P-band SAR InSAR Forested terrain Ground deformation High-gradient deformation Landslides Radar penetration depth Snow Global success of utilizing X/C/L-band InSAR (Interferometric Synthetic Aperture Radar) to survey ground deformation over non-forested terrain in the past two decades, has raised interest in monitoring forested lands, where relatively short-wavelength X/C/L SAR acquisitions often experience strong decorrelation and down­ graded InSAR quality. To address this challenge, we considered the long-wavelength P-band SAR and conducted a large-area experiment over diverse terrains of the U.S. West Coast to comprehensively assess P-band SAR’s capability for ground deformation surveying. Our results show that P-band InSAR observations greatly out­ performed L-band data for identifying ground deformation within forested regions and for measuring spatially high-gradient displacements, such as for slow-moving landslides. Over the entire study area, P-band InSAR helped to discover >200 new landslides that were missing from existing landslide inventories. It also demon­ strated high capability of penetrating through shallow snowpack to collect SAR signals from the ground surface beneath. However, P-band data manifested lower sensitivity to subtle deformation, as expected theoretically, and encountered coherence loss resulting from heavy snowpack. Overall, P-band SAR demonstrated to be a highly effective tool for discovering deformation beneath dense forest canopies and for quantifying spatially highgradient displacements. These findings provide an experimental basis for planning future satellite and airborne P-band SAR missions to enhance the capability to monitor changes of the Earth’s surface.