In seed saving, it’s essential to prevent cross-pollination of crops that are not self-pollinating by keeping plants of the same species separated by distance. This keeps seeds collected from open-pollinated varieties true-to-type. Isolating by distance is the most fail-proof way to prevent cross-pollination, but you can get creative when the such isolation distances are not feasible.
Isolation by Containment
When space is limited and isolation by distance is not possible, you can physically isolate plants from each other through the use of isolation tents or blossom bags. Growing insect-pollinated plants inside tents with introduced pollinators prevents cross-pollination. Isolation by containment cannot be used on wind-pollinated crops like corn or beets, as physical barriers can keep insects out, but allows pollen carried by the wind to enter and cross-pollinate these crops.
Isolation Tents and Mosquito Netting
Isolation tents are often used by larger-scale growers. This method can be more challenging for home growers, because generally, pollinators are purchased and introduced inside the cages. Another approach to utilizing isolation tents is known as alternate-day caging. This method involves moving isolation cages between two different varieties in the same species on alternating days to prevent them from cross-pollinating, while still providing them with access to pollinators on the days that they’re not covered. Once fruit is set, you don’t need the plants to have access to pollinators, so you can leave them caged. You don’t need fancy supplies to make an isolation tent; you can make one yourself using mosquito netting and posts (the post help keep the netting off of the plants).
This is exactly what it sounds like—placing a bag made from quick-drying, permeable material over individual flowers or flowering branches. Crop types that can be easily blossom-bagged includes eggplants, peppers, and tomatoes.
Physical Barriers and Landscape Features
Physical barriers such as buildings, stockade fences, tree lines, and shrub hedges affect how pollen moves through the landscape and may hinder the flow of windborne pollen. They may also impact the travel patterns of insect pollinators. Some seed savers plant different varieties of the same species on opposite sides of their property, believing that if the two plants are not in view of each other, it is unlikely for an insect to travel between them without stopping along the way and randomly dispersing the pollen it is carrying.
Additionally, growing dense plantings of nectar- and pollen-rich flowers between different varieties of the same species may distract pollinators and prevent them from making unwanted crosses. However, while barriers and distractions help minimize pollen dispersal within a garden, they are not guaranteed isolation methods. They only help to decrease the probability of unwanted cross-pollination and may allow for a reduction of isolation distances.
Isoliation by Timing
You can also plant varieties in sequence so that their flowering times don’t overlap, although we recommend using this method only with crops, such as corn, that have a short and uniform flowering time, as timing is difficult to coordinate. Stagger plantings of different varieties of corn in order to save seeds from multiple varieties in the same season.
Crops that are isolated to prevent cross-pollination still require pollination to successfully set seeds. Because of this, you can pollinate plants by hand to take the place of the insect pollinators. This can be easily practiced with crops like corn and squash, however hand pollination is generally not practiced on larger-scale gardens/farms because it’s quite time-intensive.