Purple loosestrife is identified by its spike of pink-purple flowers with five or six petals and opposite leaves on a square stalk. There are a few native flowers that are often confused with purple loosestrife including swamp loosestrife, winged loosestrife, fireweed and blue vervain. Swamp loosestrife's pink flowers are clustered in tufts instead of on a continuous spike. Winged loosestrife's pink flowers are scattered along the stem. Fireweed tends to be found in drier sites and its pink flowers have four petals each.
Blue vervain has more numerous more slender flower spikes that are more blue-purple.
If you find purple loosestrife, pull the plant out if you can or cut the plant at ground level or below the water level. Dispose of the plant in a closed plastic bag. This prevents seeds from being dispersed to other sites. Now is a good time to get rid of loosestrife, before it produces seeds. Researchers have found an insect that feeds on purple loosestrife, killing it.