I have discovered a ground wasp(?) nest near my house. So I went
to HD and got a spray can, but when I read the instructions it
seemed to deal with nests in trees. The instructions say to
"saturate the nest" but I only have a small opening.
What's the best strategy?

After spraying the nest for several nights without any visible effect,
I gave up and paid a professional, and I'm glad I did.