Previous studies with agronomic crops suggest that the neonicotinoid insecticide imidacloprid can have direct positive physiological effects on plant growth and stress tolerance that are independent of its insecticidal properties, but few studies have tested this effect on woody plants. In a controlled-environment experiment, we tested effects of imidacloprid on growth, biomass allocation, and gas exchange the poplar (Populus nigra) clone NC5271. In a factorial experiment, plants were grown under three fertility (high, medium, and low) and two irrigation regimes (optimal, moderate drought). Half the replicates in each treatment were also treated with imidacloprid (Merit^® 2F). As fertility increased, so did plant growth rate, total leaf area, and total biomass, while percent root mass decreased. Moderate drought stress had opposite effect, decreasing growth, total leaf area, and total biomass, while increasing percent root mass. Imidacloprid increased plant growth and total biomass. The positive growth effect was due to increased total leaf area, as repeated measures ANOVA showed that imidacloprid actually had a small negative effect on light saturated net photosynthesis. The positive effect of imidacloprid on plant growth was independent of fertility or irrigation level, occurring across all environments. These data are consistent with the hypothesis that imidacloprid can have beneficial physiological effects on woody plants that are independent of its insecticidal properties.