Have you read information at gentlebirth.org (the midwives archives section)?
I have reasearched just about every form of induction. Natural, pharmacutical, physical etc. I personally feel VERY safe with Black and Blue cohosh. I used it with my last pregnancy. I don't believe tea is "strong enough" to really do the work but that is my personal opinion. If you really want to get things going, I don't think anything is appropriate before 40 or 41 weeks, but then it's better to use the tinctues because just a dose or two under the tounge should get things moving at least.
Black cohosh is a uterine toner. It alone WILL NOT send you into labor. Blue cohosh IS a stimulant and will absolutely bring on contractions. That is why it should be teemed with black cohosh or another regulating type herb.
Herbs are medicine and should be treated seriously. ON the other hand, while many herbs have not gone through gold standard trials, they have been used in all of human history and have an excellent track record.
You should speak with your care provider and do it with their help and supervision.
Again, these are my opinions and the result of my personal research.
You may come to entirely different conclusions.