I've found the speckled flower cultivars not very fecund.
It's my observation that during cooking hot summer temperatures flowers do not set seeds well. I guess the heat cooks the pollen.
I used to be concerned about unwanted crossing by bees but I don't bother about preventing it. You can always tie off a flower bud the night before, then in the following morning open and hand pollinate it, then tie it again if you want to ensure no crossings.
I don't know about the root size question. You could experiment to learn directly.
