Ground Hornets

HELP!!!!! We have an infestation of black wasps with white stripes.  The more I read, the more it seems they are actually ground hornets. Their nests are holes in the ground. All of a sudden there are hundreds of them in one section of the garden and they're kind of scary. They apparently are quite vicious and can sting repeatedly.

Has anyone ever had these? How did you get rid of them? Google answers suggest anything from gasoline down the hole and lighting it (yikes!) to pouring ammonia on the nest. These methods do not sound environmentally friendly at all. Also, do I have to wait until autumn to try extermination?

Any advice would be much appreciated - thanks!