Flower:

Flowers are in a spike up to 20 inches long, densely packed with purple or pinkish-purple flowers. Individual flowers are ½ to ¾ inch across, have 5 to 7 petals (6 is most common), about 10 purple-tipped stamens. The petals have pointed or slightly rounded tips, a dark vein down the middle, and a wrinkled texture like crumpled tissue paper. The tubular calyx holding the flower is yellowish green, ridged, hairy, and has several long prong-like appendages at the tip end. One plant has numerous spikes.

Leaves and stem:

Leaves are up to 4 inches long and 1 inch wide, toothless, gradually tapering to a pointed tip, with a rounded or heart-shaped base and no leaf stalk. Attachment is opposite, or may be in whorls of 3 or 4. The main stem is square and covered in downy hair. Established plants can have dozens of shoots and take on a bushy appearance.

Fruit:

The calyx becomes a receptable for the seed, and turns purplish as it ripens, then drying to brown. A single mature plant can produce well over a million seeds a year.

Notes:

Purple Loosestrife is on the prohibited weed list for Minnesota and was introduced to the US by the nursery industry. It quickly escaped cultivation and has been ravaging wetland habitats ever since. It is exceedingly aggressive and can overtake native plants very quickly. Purple Loosestrife is sometimes mistaken for Fireweed (Chamerian angustifolium), which has 4 broad paddle-shaped petals and alternate leaves.