We evaluated the effectiveness of an enhanced tree trimming (ETT) program for its ability to reduce tree-related power outages, and thereby improve resilience, on an electric utility distribution system during storm events. Evaluations encompassed thirteen years of trimming (i.e. 2005-2017) data and were performed for both backbone and lateral utility lines. Backbones included all three phase lines between a substation and a faultable device whereas all other lines were considered laterals. The study site spanned the entire state of Connecticut, where the dominant vegetation is temperate deciduous forest. We controlled for variations in weather, tree cover, and wire type, by pairing ETT-treated zones with nearby untreated zones. ETT-treated conductors had storm outage rates that were 0.07-0.36 outages/km/year lower than untreated conductors or 35-180% lower than the service-area's average annual outage rate for untreated conductors. ETT-treatment was associated with lower outage rates for "minor" outage types (i.e., blown fuse, tripped recloser, etc.) but the treatment effect was not statistically significant for "major" outage types (damaged poles or wires). System-wide ETT application, for the approximately 27,000 km of conductors in the study area, was predicted to reduce annual storm-related outages by an average of 81-104 and 318-759 outages/year for backbone and lateral lines, respectively. Our study provided a robust empirical evaluation of ETT and also proposes a geospatial methodology that controls for variations in weather and environment.