Root length may be important in determining research plot border width. Measurements of hand excavated hybrid Populus roots showed root length increased with tree age up to at least 3 years. Maximum root length ranged from 2.2 m for 1-year-old trees to 5.2 m for 3-year-old trees. It appears that in some studies, e.g., irrigation and fertilization treatments, border width may be considerable and probably increases with tree age. Border width sufficient to avoid edge effects from a plot that had an irrigation and fertilization treatment applied to it was at least 2-3 m. Total tree height decreased significantly toward the plot edge across a 6 m wide untreated border zone. Published data from spacing and yield studies shows tree biomass increases toward the edge of plots, probably as a response to light. This study showed that height and diameter decreased toward the plot edge when an irrigation and fertilization treatment was applied to the plot. These contrasting results in edge effect are discussed in terms of canopy vs. root competition. Some possible effects of different experimental treatments on the necessary plot border width are explored. Forest Sci. 27:808-814.