One of the first questions you should ask yourself when thinking about your lawn is whether or not you should overseed or fertilize first. This is a very important decision to make. If you fertilize first, you will see immediate results and you’ll likely feel that you can relax for a while after fertilizing. However, overseeding is often more effective. The reason why is that the fertilizer will take several weeks to begin working its magic. So by the time you fertilize, there will be an even stronger root system in place.