Purple loosestrife is an herbaceous, erect long-lived perennial typically growing numerous stems 2½–8 ft (¾–2½ m) tall from a spreading, woody root. Stems are squarish in cross-section with 4–6 sides. Leaves have smooth margins and are lance-shaped, stalk-less, and 2–5 in (5–12 cm) long. Flowers are less than 1 in (2½ cm) across with 5–7 pink to purple (sometimes crumpled-looking) petals. Flowers occur in spiked clusters from early summer to early fall. Each flower can produce well over 100 small, light-colored seeds.