Purple loosestrife (Lythrum salicaria) is found in Eurasia, Africa, and Australia. It attracts bees and butterflies as well as the loosestrife beetle which specifically targets the leaves of this plant. In some regions, the fast-growing purple loosestrife has become an invasive species that can outcompete native species.
I. Appearance and Characteristics
Lythrum salicaria or purple loosestrife is a flowering plant belonging to the family Lythraceae. It should not be confused with other plants sharing the name loosestrife that are members of the family Primulaceae. Other names include spiked loosestrife and purple Lythrum. This herbaceous perennial is native to Europe and Asia, and possibly Australia.
The generic name Lythrum is derived from the Greek ‘lythron’, meaning blood, in reference to the flower colour in some species. However, Pliny (A.D. 23–79) stated that Lythrum is named for Lysimachus, an army general and friend of Alexander the Great. Lysimachus is derived from the Greek lysis, meaning “loosing” and mache, meaning “strife”.
The specific epithet salicaria derives from the similarity of the leaves to those of willows or Salix species.
The English word loosestrife first appeared in written form in the 16th century. It was coined by English naturalist William Turner in his 1548 work A New Herball, in which he states: “[The plant] may in englishe be called red loosestryfe or purple loosestryfe.”
Lythrum salicaria can grow 1–2 m (3 ft 3 in – 6 ft 7 in) tall, forming clonal colonies 1.5 m (4 ft 11 in) or more in width, with numerous erect stems growing from a single woody root mass. The stems are reddish-purple and square in cross-section. The leaves are lanceolate, 3–10 centimetres (1–4 in) long and 5–15 millimetres (3⁄16–9⁄16 in) broad, downy and sessile, and arranged opposite or in whorls of three.
The flowers are reddish purple, 10–20 millimetres (1⁄2–3⁄4 in) in diameter, with six petals (occasionally five) and 12 stamens, and are clustered tightly in the axils of bracts or leaves; there are three different flower types, with the stamens and style of different lengths, short, medium or long; each flower type can only be pollinated by one of the other types, not the same type, thus ensuring cross-pollination between different plants. The flowers are visited by many types of insects, and can be characterized by a generalized pollination syndrome.
The fruit is a small 3–4 millimetres (1⁄8–5⁄32 in) capsule containing numerous minute seeds. Flowering lasts throughout the summer. When the seeds are mature, the leaves often turn bright red through dehydration in early autumn; the red colour may last for almost two weeks. The dead stalks from previous growing seasons are brown.
L. salicaria is very variable in leaf shape and degree of hairiness, and a number of subspecies and varieties have been described, but it is now generally regarded as monotypic with none of these variants being considered of botanical significance. The species Lythrum intermedium Ledeb. ex Colla is also now considered synonymous.
Associated insects
The flowers are pollinated by long-tongued insects, including bees and butterflies.
A number of insects use Lythrum salicaria as a food resource.
The black-margined loosestrife beetle Galerucella calmariensis is a brown beetle with a black line on its thorax. The adult feeds on the leaves of the plant, producing characteristic round holes. Its larvae destroy tender leaf buds and strip the tissue from the leaves. The golden loosestrife beetle Galerucella pusilla is nearly identical to G. calmariensis, but usually lacks the black thoracic line. Its feeding habits are also quite similar to the other leaf beetle.
The loosestrife root weevil Hylobius transversovittatus is a large red nocturnal weevil, which spends its nights feeding on leaves and leaf buds. The larvae emerge from their eggs and immediately burrow into the root of the plant, which they feed on continuously for over a year. This root damage stunts the plant’s growth and ability to create seeds. If several larvae inhabit the same root, the plant can be killed.
The loosestrife flower weevil Nanophyes marmoratus is a tiny weevil which lays a single egg in each flower. When the larvae emerge they eat the flowers’ ovaries, and the plant is unable to create seeds. The larvae usually proceed to hollow out the flower buds and use them as safe places to pupate.
Caterpillars of the engrailed moth (Ectropis crepuscularia), a polyphagous geometer moth, also feed on purple loosestrife.
II. How to Grow and Care
Sunlight
Purple loosestrife prefers full sun but can also grow in slight shade. A lack of sunlight can slow its growth. When the sun is strong in summer, give it some shade to prevent it from wilting.
Temperature
Purple loosestrife is widely distributed in temperate and subtropical regions, preferring warm, moist environments. It has strong drought resistance and can tolerate temperatures as low as -25 ℃. Purple loosestrife prefers moist soil, so it’s best planted near a pond or a ditch. A lack of moisture can cause it to grow slowly.
Watering
Thriving in wetlands, purple loosestrife has adapted to environments with high moisture levels and can tolerate occasional flooding. This species exhibits a preference for consistent hydration, aligning with its natural habitat’s conditions. Watering every week is optimal to maintain its health. As an herbaceous perennial commonly grown outdoors, purple loosestrife benefits significantly from rainwater during its active growing season, which complements its water needs.
Soil
Purple loosestrife can grow in many types of soil. It grows well in both slightly acidic and slightly alkaline soils, with an optimum pH range of 5.5-8.5. Purple loosestrife prefers moist soil and can even grow in water, so choose soil that is rich in nutrients and has good water retention.
Fertilizing
For optimal growth, purple loosestrife benefits from balanced nutrition fertilizers, while for enhanced flowering, high phosphorus options are preferred. Apply fertilizer sparingly in early spring and repeat mid-summer to support vigorous growth and robust flowering, being cautious of over-fertilization. Seasonal variations influence purple loosestrife’s needs; less is required in dormant periods. A light hand is key—excessive use can harm local ecosystems. Practical tip: Dilute fertilizer to half-strength to prevent root burn and encourage healthy development.
Planting Instructions
Propagation
Purple loosestrife propagates best in spring through seed sowing. Its propagation is relatively easy, with signs of successful propagation being young sprouts and vigorous growth. Ensure a moist, well-drained environment for optimal propagation results.
Transplanting
The best time to transplant purple loosestrife is during early to late spring, as it gives the plant ample time to establish before the growing season. Choose a sunny, well-drained location for optimal growth. Gently loosen the root ball when transplanting, and ensure proper spacing for healthy development.
Repotting
Repot purple loosestrife every 2-3 years in early spring to accommodate its vigorous growth habit. As a large, upright perennial, a spacious pot is crucial for the root system’s development. Post-repotting, maintain moist soil and provide full sun to part shade for optimal recovery. Use a loamy, well-draining mixture to ensure healthy regrowth. Extra care during the initial weeks assures steady establishment.
III. Uses and Benefits
- Ornamental uses
Purple loosestrife is a showy ornamental garden often cultivated in wetlands or bog gardens in controlled environments, where its invasive nature can be monitored. This plant will thrive along ponds and streams as a border plant and will attract pollinators.
- Medicinal uses
It has been used as an astringent medicinal herb to treat diarrhea and dysentery; it is considered safe to use for all ages, including babies.
