True-to-type seeds can be saved by spatially isolating a single variety of a species. The distances required vary from species to species but must be large enough to prevent contamination from other plants’ pollen. However, those distances are not absolute as they will mainly depend on factors specific to your environment (garden/land size, weather, pollinator population density, insect forage sources, natural physical barriers, presence of other gardens/farms in the area)–hence the importance of having a good understanding of your surroundings and “pollen landscape”–as well as the breeding tendency of the seed crops (read Plant Inbreeding and Outbreeding Spectrum). While general recommendations exist both for home gardens and commercial production sites, they vary widely and should therefore be adapted accordingly to your local conditions and crops’ breeding method.
Based on the type of pollination used by your crops to reproduce, some might require longer isolation distances (wind-pollinated plants) than others (insect-pollinated plants). A greater number of plants and varieties, higher levels of pollinator activity, and an open landscape exposed to high winds are also variables that usually require increasing the isolation distances (even for inbreeders). On the other hand, the presence of geographical or vegetative barriers, a vicinity free from other gardens and farms, as well as alternative pollen sources and staggered flowering time might reduce the need for spatial isolation. In any case, careful consideration should be made whether the isolation distances need to be lessened or increased in order to limit risks of crossing.
Note that even if the recommended distances are respected, cross-pollination can still occur and other more efficient isolation techniques might still be needed to ensure seed purity. In the event of accidental or unwanted crosses, the purity of a variety can still be regained by removing off-type plants in following generations. The number of generations required to remove an undesired cross will depend on the degree of crossing and whether your crop is predominantly a self or cross-pollinated species. Ultimately, while isolation by distance is a flexible and easy technique to implement, it is still a risk management tool subject to the unpredictability of nature’s ever-changing biological and environmental interactions.