PURPLE LOOSESTRIFE

Purple loosestrife (Lythrum salicaria L.) is a plant that has been invading North American wetlands since its introduction from Europe in the early 1800's. It is a vigorous perennial that first arrived along the New England seaboard probably in the ballast of ships. Since its introduction, it has proliferated throughout the northeastern United States, Quebec's St. Lawrence basin, and the entire Great Lakes area south of Sault Ste. Marie. It has also spread into the northern reaches of the Mississippi river system into southern Manitoba and westward to southern B.C., Washington and Oregon.

Purple loosestrife is most easily recognized during its flowering period, which lasts from July to September in Ontario. It is a tall erect plant with stems reaching heights over 2 meters topped by the reddish-purple inflorescence. The flowers are made up of usually 6 petals about 10mm long, reddish-purple in colour, with 12 stamens, some or all of which protrude from the flower. These flowers are packed tightly into whorls around the stem forming the spike, which makes up the top 30 cm of the stem. The lowest flowers on the spike bloom first and blooming continues upwards throughout the season producing mature seeds from the lower flowers while the plant is still green and growing.

The stems and leaves are covered in a dense mat of short fine hairs. There is very little branching in the stems as they grow from the overwintering rootstock. The leaves are long (4-7cm), untoothed, opposite and stalkless. They are rounded to blunt at the base where they join the stem and taper to a point at the tip. They get progressively smaller towards the top of the stem and in some cases become alternate in arrangement. The plants produce a strong main stalk and several secondary stems from a single rootstock. The rootstalk consists of a taproot with many secondary roots and much branching but shows no evidence of horizontal creeping ability. The single rootstalk of an older plant can produce a root-crown up to 0.5m across and give rise to as many as 50 stems. These stems form a colourful wide-topped clump that is capable of producing over 2 million seeds in a season.

Purple loosestrife may most commonly be confused with three other wildflowers: fireweed (Epilobium augustifolium), blue vervain (Verbena hastata), and blazing stars (Liatris spp.). Fireweed has violet-pink coloured flowers consisting of 4 petals and 8 stamens, and has very narrow, alternate and toothed leaves. Blue vervain is slightly shorter, about 1.5m tall, with much smaller flowers made up of 5 blue or purple petals and 2 pairs of stamens. It rough hairy leaves are coarsely toothed and stalked. Blazing stars have many tiny purple florets compressed together to form fuzzy flowering heads on stems slightly more than a meter tall. The leaves are alternate, very narrow and often speckled with purple resinous dots.

ESTABLISHMENT

The flat thin-walled seeds of purple loosestrife must settle on open moist ground or saturated organic debris to germinate and take root. Seeds that germinate in the same season they are produced, rarely survive the winter, but seeds that do not germinate can remain viable for many years. The seeds will germinate in soils with a pH 4-9.1 but they are more successful in soils from slightly acidic to neutral pH. Seeds will not germinate below 14 C, with the critical temperature lying somewhere between 15 C and 20 C. The new seedling needs at least 50% of full sunlight for vigorous growth and seed production, with flowering beginning 8-10 weeks after germination.

DISPERSAL

Seed dispersal occurs by floating seeds being carried by water currents and is the most common method of dispersal. Seeds can also be carried in mud and debris adhering to off-road vehicles, the bodies of waterfowl, turtles and muskrats, or in the cooling systems of outboard engines. The plant can also be spread as parts of broken or cut plants that float away or are carried away by animals or equipment. These pieces can take root and produce new plants. Unintentional seeding can occur from seeds of loosestrife contaminating commercial packages of native wildflower seeds that have been sold through seed companies.

ADAPTATION/HABITAT

Purple loosestrife is well suited to most freshwater margin areas including marshes, bogs, ditches, stream and riverbanks, and reed swamps. It establishes best in disturbed areas where local vegetation is highly stressed or non-existent, and it can adapt well to stress itself. Loosestrife can survive rising water levels, lower light levels, changes in nitrogen and phosphorous levels and trampling or cutting, all by making morphological or physiological adjustments. On undisturbed sites it is slow to invade because of competition from local vegetation. Loosestrife is also capable of summer dormancy whereby a rootstock will not send up shoots for a season but will again sprout shoots the following season.

THREAT

The threat from invasion of local wetlands by purple loosestrife comes from the aggressive manner by which it replaces native vegetation. Where it grows, it produces a dense mass of roots that out competes other plants for nutrients and more importantly, space. The woody root systems quickly choke out the original species of plants, forming thick stands that can maintain themselves for up to 20 years. In the process, this alters the ecosystem of the area resulting in losses of fish and wildlife as well as native flora. Because the habitat is not compatible as a food source or a breeding ground, the inhabitants, whether permanent or migratory, move away or perish. In either case, many species are lost to the area for good.

CONTROL

Prevention of invasion is the most effective method for control because once loosestrife is established it is very difficult to eradicate. Minimize disturbance of susceptible areas, such as soil scraping, trampling, or drastic changes in water level. Monitor these areas at least on a seasonal basis checking for the first appearance of the plant. Hand-pull any new young seedlings being sure to remove the entire plant from the area. Avoid cutting, mowing, or trampling the plants, as this stimulates shoot growth from buds at the stem base. Flooding is not an entirely satisfactory control method. Currently there are no chemicals registered for the control of purple loosestrife.

Peter F. Walsh

PDCF-0022 (Revised 2003/11/07)