If you have a garden in your front yard, and another bed in your back yard, they won't cross VERY much, but even 20% cross-pollination means that you can't really PRESERVE the heirloom variety.
You can still use saved seeds that are "slightly cross-pollinated" by planting them thicker, and then cutting down "rogues" once they show themselves to be noticeably different from the heirloom variety. After all, if 80% of the plants were NOT cross-pollinated, you will have plenty of "the real thing" for the first few years.
But the population of saved seeds will lose and acquire characteristics from other strains until it is a mixture, adapted to your conditions, and selected by your "roguing" preferences. Of course you can start over every few years with one new packet of the "pure" heirloom. Stored dry, cool and dark, the first packet could last 5-10 or more years.
Some people take the trouble to "bag and daub" a few branches of a favorite variety. An organza bag or tulle or floating row cover material can be made insect-proof. It has to be an insect-pollinated, not wind-pollinated variety. They have to hand-pollinate those branches, but it does get them enough saved seeds that are 100% not-cross-pollinated to carry on a line.
If you cover an entire, small seed bed
tightly with floating row covers (so that pollinators can't get in), the seed bed won't be cross-pollinated unless the species is pollinated by wind. However, it also won;t be pollinated at all, unless you do it by hand.
I read one suggestion where the seed bed was covered tightly 6 days out of 7. Once per week (or twice per week), he would go cover every OTHER plant tightly. Then, after nightfall when all pollinators were (supposedly) asleep, he would go out and uncover the seed crop. The next morning, insects would find the seed crop and pollinate it, but not be able to reach the pollen from other plants of the same species.
I don't know for sure whether that would work, and it seems like a lot of work!
Or you might do a rotating system, where one year you only grow one kind of pea, and one kind of pumpkin. Save lots of seed from them that year. Grow as many kinds of squash that year as you please.
Next year grow just one kind of squash and one different kind of pea, saving seeds from them.
The third year, grow all kinds of saved seeds of several varieties, but maybe not collect any seeds that year.
I think you could save enough seed from one variety to grow it out for 3-5 years, so you could maintain an heirloom line by growing it (and only it) every third, fourth, or fifth year. Store the seeds very dry so they last as long as possible.
There is an idea catching on called "seed libraries". Some group accepts donations of seeds and then "loans" them out to gardeners who will grow them out in isolation from other varieties of that species. At the end of the year, the gardeners "return" more seed than they "borrowed".
There is usually some genetic drift or selection when a line is propagated by hobbyists for many years in one location. These "seed libraries" call that an advantage because the original strain becomes selected for and adapted for that locality. Climate, soil, daylength, insect pests and soil diseases all vary from region to re4gion and county tom county. After 5 or ten years growing in one area, unintentional genetic drift selects for gene combinations best suited to that area. Natural variation existing in the original seed sample recombines and the best-adapted traits gradually become a larger % of the seed.
Global Seed Library Locator Map (mostly USA and Europe)
http://www.seedlibrarian.com/
Olympia WA Seed Exchange
http://www.olympiaseedexchange...
King County Seed Lending Library
http://kingcoseed.org/
