Similar Species: Lady's Thumb or Spotted Ladysthumb (P. Persicaria) is a European species that has become naturalized throughout the US. It can reach 1m (3') in height and can be easily distinguished from P. Pensylvanicum by the fringed stem sheath and the dark spot present on the center of most leaves. Less similar is Dock-Leaved Smartweed also know as Curlytop Knotweed, Nodding or Pale Smartweed (P. Lapathifolium) can reach 2m (6') in height and has longer, slender flower spikes that droop and are white, pink or green.

There are about 30 other species of Polygonum in the Eastern US.

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One of the best general guides to wildflowers of the North Eastern and North Central United States. Newcomb's key is an excellent, simple method for identifying plants. Newcomb has drawings for almost every plant mentioned that are excellent aids to identifying the species. Though only the more common plants are covered this is often the first book I pick up when trying to identify a wildflower.

Wildflo wers of Tennessee the Ohio Valley and the Southern Appalachians

This is perhaps the best of many field guides covering this region. Featuring 446 excellent color photographs (located with the text) and mentioning as similar to those illustrated are another 800 or so species for a total coverage of over 1,200 species. The start of each family section includes line drawings of some of the species showing important features. The text includes the usual description, bloom season, range, habitat and additionally includes information such as medical uses and lore and how the species was named. This is the official field guide of the Tennessee Native Plant Society.