As spring rolls near, I've got a question about pre-emergent weed killers. I live in North Carolina Zone 7 (Triangle area). When should I apply pre-emergents, how many times should this be done (and how often)? Any specific kinds that work exceptionally well? I know we have problems with dandelions and clover. Also, how long will I have to wait to put down new seed in these areas? Thanks for your input.