The time-course of development of the roots and endomycorrhizas of five common herbaceous plants in a southern Ontario hardwood forest (Arisaema atrorubens, Erythronium americanum, Asarum canadense, Smilacina racemosa, and Trillium grandiflorum) was examined. Root growth of these species was very slow. Formation of vesicular-arbuscular (VA) mycorrhizas was quantified by measuring the average distance from growing root tips at which (i) hyphal contact, (ii) root penetration, and (iii) arbuscule formation by hyphae of VA mycorrhizal fungi first occurred. The rate at which mycorrhizal colonies within roots expanded was also quantified. These measurements allowed the rate of mycorrhizal colonization of roots of species to be compared. All events were slower in woodland plant roots than in other previously investigated species. The rate of VA mycorrhizal colony-expansion was found to be significantly faster in roots containing longitudinal air channels, which apparently facilitated the spread of hyphae. Environmental factors may also have been important since events were even slower in those roots produced by Erythronium in the autumn. Reasons why slow, steady, root and mycorrhiza formation could be advantageous to woodland plants are considered.