Due to public and regulatory restrictions, genetic containment technology is critical for research and development of transgenic trees in forestry. A multi-year field trial demonstrated that the RNAi-LFY construct successfully induces sexual sterility in 6K10 poplar (Populus alba) without visible effects on vegetative development. However, its limited number of replicates prevented strong inferences about possible vegetative impacts. We therefore established a new, replicated field trial to reexamine possible effects. Nine to 40 replicates of two sterile LFY-suppressed, four non-sterile transgenic, and wild-type control lines were studied over three years in a field trial in Oregon, USA. Traits measured included height, diameter, senescence, and several leaf and petiole traits. Though lines (insertion events) varied widely, the sterile and control lines were statistically and substantially different in stem growth and petiole length–25.5% and 18% (respectively) smaller in sterile lines. Our results suggest LFY suppression may cause vegetative penalties and therefore deserves further scrutiny.