Conducting manipulative climate change experiments in complex vegetation is challenging, given considerable temporal and spatial heterogeneity. One specific challenge involves warming both plants and soils to depth. We describe the design, and performance of an open-air warming experiment called Boreal Forest Warming at an Ecotone in Danger (B4WarmED) that addresses the potential for projected climate warming to alter tree function, species composition, and ecosystem processes at the boreal-temperate ecotone. The experiment includes two forested sites in northern Minnesota, USA, with plots in both open (recently clearcut) and closed canopy habitats, where seedlings of 11 tree species were planted into native ground vegetation. Treatments include three target levels of plant canopy and soil warming (ambient, + 1.7 °C, + 3.4 °C). Warming was achieved by independent feedback control of voltage input to infrared heaters aboveground and buried resistance heating cables belowground in each of 72-7.0 m2 plots. The treatments emulated patterns of observed diurnal, seasonal, and annual temperatures but with superimposed warming. For the 2009 to 2011 field seasons, we achieved temperature elevations near our targets with growing season overall mean differences (∆Tbelow) of +1.84°C and +3.66°C at 10-cm soil depth and (∆Tabove) of +1.82°C and +3.45°C for the plant canopies. We also achieved measured soil warming to at least 1-m depth. Aboveground treatment stability and control were better during nighttime than daytime and in closed versus open canopy sites in part due to calmer conditions. Heating efficacy in open canopy areas was reduced with increasing canopy complexity and size. Results of this study suggest the warming approach is scalable: it should work well in small-statured vegetation such as grasslands, desert, agricultural crops, and tree saplings (<5 meter tall).This article is protected by copyright. All rights reserved.