New shoots grow from the expanded crown in spring. Roots can associate with mycorrhizae. Lower stems sometimes develop adventitious roots. Ovary superior but appears inferior, surrounded by and not fused to calyx tube. Calyx tube appendages linear, 2-3 mm long .
Flowers solitary in axils.
Petals pale pink to lavender, 2-5 mm long . Capsules oblong-ovoid, surrounded by persistent calyx tube, open into halves at tip. Above ground parts of purple loosestrife typically die in late fall. Senescing foliage often turns red. Dead brown stems often persist through winter, are oppositely branched, and may retain a few capsules.