Ectomycorrhizal and nonmycorrhizal, glasshouse-grown northern red oak seedlings (Quercus rubra L.) received root treatments of IBA in starch, fired-montmorillonite clay, or starch-encapsulated montmorillonite clay. Clay proved to be superior to starch as a carrier for IBA, inducing significant increases in diameter, root length, leaf area, and shoot dry weight. Positive growth interactions between mycorrhizae and IBA were found with the clay carrier. The typical bare-rooted red oak seedling (grown for 1 year in nurseries and outplanted) performs poorly because of insufficient root size. Container-grown seedlings produced using clay/IBA treatments may perform better under field conditions than stock grown conventionally. Chemical name used: indole-3-butyric acid (IBA).