How to prevent cross pollination?

[ken_]

I want to grow 2 different kinds of heirloom tomatoes this year. They're both open pollinated and I'd like to save some seeds for next year. They won't have a lot of physical separation, is there any way to keep them from cross pollinating and producing frankenseeds that won't grow (or produce the same tomatoes as the parent plant)?

[TWP]

If you allow bees to pollinate your plants, you can expect cross-pollination.

If you manually pollinate, you may be able to put plastic backs over all blossoms.  That is a labor intensive process...

Perhaps doing a single variety planting, rather than two, might address this.

If your plants are "heirloom" then cross breeding is much less likely to produce non-viable seeds.  The seeds will be a mixed hybrid of the two heirloom varietals, but should still be viable.  Some seeds will be true to the original plant and (on the same plant) some seeds will be hybrid.  Bees don't discriminate.

Let us know what happens Next year, when you harvest from your collected seeds.  Needless to say, do some test sprouting and have a backup set of seeds in case the harvested seeds have too low a sprouting rate.