European origin, spread & degraded temperate North American wetlands since the early 1800s. Introduced both as a contaminant of European ship ballast and as medicinal herb for treatment of diarrhea, dysentery, bleeding, wounds, ulcers and sores.
By the 1830's, it was well established along New England seaboard. Construction of inland canals & waterways in the 1880's favored expansion of the plant into NY and the St. Lawrence River Valley. The continued expansion of it coincided with increased development and road systems, commercial dist. of the plant for horticultural purposes, and regional propagation of seed for bee forage. As of 1996, it is found in all contiguous states (except FL) and all Canadian provinces.
Anatomy & Physiology:
Perennial, emergent aquatic plant. 30 -50 herbaceous, erect, annual stems rise about 9 feet tall, from a persistent perennial tap root and spreading rootstock. Short, slender branches spread out to form a crown 5 ft wide on plants. Somewhat squarish stems are 4 to 6 sided, w/ nodes evenly spaced. Stems submerged under water develop aerenchyma tissue characteristic of aquatic plants. The stalkless leaves can be opposite or opposite w/alternating pairs at 90 degree angles or sometimes in whorls of 3, near the base. Upper leaves & floral bracts can be alternate. Leaves are one ½ to 4 inches long, wider and rounded or heart-shaped at the base. Leaf shape varies from lanceolate to narrowly oblong, and is sometimes covered w/fine hairs. Variability in pubescence & leaf shape is influenced by light levels - leaf area increases and fine hairs decrease w/ lower light levels.
Magenta flowering stems end in a 4-16 inch flowering spike. Flowers appear from July to early October. The flowers are in pairs or clusters of the upper leaf axils. Each flower is complete, containing 5 - 7 petals, w/ the same number of sepals as petals, and 2x as many stamens as petals. Typical flowers have 6 sepals, 6 petals and 12 stamens. Ovary is superior, w/ 2 fused carpels. The narrow, wrinkled petals are from 1/4 to 5/8 in long. Petal color can range from white to pink to red to purple. Fruit is a 2-valved capsule enclosed in the pubescent calyx. Pollen grain color & size varies. Each flower exhibits one of three style lengths (tristylous). Flowers are short-styled (w/medium and long stamens), medium-styled (w/short and long stamens), or long-styled (w/short and medium stamens). Individual plants produce 1 style-type.
Seed Production and Dispersal: A mature plant can produce 2.7 million thin-walled, flat seeds. Some seeds sink in the water, and resurface after germination. Water dispersal includes floating seedlings and floating ungerminated seeds. The seeds are small and light enough for wind dispersal. Most dispersal is down slope, (not downwind). Transport through wetland mud by animals, humans, boats, or vehicles. Spread also occurs when seeds eaten. Also vegetatively. Disturbance to plant (stomping/ breaking underground stems, or breaking off stems or roots during incomplete plant removal) initiates bud growth.
Freshwater & brackish wetlands. Successful colonizer & potential invader of any wet, disturbed sites in North America. Can sometimes grow in upland sites.
No effective method is available to control it. Uprooting plant by hand & ensuring removal of all vegetative parts can eliminate it. Water-level manipulation, mowing or cutting, burning, and herbicide application. These are costly, require continued long-term maintenance and are non-selective and environmentally degrading.
Biological Control: 4 host specific insect species have been released in the US. These species are a root-mining weevil, two leaf-eating beetles, and a flower-feeding weevil.