What risks are involved in taking blood thinning herbal supplements before surgery?

Bleeding..... Do not know what blood thinning supplements you are describing, and depends on what surgery is being performed. But the risks of taking any blood thinners before surgery, can be bleeding, bruising, infection, poor healing or scarring, etc. Be very sure you discuss all of your medications with your surgeon.
Bleeding. You can significantly increase your chance of serious bleeding during and after surgery. Do not take blood thinning supplements (or any other supplements) without letting your surgeon know exactly what you're taking.

Related Questions

Are blood thinning herbal supplements good for otherwise healthy people?

Maybe. Healthy people should be able to take herbal supplements that may have blood thinning potential safely, although a small percentage of people may have a hereditary blood condition that has not yet been diagnosed.
Supplements. Are any herbal supplements proven to have benefits enough to recommend otherwise healthy or ill patients take them? Actually very few have been studied with rigorous scientific methodology and of those I'm not aware of any that consistently yield benefits.
Almost certainly but. It is interesting that most doctors would say "yes" if the question was about using low dose Aspirin to thin blood but not so positive when in comes to herbs. Herbs like ginger, garlic & turmeric thin blood & are likely to reduce risk of heart attack & stroke, but just like Aspirin are not suited for everyone. Some with thin blood/bleeding tendencies should be cautious re: their use.