Purple loosestrife is a stout, erect perennial herb with a strongly developed taproot and showy spikes of rose-purple flowers. Blooms June through September. Purple loosestrife has flowers with 5-7 purple petals (petals can also be pink or white); the leaves are opposite or in whorls of 3, all the leaves lack teeth on their margins; the stem is stiff and 4-sided, often appearing woody at the base of large plants.
Similar species: There are several native species that also produce purple spikes of flowers. Purple loosestrife can be differentiated from them the combination of other characteristics.