Here is an excellent video from the guys at Sigma 3 Survival School that explains the uses for greenbriar and yarrow as a primitive coagulant to stop blood flow. For more information on Yarrow and Greenbriar see the links below.
Yarrow – http://www.foragingtexas.com/2008/09/yarrow.html
Greenbriar – http://www.foragingtexas.com/2008/08/greenbriar.html