James 3:18

With the words "harvest" and "sown", we get the idea of farming. There is some sort of yield that is produced from what has been planted. In this case, peace has been planted, and righteousness is harvested. James is still talking about the wisdom from above which is contrasted by a different wisdom which has jealousy and selfish ambition. Not only does the wisdom from above have the qualities mentioned earlier, but a righteousness producing peace is a part of this wisdom. It's almost as if explaining the goal of this wisdom that is from above, not just these qualities as an end goal. The goal is not to hoard these qualities which are what describes the wisdom that is from above, but the result should be righteousness that is harvested from peace. Righteousness is not attained by trying to be righteous, but as a result by those who make peace.