Purple Loosestrife

Purple loosestrife is a prohibited noxious weed that is found in wetlands, canals, and ditches. As a prohibited noxious weed, it must be eradicated if found in Alberta. Purple loosestrife can quickly spread, clogging waterways and choking out native plants. As it spreads, it reduces biodiversity and degrades habitat for many native birds, insects, and other species.

Purple loosestrife has a noticeably square, woody stalk. The smooth edged leaves are pointed and are attached directly to the stalk on opposite sides. The plant can reach heights of 1-2 meters (3-6 feet). The flowers of purple loosestrife are pink or purple and form a long flower spike at the top of the stems. Flowering usually occurs in July and August.

A single purple loosestrife plant can grow as many as 30 flowering stems that will produce close to 3 million seeds each year. The seeds are very small and are easily spread by wind, water, animals and human activity. It is very important to control this invasive weed to protect farmland and waterways as well as natural wildlife habitat.

Purple loosestrife is sometimes sold in greenhouses under different names. These names include, but are not limited to: spiked loosestrife, lythrum, and purple lythrum. If you have purchased purple loosestrife in the past and planted it in your yard, you are required to remove it as it is a prohibited noxious weed in Alberta. You can replace it with a non-invasive flower species that does not pose a threat to the environment. Some alternative flowers are:

• Lilies (Lilium)
• Spiked Speedwell (Veronica spicata)
• Siberian Iris (Iris sibirica)
• Spiked Gayfeather, Blazing Star (Liatrus spicata)
• Garden Sage (Salvia)
• Purple Coneflower (Echinacea purpurea)


The best control method for purple loosestrife is to limit seed production. Clipping the flowering spikes before they can set seed will eliminate millions of seeds from spreading into the environment. This is best done in July or early August. The remainder of the plant should then be dug up so as to not break off the roots. Purple loosestrife can regenerate from broken roots, so it is important to dig up the whole root. Pulling the plants generally results in a broken stem and new plants growing the following year. Cut stems and removed plants should be burned or taken to the landfill in a durable garbage bag. Do not compost purple loosestrife as it can regenerate from roots and stems. There are several herbicides that are registered in Canada for control of purple loosestrife; however they are difficult to apply because the weed generally grows in water. No herbicide is registered for the control of purple loosestrife if it is growing in water. Always follow the label directions when applying any herbicide. If you find purple loosestrife in the MD of Taber, please contact the Agricultural Service Board for more information.
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