Purple Loosestrife (Lythrum salicaria L.)
Purple loosestrife is an aggressive wetland perennial that invades marshes, streambanks, irrigation ditches, and other moist areas. It grows upright from a thick, woody rootstock and can reach 6–8 feet tall under favorable conditions.
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Stems are erect and often branched, terminating in tall flowering spikes. Leaves are simple, lance-shaped, and have smooth margins. They are arranged opposite each other or in whorls along the stem. Leaves may feel slightly downy, particularly on younger growth.
Flowering occurs in mid- to late summer. Showy rose-purple flowers have 5–7 rounded petals and numerous stamens. Flowers are densely arranged in long, vertical spikes (racemes) at the tops of stems, creating a striking display.
Purple loosestrife reproduces by both seed and vegetative growth. Each plant can produce millions of tiny seeds annually. Seeds develop in small, two-valved capsules and are easily spread by water, wildlife, and human activity. Established plants can also produce new shoots from the root system, forming dense stands that displace native wetland vegetation.
Quick ID Guide
Life Cycle:
• Herbaceous perennial
• Thick, woody rootstock
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Height:
• 6–8 feet tall
• Upright, branched stems
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Leaves:
• Lance-shaped
• Smooth margins
• Opposite or whorled arrangement
• Slightly downy texture
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Flowers (Mid–Late Summer):
• Rose-purple
• 5–7 petals
• Numerous stamens
• Dense vertical spikes at stem tips
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Seeds/Reproduction:
• Tiny seeds in two-valved capsules
• Extremely prolific seed production
• Also spreads by root shoots
Management Options
Prevention and early detection are critical for managing purple loosestrife, particularly in wetland environments. Avoid planting ornamental varieties and monitor riparian and wetland areas regularly for new infestations.
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Small infestations may be controlled by hand pulling or digging, provided the entire root crown is removed. Removal is most effective when soils are moist. Care must be taken to prevent soil disturbance and the spread of plant fragments. All flowering heads should be bagged and removed from the site to prevent seed dispersal.
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Mowing or cutting before seed set may reduce seed production, but will not eliminate established root systems. Repeated treatments are necessary to prevent regrowth.
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Large or established infestations are typically managed with herbicides labeled for aquatic or riparian use. Applications are most effective when made during active growth, often at early flowering. Always read and follow the herbicide label before application. Contact Sublette County Weed & Pest for guidance on appropriate products, rates, and timing for wetland environments.
