Organic fillers derived from biomass waste streams are commonly formulated with phenol-formaldehyde resoles in the production of veneer-based wood composites. The surface characteristics of three fillers, flours of walnut shell (Juglans regia), red alder (Alnus rubra) bark, and corn cob (Zea mays) furfural production residue, were investigated with the column wicking method using the Washburn equation. A series of n-alkanes (spreading liquids) provided reliable estimates of interstitial pore radii, which then enabled contact angle estimates for a variety of non-spreading liquids. Among the three fillers, only alder bark data were complicated by swelling effects, which were eliminated with strict data quality criteria. Corn cob residue exhibited the highest total surface free energy, but all fillers had low energies that were dominated by dispersive effects. The minor polar contributions were basic. Specific surface areas determined from wicking substantially disagreed with Brunauer-Emmett-Teller measurements using N-2 adsorption, where the former were more consistent with measured particle size.