Purple Loosestrife is a herbaceous perennial plant that grows 3 - 6 feet 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. Purple Loosestrife has reddish-purple flowers with six petals. Once this plant begins growing, it spreads rapidly. Purple Loosestrife enjoys growing in areas that are moist.