There is no absolute safe distance because bees can carry MG pollen over 10 miles,but the farther away the plants are from each other the better...and hand pollinating at least some of the blooms to insure that you get some of the same type cultivar is recommended...

You can help to prevent unwanted crossings by planting different species of Morning Glories inbetween the types that you want to remain true to type because different species will not cross fertilize...and each time a pollinator visits a flower,some pollen is picked up and some pollen is lost,so the more flowers of different species that a pollinator visits inbetween the flowers that you want to remain true to type the better the chances of only intended cross fertilizations occurring...

Hope that helps...

Ron

Thanks for the help Ron. This flower may become an addiction for me. I just wish there was more information out there. Everyone is so into showing the blooms that the info about growth is limited. I have several types and only one pack gives the information on how long the vine grows.

There is some of the best all around MG info here:

Here is a link that might be useful: MG forum