Well, it depends on how many different species you have and it can get complicated but I will try to present it as basic as possible.

Cross pollination is not the same as cross fertilization...an apple and a cucumber may cross pollinate , but they won't cross fertilize.

1) Species which are the same can cross fertilize...cultivars and varieties of the same species can cross fertilize.

2) species which are different will not cross fertilize

So, if you have "Crimson Rambler" and "Milky Way" which are both Ipomoea purpurea , they can cross fertilize but they will not cross fertilize with your "Flying Saucers" because FS is Ipomoea tricolor.

Your "Flying Saucers" can cross with other cultivars of the same species (Ipomoea tricolor) like Heavenly Blue or Pearly Gates.

There is no guarantee that cultivars of the same species will or won't cross, as it depends on how close they are ( the closer , the more chance of crossing and although 25 to 50 feet apart would be best, there are no guarantees as bees have been proven to be able to carry Ipomoea pollen for over several miles) and other related factors...

Hope that helps...

Ron

This post was edited by ron_convolvulaceae on Mon, Oct 7, 13 at 18:40