). Also, Virginia knotweed’s flowers have 4 (not 5) petal-like sepals.
Stem length: 16–24 inches (not counting the flowering stalks).
Statewide. Found throughout much of eastern North America.
Habitat and Conservation
While many smartweeds and knotweeds grow in moist soils in open, sunny areas, this species is notable for living in shaded and half-shaded areas, particularly woodlands. Look for it in rich, moist soils in upland and lowland woods, bottomland forests, woodland edges, and in thickets. Keep in mind that young leaves in spring often have the distinctive Vs of brown, but by midsummer the leaves are usually mostly all green.
Native Missouri wildflower.
This and many other smartweeds or knotweeds were, for a long time, included in a very large genus Polygonum. Botanists decided to break that group into separate genera. Thus Virginia knotweed and its close relatives are now segregated into the genus Persicaria, in part, because of their shared characteristic of having spikelike flower clusters. Additional older names for this species include Antenoron virginianum and Tovara virginiana.
Native plant gardeners use Virginia knotweed as a hardy, clump-forming foliage plant that can be naturalized to help stabilize soils in low, wet areas. Note that it can spread aggressively, both by roots and from seeds. This is one of the few smartweeds to find a place in landscaping, as the rest are generally considered weeds.
Among gardeners, the most popular Virginia knotweeds are showy, variegated (white-marked) cultivars, including a group called Variegata, which have white-splotched leaves and pink flowers. Cultivated varieties of native plants are sometimes called “nativars.” Scientists are working to determine how these native species with unusual genetics interact with insect herbivores — do insects prefer them or avoid them? Do they offer better nutrition to the insects, or less? The answers to these questions can influence the fortunes of animals, such as birds, that feed on the insects.
The Cherokee made a hot tea from Virginia knotweed leaves and honey locust bark and used it as a medicine for whooping cough.
The plant is named “jumpseed” for the way the mature fruits (achenes) are expelled from of the plant at the slightest touch — they can be flung up to 13 feet away from the parent plant.
The family name, Polygonaceae, is from Greek poly (many) and gony (knee) and means “many knees”; it refers to the thickened, usually bent nodes where the leaves join the stem; they look like knobby little knees. In some members of the family, the plants can develop roots at these nodes when they touch the ground. In Virginia knotweed, however, the nodes do not develop roots.
You might think that the little flowers of Virginia knotweed would be too tiny to be useful to anything, but guess again: They offer nectar to a wide variety of bees, wasps, and ants. The various kinds of bees and wasps (including leaf-cutter bees, halictids, bumble bees, honey bees, thread-waisted wasps, and potter and mason wasps) serve the plant as pollinators. The ants, however, simply eat the nectar without pollinating.
By midsummer, it is common to see plenty of little holes in the leaves of Virginia knotweed — a testament to the plant’s role in a healthy food web. Many insects feed on the foliage, the roots, the sap, or other parts of Virginia knotweed, including beetles, true bugs, aphids, grasshoppers, moth and butterfly caterpillars, and more. Many of these insects become food for larger animals, from spiders and assassin bugs to fish, amphibians, reptiles, mammals, and birds.
The mechanism that causes the ripe seeds to be flung away from the plant at the slightest bump can also help the seeds to be dispersed as sticktights by animals. A passing mammal can easily trigger the jumpseed’s trademarked fruit dispersal. Upon deployment, the fruit’s hook-shaped beak becomes tangled in the fur of the animal, which then carries the seed to a new location.