The purpose of the work described in this paper is to investigate the effectiveness of pre-wetting structures, dead fuels, and landscaping plants in preventing fire spread from wildland fires to structures. Critical fluxes for fire growth were determined using intermediate-scale testing for three wetting agents (water, type A foam, and gel) applied to 10 landscaping plants conditioned to 20% moisture, a mulch material, and four external structural materials (vinyl siding, plywood siding, asphalt shingle roofing, and cedar shake roofing). The critical flux for fire growth values was determined at 3-min heat radiation exposure and simultaneous 300-mm long flame exposure. Test specimens were exposed to various durations and intensities of drying prior to exposing them to heat radiation. Application of water or foam provided no noticeable protection. Gel was effective in providing protection even after 60 min of laboratory condition drying but was less successful when exposed to fire weather simulating accelerated drying. Some uncertainty is associated with the results of this work because of the variability of landscaping plants and gel wetting agent application uniformity. The intermediate-scale test results were verified using full-scale testing. Copyright © 2012 John Wiley & Sons, Ltd.