The learning curve in gardening (as with most worthwhile pursuits) is steep at first, as novices succeed or fail through trial and error. Rules and guidelines are helpful, but they're open to interpretation. Anyway, because gardens are so extraordinarily personal, garden builders are stubbornly inclined to do it their way. Still, mistakes can be costly. And the bigger it is, in terms of time, money or both, the more difficult an error can be to undo. Here are several mistakes that gardeners, including myself, seem to spend a lot of time correcting.