In the past, I've just purchased my heirloom tomato plants or started from purchased seeds, but I'd like to start saving my seeds. Problem is that I grow three different varieties... this year I'm growing black cherry tomato, roma plum tomato, and hillbilly (beefsteak) tomato. They are all different types (cherry, plum, beefsteak), but that doesn't matter in terms of seed saving for tomatoes because tomatoes of any type can cross pollinate, right?

They are all grown in the same area and I can't change that because we rent and have limited gardening space. So, what can I do to keep the seeds pure?

Melissa