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.
New shoots arise the following spring from buds at the top of the rootstocks.
Purple loosestrife can spread vegetatively by resprouting from cut stems and regenerating from pieces of root stock. These plants are found in regularly disturbed sites. Currently, populations occur in the North Unit along the Mississippi River. Purple loosestrife has the potential to invade and replace native communities and poses an endangerment to the areas' primary resources. Several control methods have been used on purple loosestrife with varying levels of success.