If you cut off the stalk before the seeds are ready this will prevent it from self-seeding but if you want them to spread, the best way is to wait until they go brown, collect the seeds and plant them where you want them to grow, otherwise the wind carries them all over the place (including the lawn!). Hope this makes sense!

Thanks Sheilar. Are these plants biannual? The seeds already look brown. How about if I cut them off and run my hands down the stalks to scatter the seed where I want it. Will that work?

Mine come up every year but it may be new plants that are re-seeding rather than the same plants coming back. I think it will work if you scatter them like you said.

Thanks Sheilar. I will give it a try and hope for the best. I'm not sure I know how to gather the seed.