Deciding which edges in the forest really need to be hard is tricky when we are all wrapped up in doing justice to the individual trees.
Working under the assumption that the best way to see if something needs to be in the painting is to leave it out, I recommend making a study that has no hard edges at all. When it is finished, the study will tell you where more focus is required. It also helps to have a limit in mind, say, half a dozen strokes, so you can identify the most important spots.
You may have to wet both sides of your paper to get it to stay damp long enough
For homework, choose an image that has lots of shapes, and paint a version that is all soft edges. Part of the exercise is to practice the techniques involved in keeping the shapes blurry without losing definition altogether. The awareness skills that are at work include noticing as soon as a hard edge appears, and stopping right there. Then dry the paper, re-wet the relevant areas and continue.
Assess the study, with an eye toward where it needs greater definition. Make the hard-edged additions one at a time, and stand back each time to see if that's enough.