Ornamental plant species are frequently planted well outside of their native ranges and, in some cases, escape from cultivation to develop naturalized populations. Understanding the causes and consequences of naturalization resulting from intra-continental introductions will shed light both on mechanisms of invasion and on the potential for facilitated migration as a response to global climate change. In this study, we characterized a naturalized population of pin oak (Quercus palustris) that has recently arisen in the woodlands of Colby College in central Maine, 275 km from its native range limit. The population stemmed from trees in two small horticultural plantings, one from 1950 and one from the mid-1980′s. As an initial indication of potential invasiveness, we compared growth rates of individual pin oak trees to those of red oak (Quercus rubra), the most closely related native species and likely competitor. We also carried out genetic testing to determine how many parents gave rise to the naturalized population. Growth rates of pin oak saplings were only marginally greater than those of red oak saplings. Genetic analysis showed that at least 75% of sampled progeny descended from the six trees of the original 1950 population, with at least one-third of the progeny descending from two individual trees. Our results demonstrate that a small number of founders can be sufficient to establish a naturalized population beyond the native range limit. It is too soon to tell whether naturalized populations of pin oak in Maine will behave invasively, as might be expected with a trans-oceanic introduction, or will behave more similarly to populations at a naturally expanding range limit. There is no evidence to date that the naturalized pin oak population is detrimentally impacting the native forest community.