We analyzed the recent (< 25 yr) spread in New Hampshire, USA, of the exotic tree Kalopanax septemlobus, native to Asia. The invasion was likely initiated by a single tree planted ca. 1972. Our objective was to assess the viability of the invasion, especially in light of the small propagule size. We tallied, mapped, aged, and measured the height and growth of K. septemlobus individuals at two sites, the University of New Hampshire campus (UC) and Thompson Farm (TF), both in Durham. We found over 3,800 plants at UC and 270 at TF in < 120 ha (296 ac) total area. Plant age ranged from 0 to 22 yr, and UC plants were as far as 775 m (2,543 ft) from the purported parent tree. Annual height growth was comparable to midtolerant native trees. Plants occurred in both open and forested habitats, and the mean level of photosynthetically active radiation incident on understory plants was 4 to 6% of full sun. The large population size, shade tolerance, rapid height growth, and ability to sprout from damaged stems suggest potential for K. septemlobus to invade and persist in forests, the most common natural ecosystem in the northeastern United States. We further suggest that small propagule size, likely a single tree, has not prevented K. septemlobus from initiating a spatially extensive and vigorous population. Kalopanax septemlobus has been planted as an ornamental in the northeastern United States, and prevention of region-wide invasion might depend on removal of these trees, even when they occur as single individuals.