James: Congratulations on your interest in growing your own vegetable seeds. I highly recommend the practice!
Pollenation is generally a highly localized event with an occasional pollenation from far away. That occasional pollenation from far away can create a huge problem for mega seed companies who are growing enough seed for an entire nation. An occasional cross isn't as dramatic to a backyard farmer. As an example, some years ago, the farm that was growing Delicata squash seed for the world got contaminated by a poison producing gene. They then sent seed for poisonous squash out into the world. I think that the reputation of Delicata squash still hasn't recovered. They were not tasting each squash fruit before saving seed from it, so they didn't have a clue that they were distributing poisonous squash to the world. I had something similar happen in my garden. I inadvertently grew a poisonous melon in with my muskmelons. Because I am a small scale grower, I taste every melon before saving seeds from it. When I found out that I had a poisonous melon growing in my garden, I threw away a whole year's worth of seed production rather than risk distributing poisonous melons. So the mega-seed companies have to be extremely conservative about variety purity. I don't have to be, because my quality control checks are different.
The mathematical probability of cross pollination looks like this. Extremely likely to be pollinated by nearby plants. Extremely unlikely to be pollinated by plants from further away.
In my garden at a separation distance of 100 feet, I get about a 5% cross pollination rate on small plantings of squash. I get about a 5% or less cross pollination rate on corn varieties grown 3 to 11 feet from each other.
Here's an estimation of cross pollination between two 100 square foot patches of carrots growing ten feet apart from each other. Along the edges of the patch next to each other, there is about a 1% chance of cross pollination. That's too much for a mega seed company, but it's just fine on my farm.
Those sorts of low cross pollination rates are something that I can easily live with. Some cross pollenations are more difficult to deal with. For example, I really don't like surprize hot peppers in my sweet peppers, so I'm very careful about not growing sweet peppers and hot peppers in the same field. But I don't care much if the shape or color of a winter squash varies a bit, as long as it tastes good.
Preventing cross pollination is difficult. Minimizing cross pollination is easy. It's difficult to keep something 99.9999% pure. It's easy to keep something 95% pure. In my garden 95% purity is good enough for most crops. If I don't like a naturally occurring cross, I cull the crosses when they show up. I'm much too lazy to be hand pollinating crops when wind or animals will do it for me.
Hedges help reduce pollen flow. Distance helps. Staggered flowering times help. The more diversity surrounding a paddock, the more pollinators there will be, and the more likely they will be to produce cross pollination. For example, at my place in the desert, the cross pollination rate on common beans is around 1:200. A collaborator's garden next to a swamp has a cross pollenation rate closer to 1:20. (That's for beans grown
side by side, not in separate paddocks.)
Even stored at room temperature, I'm still getting close to 100% germination on many species of seed that I grew 5 years ago. In one growing season I grew enough Bok choi seed to plant 24 acres. That's enough to provide my community with seed for decades.
I grow seeds for every crop on my farm. A few years ago I counted 70 varieties in 55 different species. I've added many dozens of species since then, and a number of varieties.
I don't try to maintain heirlooms. I plant plenty of heirloom varieties, but I don't try to keep them pure. I let them get open pollinated by other varieties (to the extent that they will). The genes are still hanging around, just not a particular arrangement of the genes. My
experience is that cross-pollenation gives me more opportunities to select for varieties that thrive on my farm.
I have 7 isolated fields, so in theory, I could grow seed for 7 varieties of brassicas, and pepo squash per year. If seed is good for 10 years when stored at room temperature, then I could maintain 70 varieties of each species.
I allow impurity in my squash: Taste has to be magnificent.