To understand “adjacency arrangement” (configuration of a patch and its adjoining elements) as a basic unit of landscape pattern, we studied 30 woodland patches (Populus tremuloides) in Alberta, Canada, with adjacent vegetation varying from 0% shrubland (100% grassland) to 100% shrubland (0% grassland). We evaluated (1) how important adjacency effects are relative to resource availability and disturbance in affecting plant species richness and composition in the woodland understory, and (2) which species and species groups change in abundance and presence with increasing percent shrubland in the adjacency arrangement. We found that, in addition to topography and previous grazing, adjacency arrangement is a major significant variable affecting the vegetation composition of woodland patches. Along a gradient from 0% to 100% adjacent shrubland, the species composition in woodland patches changed from an abundance of weedy and introduced plants to an abundance of moist‐environment plants, the proportion of native species and perennials increased linearly, the number of “shrubland species” increased linearly, “grassland species” decreased linearly, and total species richness remained constant. Woodland vegetation did not differ significantly where adjacent shrubland was to the east or to the west of the wooded patch, nor between patches which did or did not experience a change in the adjacent shrubland in the recent past. We conclude that adjacency arrangement is an easily measured, highly promising concept for ecological understanding, as well as for land planning, design, conservation and management.