Lythrum virgatum, Purple loosestrife is an erect, herbaceous, perennial wetland weed. One of the more recognizable features of purple loosestrife is its square-shaped stems. They are woody, and can be either smooth or covered with downy hairs. Stems have short, slender branches and evenly spaced nodes. Mature plants have up to 30 flowering stems, which can reach a height of 1.6-3.3 ft. (50-100 cm).
Foliage
Purple loosestrife leaves are 2-5 in. (5-12 cm) long and narrow with a rounded or heart-shaped base, and smooth-edged. The stalkless leaves are arranged opposite or sometimes alternate along the stem. The lower leaves often form a whorl around the stem.
Flower
The inflorescence of purple loosestrife is a spike of numerous, showy, reddish-purple or magenta flowers set in clusters. Each flower measures about 0.6-0.8 in. (15-20 mm) across, has five to seven petals, and a small, yellow center. Flowering occurs from about April to August.
Fruit
Seeds are produced in rounded capsules about 0.24 in. (6 mm) in length. The capsules open to release more than a hundred tiny, light brown seeds about the size of poppy seeds. Water, wind, wildlife, and humans easily spread the lightweight seeds that are shed throughout the winter. Purple loosestrife is a prolific seed producer; a single mature plant can produce several million seeds. When purple loosestrife densities are high, billions of seeds are produced per acre. Seed viability is greater than 90 percent and seeds can remain viable in the soil for many years.
Ecological Threat
L. virgatum is commonly found along waterways and other wetland habitats where it replaces native vegetation. It was introduced from Europe during the late 1800s. L. virgatum is native to parts of Europe and temperate Asia.