Go to the dollar store and get a few shakers of ground cayenne pepper. After you plant something generously shake pepper all over the ground. The squirrels don't like to sniff the pepper, it bothers their noses. This works better than trying liquid pepper spray because the squirrels aren't detered unless they taste it.

The squirrels feel compelled to rummage through any freshly-dug earth as they think another squirrel (or themselves) have buried something tasty there.

The pepper isn't 100% and does need to be reapplied after rain, but it is the only thing that has worked for me.