Answer: Purple loosestrife is illegal to sell or transport in much of North America because of its invasive nature. Lythrum salicaria is the culprit, but many states also ban Lythrum virgatum and hybrids. Some species which are similar in appearance to purple loosestrife but are not invasive to natural areas include the following: obedient plant (Physostegia virginiana) produces flowers shaped like dragon heads and the seed are a source of food for songbirds. The pink flowers of spiked blazing star (Liatris spicata) and button blazing star (Liatris squarrosa) provide food for butterflies and hummingbirds, and the seeds are eaten by songbirds.
Given the reputation of loosestrife, I'm glad you asked about substitutes!
Q&A Library Searching Tips