However there is one herb that can be used safely for headaches and migraine without concern of overdose from moderate ingestion. Wood Betony (Woundwort) can be taken in tea or capsule form and can be bought in some health food shops, often in conjunction with other herbs. You can grow Betony legally and safely in your own garden. It will grow in full sun or partial shade so it will hopefully thrive in whatever conditions you have to grow it in.
There's a good article here that will give you more information and some detail on dosage.
Disclaimer - Not only am I not a doctor and not medically qualified at all, I don't even identify as a green/herbal witch. So obviously take my information as a starting point and discuss with your own doctor before taking a herbal remedy that anybody recommends over the internet.