To help keep your dreams nightmare free I have a few things you can try.
First, get a bunch of Betony, and stuff it into your pillow, like into the stuffing, and sleep on this pillow. That should help prevent nightmares.
I recommend NOT doing anything with Cowslip if I were you guys. It can be used to contact loved ones who have... departed, while you're asleep. If that's something you're interested in, try grinding some up, putting it into a teabag, make tea from it and after drinking that, go to bed and see how it goes. I've never tried this, because I don't have anyone who is dead that I'd like to talk to.
Hell... i think that's about it for now. EveN though i only gave that one example of how to keep out nightmares, Believe me, it's effective. A last miNutE word of advIce Save any left over Parts Of plants that you don't use durIng your Spells. Other thaN that i believe that's it.
Do with this information what you will.
Forever Watchful~ Tikka