Getting to Know Purple Loosestrife
Wilson, L.M.; Schwarzlaender, M.; Blossey, B.; Randall, C.B. Biology and Biological Control of Purple Loosestrife. United States Department of Agriculture, Forest Service. Forest Health Technology Enterprise Team. FHTET-2004-12. 78 p.
Purple loosestrife (Figure 3a) is an erect, herbaceous, perennial wetland weed that is commonly found along waterways and other wetland habitats. It was introduced from Europe during the late 1800s.
Purple loosestrife leaves are 2-5 inches (5-12 cm) long and narrow with a rounded or heart-shaped base, and smooth-edged. The stalkless leaves are arranged opposite or alternate along the stem, and lower leaves often form a whorl around the stem.
One of the more recognizable features of purple loosestrife is its square-shaped stems. They are five- or six-sided (you can feel the edges of the stem when you roll it between your fingers), 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 5-9 feet (1.5 to 3 m). Dead stems persist through the winter and often decay slowly over several years, and new shoots are produced each spring from buds on the persistent, woody rootstock.
The inflorescence of purple loosestrife is a spike of numerous, showy, reddish-purple or magenta flowers set in clusters (Figure 3b). Each flower measures about 0.6-0.8 inches (15-20 mm) across, has five to seven petals, and a small, yellow center. Flowering occurs from mid-June to September, depending on location.
Seeds are produced in rounded capsules about 0.24 inches (6 mm) in length. The capsules open to release more than a hundred tiny, light brown seeds about the size of poppy seeds (Figure 3c). 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.
Biology and Ecology
Seeds germinate in late spring and early summer in open, sunny places when soil temperatures reach 68°F (20°C). Seedlings lodge into moist soil and quickly grow to over 3 feet (1 m) tall in their first growing season, and many even flower (Figure 4). Even young plants develop a large taproot, and have an extensive rootstock by the end of the first growing season. The mature size of the plant will depend, in part, on growing conditions, soil type, water level, the genetic potential of the plant, and plant density. After the first extreme frosts in fall, all above-ground plant parts will die back. Plants resprout each spring from their large rootstocks (Figure 5).
Any sunny or partially shaded wetland is susceptible to purple loosestrife invasion. Although purple loosestrife generally prefers moist soils, mature purple loosestrife plants can tolerate a wide range of water levels, pH and climatic conditions, soil, and vegetation types. In flooded areas, the plant forms dense, fibrous rootmats.
Purple loosestrife spreads primarily by seeds. Seeds are dispersed by floating on streams, by birds, wildlife and livestock, or in the mud of vehicle tires or boots. In addition to spread by seeds, purple loosestrife also spreads vegetatively. Root fragments cut from the plant can produce new plants and stem pieces may generate new infestations when they float downstream and lodge against a streambank.
Although still sold and planted for its beauty as an ornamental, purple loosestrife’s habit of devastating waterways and wetlands has caused its sale to become restricted or, in some states, illegal. It is especially important to know that the species called European wand loosestrife, L. virgatum, and widely sold as an ornamental, is the same species as purple loosestrife (L. salicaria). Thus, garden and pond plants that are being sold as L. virgatum are in fact L. salicaria, and may be illegal for sale in your area. Studies have shown that other ‘varieties’ of loosestrife sold as ornamentals are generally not sterile as advertised.
In addition to purple loosestrife, there are a number of other Lythrum species in the United States and Canada. Some species are native (see Table 1a) and some, like purple loosestrife, are introduced (see Table 1b).
Other plant species are often confused with purple loosestrife. While they are not related to purple loosestrife, they may be similar in appearance. They include hardhack (Spirea douglasii), fireweed (Epilobium angustifolium), and blue vervain (Verbena hastata) (see Table 2).
Table 1a. Species of Lythrum in the United States and Canada: native species
Table 1b. Species of Lythrum in the United States and Canada: introduced species. Not shown: spatula-leaf loosestrife (L. portula) and thyme-leaf loosestrife (L. thymifolia).
Table 2. Comparison of purple loosestrife and similar looking plants. (Left to right, top to bottom: UGA1291040, UGA1291041, UGA1291042, UGA1291043)