Isolation involves – through the use of several techniques and mechanisms – the protection of your seed crop from the pollen of plants belonging to the same specie that you do not intend to cross with. This helps to prevent the contamination of your crop’s genetics with wild plants, other plant varieties of the same species that you are growing, and other plant varieties of the same species that others may be growing in the vicinity. The common isolation methods used by seed savers are isolation by distance, mechanical isolation, and isolation by time. Isolation standards will often depend on each individual’s requirements in relation with the seed crops grown as well as the specific landscape configuration of the area.
The growing and reproductive peculiarities of each seed crop for your environment as well as their botanical name will have to be learned through personal observation and experience in order to reliably identify related species and varieties that might potentially compromise the genetic purity of your plant population through their pollen. Plants that share your crop’s binomial name belong to the same species as your crop. For instance, cabbage, cauliflower, broccoli, kale and Brussels sprouts are all from the same species, sharing the binomial name Brassica oleracea. Therefore, if kale seeds were to be saved, isolating it from cabbage, cauliflower, broccoli and Brussels sprouts would be necessary in order to avoid any cross-pollination and contamination. There are however rare exceptions where varieties of a species have the ability to cross with individuals (wild and domesticated) of another species of the same botanical family (i.e., Cucurbita argyrosperma with Curcubita moschata).
Floral characteristics such as the number of flowers per plant, duration of flowering, or the amount of pollen shed are all factors that can have an impact on the potential outcrosses in a specific environment. An important production of pollen would certainly increase the chances of wind or insects carrying it to another location, resulting in an increase risk of contamination. Studying the wind patterns specific to your environment–especially when wind-pollinated crops are grown–is therefore desirable as it will help you to adapt your isolation strategy to prevent accidental outcrosses resulting from the presence of other species located in the region.
If different varieties of the same species are grown in the same seed production area, it is of critical importance to ensure that no volunteer plants from an earlier planting appear in the same field. Those will often come from previous plants with seedpods that have shattered/self-sowed and germinated, which stresses the necessity to conduct timely harvests when the situation requires it. If left unnoticed and allowed to flower, those volunteers may reduce to nothing your isolation efforts by generating undesired crosses, lead to unintended seed selections, and therefore irremediably compromise the purity of the whole variety. In case volunteers are spotted, they can be either removed, consumed before the emergence of blossoms (i.e. greens), or replanted in a more appropriate and isolated place.
One of the biggest challenges when isolating seed crops remains pollen contamination from genetically modified (GM) crop production, especially with wind-pollinated crops such as corn in regions where GM fields are prevalent. Genetic mixes resulting from outcrosses will put at risk the integrity of the germplasm while potentially damaging the local ecological balance. Should such fields of the same species are present in the vicinity, isolation distances must be significantly increased (at least twofold) in order to reduce deleterious crosses as much as possible.
Lastly, should the space in your garden is limited, it is always recommended and safe to grow one variety of a given crop species at a time, which will also be easier to isolate if applicable (or require no isolation at all if no other varieties are grown in your neighborhood). In case you are looking to grow a variety regardless of the risk of contamination from foreign pollen coming from the vicinity, you can decide to simply rogue out any off-types instead. This might be enough based on your situation, provided you have a good understanding of the variety and are able to identity potential crosses when they emerge. Planting in friends’ or neighbors’ gardens can also be considered as a viable alternative whenever the isolation of one or several varieties if needed, and that your community has the space required to do so.
Note: Varietal purity should not be confused with specific purity, which characterizes a seed lot containing no inert matter (mineral or vegetal) and no foreign seeds.