Nasturtium and marigolds will not cross pollinate one another. But among your varieties of marigolds and among the nasturtiums themselves they will.

Oh dear! I reread my post and it does look like I am wondering if they cross pollinate with eachother!! OK, I know they dont, but I am wondering about the two species amongst their own varieties. But I guess I now know the answer to that one ,too. Should I pollinate by hand? Are the flowers "perfect" or do I need to use pollen from different plants? Thanks!

Hi,
I'm not sure there's an easy way to pollinate them and keep the cross-pollination to a minimum. Before all that though I want to clear up something when flowers are not perfect, you don't always need flowers from another plant, you can still use flowers from the same plant. There are a few plants that do need pollen from another plant, but that's not as common of an occurrence with plants.
Ok, hand pollination would work. You would need to pollinate after the flower opens and then bag the blossom. Either tape a small brown bag around the flowers or use those organza gift bags. You need to bag after pollinating because a bee could still come by and pollinate after you do causing a mix of offspring. Having bagged blossoms in the flower garden isn't too attractive, well organza isn't so bad.
I prefer to just pick one variety for the year. It makes it easier for seed saving. Some flowers though have a long distance of crossing chances like sunflowers. It is a few miles! For those I still do save seeds, but I know I might have some odd, but still pretty, babies.
Remy