If you send this question over to the tomatoes forum, you'll find a few true tomato experts who I'm sure can help you.

It certainly does not make sense that hybrids cannot cross with open pollinated (heirloom) tomatoes. They can. Any tomato can cross with any other tomato. They don't cross very often though because of their pollination mechanism. They are self pollinating and most do not get pollinated by insects, who are collecting pollen by shaking it out of the flower which encloses the anthers in most cases.

Pollen does sometimes get transferred by insects however. So if you want to be certain, bag the blossoms with a little bag made of tulle. If you are willing to take the slight risk of cross pollination, just separate them from other tomatoes by planting them on opposite sides of some other crop.

Jim

garbird, Tomatoes are self pollinating. Insects, Bees and such can pollinate as well as they travel from blossom to blossom. Isolation from other tomato varieties be they Hybrid or Heirloom helps in prevention of cross pollination. The safest process is to bag the blossom before it opens and then tag it; after the tomato forms remove the bag and wait for the tomato to mature.
The best of luck to you on growing your tomato!
Gary

Garbird:

Your heirloom is "dehybridized," and as such, will render seeds that are true to the strain, if not X-pollinated. So we agree on that.

Your intuitive comments are 100% correct. Absolutely those hybrid tomatoes can provide pollen that might get to your heirlooms, and that would ruin your seed stock...but would have no effect on the taste or nature of the tomatoes coming from your heirloom vine. Only the SEEDS are affected. (Just thought I'd throw that in, as the misconception has been voiced before.) And, given your astute intuition, you should see the logic of this. While the pollination gets the fruit to set and develop, the genetic specifics are relevant only to the resulting seeds, not flesh of the fruit.

Of course, the fruit of the subsequent generation would be entirely affected by the genetic nature of the plant that produces the fruit.

But, back to your original inquiry...

As your plants develop, I suggest you closely examine the flowers of your plants. You should notice that your hybrids typically have flowers where the male part (anthers) completely envelope the female part (pistil and stigma). This virtually assures self-pollination, and pollination at a high rate. Hence the prolific yield of the typical Early Girl or Better Boy.

The heirloom, ironically, has a long female part (pistil) which tends to encourage cross-pollination, so anatomy is working against your desire to favor self-pollination. (I suppose the term "OP" (Open Pollination) comes from this anatomical aspect, but I am only surmising that...no confirmation from authoritative writings.) So, once again, common sense should be considered. And, as expected, heirlooms tend to have low yields...in my opinion, because the anthers of these flowers are often unable to "get to" the stigmas, which ride high and above the anthers.

So I manually pollinate them.

But I never collect my own seeds this way. I just want to eat the fruit!

Again, trust your senses. If pollen from a different tomato variety might pollinate your heirloom, then the seed is junk. And I suppose that pollen from a different plant that is the SAME varietal should be OK...but I have never read that anywhere.

I suppose I reserve the right to trust my common sense, too.

Bees can cross tomatoes, although the degree of crossing varies widely. Tomato flowers are not a major source of pollen or nectar, so if better sources are available, the chances of crossing are reduced. The use of isolation space, and growing flowers or flowering vegetables as "barrier crops" between different varieties, can be helpful. Using this technique with 30 feet between varieties, I have yet to find a cross. If you were to grow the heirloom on the other side of your yard, or in large pots elsewhere, chances are high that the seed would be reasonably pure.

If the heirloom is irreplaceable, however, "reasonably pure" may not be good enough. In that case, the blossom bagging suggested by several posters is the best option.