Every other question probably ought to have the tag right?
Not sure if there is a landscaping tag, but the same would go for it.
Tags should probably be more specific than gardening unless something is affecting your entire garden (over run with some kind of bug/disease).
Yes, there's no need for a gardening tag here. I've re-tagged the existing questions and added gardening to the blacklist.
Yes, 'gardening' doesn't tell me what your question is about. And I wouldn't go to the sidebar and click on the "gardening" tag to see a subset of questions, or to try and find a duplicate of your question.
Yes 'gardening' struck me as being too broad as well. However I don't think 'landscaping' is. So far landscaping questions are in a minority.