Relative size of plantings can have an impact, too. Small plantings are more vulnerable to foreign pollen than large plantings, since in a large planting foreign pollen is spread among more plants. Further, plants on the edge of a group are more vulnerable to crossing than plants in the middle. For this reason, whenever a variety is important, collect seed from plants nearest the center of a planting for greatest purity. While the USDA figures are based on large plantings, Ms. Ashworth's figures are better-adapted to the scale of the home gardener.
Caution is especially important with rare or heirloom plants.
If you experiment with smaller isolation distances, use common varieties instead of rare ones in case of accidental cross-pollination. Remember that once two varieties have crossed, their genes are inextricably crossed. Wind Pollination and Distance Isolation Distance needed for distance isolation of wind-pollinated plants varies. Wind patterns and strength, existence of windbreaks, etc. Pollen from corn, which is relatively heavy, falls to the ground within a few paces in a quiet protected spot, but can travel half a mile or more in open, windy areas.