Elephantgrass (Pennisetum purpureum Schum.) is a candidate bioenergy grass in the southern Gulf Coast Region, but it is also a weed in sugarcane (Saccharum sp.) fields, rights of ways, and natural areas of central and southern Florida. It is not known if naturalized populations (NPs) and cultivated elephantgrass types are similar genetically and in characteristics related to invasiveness. This information is needed to inform decision making regarding use of elephantgrass as a bioenergy crop in the region. Objectives were to compare 10 elephantgrass NPs collected throughout Florida with six cultivated types in terms of: i) morphology, growth characteristics, and mechanisms of vegetative spread; ii) flowering behavior and seedling recruitment; and iii) genetic relatedness. Elephantgrasses were planted in replicated 2- by 5-m plots at locations in North and South Florida in 2008. In September 2009 and 2010, cultivated types were taller (3.8 vs. 3.4 m), had longer (92 vs. 62 cm) and wider (35 vs. 20 mm) leaves, and had fewer tillers per plant (32 vs. 38) but greater tiller mass (270 vs. 160 g per tiller) than NPs. Cultivated types were more upright growing and less likely to lodge. Among all grasses, there was a 56-d range in initiation of flowering, with cultivated types flowering later than NPs. One cultivated type did not flower in either year, and another did not flower in Year 2 when the first freeze occurred on 2 December (32 d earlier than in Year 1). Random amplification of polymorphic DNA (RAPDs) and simple sequence repeat (SSRs) markers, used to assess the genetic relatedness between NPs and cultivated elephantgrasses, generated phylogenetic trees that clearly separated the two groups of entries. This clear distinction between cultivated elephantgrasses and NPs indicates that these markers may be useful in the future to efficiently select for parents in a bioenergy breeding program. Additionally, our data indicate that among elephantgrass types there is a wide range in plant growth characteristics and traits associated with potential invasiveness. Therefore, it should be possible to select types with reduced invasive potential for use in bioenergy production systems.