I'm planting a patch of Maximillian sunflowers near a neighbor's fence, and want to put in a root barrier to contain them on my side. (And keep their grass on their side.) How deep do you think I should go?
In my experience of wild sunflowers they spread by seed and don't survive mowing. If your neighbor has a grass lawn that they mow, neither of you will need to take any extra steps, to control the sunflowers.
Now controlling the grass is a different matter. Let me know if you discover an effective barrier for grass roots. I control mine by persistent pulling, but they can travel several feet underground. It's a garden task force that never really ends.