We found no Allee effect in invasive Phragmites australis in a small river system in New England.

Patch vigor (expansion rate, stem density, stem height) and genotypic richness was not related.

Patch vigor was fostered by increasing distance from the river mouth and elevated nitrogen.

Propagules were transported primarily downstream, but were often caught up in tidal creeks.

Genotypic richness was fostered by low-level human development and barren substrate.