Purple Loosestrife is an erect introduced invasive perennial grows 3 to 7 feet in height. There are multiple stems from the base and main stems have 4 or more angles and branch frequently. Stems are usually hairy within the inflorescence.
The leaves are 1 1/2 to 4 inches long, lance like, downy, smooth edges, stalkless and clasping. They are opposite but can have whorls of 3.
The inflorescence is a terminal spike that can be up to 16 inches high and on auxiliary spikes rising from the upper leaf axils. The spikes are periodically interrupted.
Flowers: The purple to reddish flowers are 1/2 to 1 inch wide and are 6-parted and have a wrinkled appearance; however, many flowers may only have 5 petals. The hairy calyx tube is green with noticeable veins and with 5 to 6 teeth, or appendages, on the tip. Flowers may have a short stalk but they are usually stalkless. There are 2 sets of stamens, 8 to 10 in total, alternately longer and shorter with the longest protruding from the flower tube and the shortest barely protruding; these have white filaments and yellow anthers. The long single style has a knob-like tip. Both the style and the stamen filaments tone to pinkish-purple at maturity. Flowers do not self-pollinate.
Seed: Fertile flowers produce a seed capsule that is surrounded by the calyx. Each capsule contains numerous small tan oblong seeds that can be blown a short distance by the wind and these will float on water for further distribution.
The plant is exceedingly invasive. Each plant can produce up to 2.7 million tiny seeds which are viable for years.
Habitat: Purple Loosestrife occupies the aquatic edges of streams, lakes, swamps, etc. It grows from a taproot with lateral rhizomes and the stem secures itself with dense woody roots that are impossible to pull by hand on mature plants.
Names: The genus Lythrum is believed to be derived from the Greek word lythron, for 'blood' and referring to the color of the flowers. The species salicaria means 'willow like' as the plant grows in the same habitat has willow with willow-like stalkless leaves. The author name for the plant classification, 'L.' refers to Carl Linnaeus (1707-1778), Swedish botanist and the developer of the binomial nomenclature of modern taxonomy.
Comparisons: In moist areas you may encounter the Winged Loosestrife, Lythrum alatum, which has flowers that are also purplish, but the stems are winged and leaves are thick and rigid. This is the only other species of Lythrum normally found in Minnesota.
Above: The inflorescence is a tall terminal spike, sometime interupted, and additional spikes from the upper leaf axils. Drawing by Dr. Otto Thomë, courtesy Kurt Steüber's Online Library,
Below: 1st photo - While petals can be six in number, five are frequently seen (as shown here) and have a wrinkled appearance. 2nd photo - The calyx tube has hair, veining and 5 to 6 "teeth" on the apex.
Below: 1st photo - This flower example has the standard 6 petals. Eight stamens are visible with 4 longer and 4 shorter, the shorter ones just slightly exerted from the corolla tube. The style has a knob-like tip (capitate). Both the style and the stamen filaments turn pinkish-purple at flower maturity as seen here. In the younger state, the style is greenish and the filaments are white as seen above. 2nd photo - A detail of one seed capsule with the dried calyx, which remains attached, and some of the small tan seeds contained therein. Each flower stem has a number of capsules (as seen below) and the plant has many flower stems, thus you can seed what a great quantity of seeds this invasive plant can produce.
Below: 1st photo - The stalkless leaves appear to clasp the stem. 2nd photo - Seed head detail. 3rd photo - In certain conditions the fall color of the leaves can be brilliant red.
Below - Historical Photo: Birch Pond, located just outside the Garden, shown surrounded by stands of Purple Loosestrife on August 5, 1950. The Garden is located up the hill behind the trees. Biological means were used in the late 1990's to successfully eradicate it. Note the white swan in the pond. For many years a pair of swans occupied the pond. Photo from a Kodachrome by Martha Crone, courtesy Minnesota Historical Society, Martha Crone Collection.
Notes: Purple Loosestrife has been present in the Garden for a long time; Eloise Butler was the first to note it, logging on July 15, 1916 that it was beginning to blossom. Martha Crone noted it in bloom on July 31, 1939 and also listed it on her 1951 Garden Census. Originally introduced from Europe as a garden perennial, it has invaded and spread across the United States and southern Canada. Only Arizona and a few gulf coast states seem to have escaped it. In Minnesota it is found in a number of counties, principally in the eastern half of the state. Noxious: In Minnesota it is listed as a "Prohibited noxious weed" which precludes it's sale by any nursery. In recent decades it has been the successful target of extirpation by biological means, using a particular beetle (Galerucella spp.) that feeds on the plant and gradually weakens it so it cannot produce seed. The Minneapolis Park and Recreation Board has successfully used this method to remove the plant from nearby Birch Pond (pictured above). Their program began in 1997 and by 2000 the pond was free of Loosestrife. In the Garden it is under strict control. Chemical control requires glyphosate. The plant has remained under strict control in the Garden and has been allowed to remain for historical reasons.
As noted above, it was not originally apparent how invasive the plant would be. The Minneapolis Sunday Tribune would even state "The Purple Loosestrife grows as high as six feet in wet meadows and margins of pools. A welcome European immigrant which usually grows in dense colonies, it rewards us for our hospitality with showy masses of color like this." (Picture Magazine, May 21, 1950, Minneapolis Sunday Tribune). The article pictured Birch Pond (above). Little did they suspect just how well we would be rewarded!.
Monet's garden at Giverny grows Purple Loosestrife, among other tall plants for summer color and height, along the Grand Allée. Monet would even bank the soil to make some of the plants look even taller. (Ref.# 36a)
Planting was still advocated into the 1960's. Former Curator Martha Crone did so, but also acknowledged the invasive tendency in this comment: "It is a good plant to grow along streams, margins of ponds or in wet meadows. Especially where the competition is too severe for less aggressive plants to grow. The Plant is a long-lived perennial and produces graceful spikes of purple or pink flowers. They bloom during July and August. When once established it is hard to eradicate and will crowd out other weaker growing plants. It can be grown in garden borders where it remains smaller and does not spread. This six-petaled flower has 12 stamens [ed. note - actually 8 or 10] of two different lengths, and the length of the single pistil varies in different flowers; this is termed by botanists as trimorphous. Only pollen from stamens of the same length can pollinate stigmas, therefore each flower is sterile to its own pollen, thus ensuring the vigor of the race." Published in the Fringed Gentian ™ April 1958, Vol. 6 #2
References: Plant characteristics are generally from sources 1A, 32, W2, W3, W7 & W8 plus others as specifically applied. Distribution principally from W1, W2 and 28C. Planting history generally from 1, 4 & 4a. Other sources by specific reference. See Reference List for details.
Friends of the Wild Flower Garden, Inc. Text and photos are by G. D. Bebeau unless otherwise credited. "www.friendsofthewildflowergarden.org"