Purple loosestrife begins to bloom in July and continues until September or October. Flowering occurs 8-10 weeks after germination and starts at the bottom of the inflorescence. The lowest capsules on the inflorescence will ripen and disperse their seeds while flowering is still occurring further up the spike. Each capsule contains approximately 120 seeds with an average plant having approximately 900 capsules. Purple loosestrife seeds are mostly dispersed by water but are also dispersed by wind, and in mud adhering to aquatic wildlife, livestock, and people.
Seed can remain viable underwater for several years.
Germination occurs with the onset of favorable conditions, usually in late spring. Seedlings that germinate in the spring grow rapidly and will produce a floral shoot up to 30 cm in length the first year, while summer-germinated seedlings develop only five or six pairs of leaves before the end of the growing season. The taproot is strongly developed in the seedling stage and persists throughout the life of the plant. In mature plants, the taproot and major root branches become thick and woody. The semi-woody aerial shoots die in the fall but persist for one to two years making stands of purple loosestrife very dense.