If the water does the trick, you'll be doing it forever. By the way, it really isn't such a chore. Find a non-oscillating sprinkler, or even better, get a soaker hose. Arrange it around the tree, turn it on for a few hours, and walk away. Soaker hoses don't shoot water up in the air like a sprinkler, so nothing's wasted. It's a slow oozing kind of thing. And, they're useful in your flower beds, too.