A flash unit can do wonders to light up a background that's too dark. Here are two examples of that from a senior session.
The before and after under the tree was lit by my Canon 600 into a Larson softbox. It was on the ground behind her aiming up at the tree. Using a softbox avoided the problem of hard shadows across leaves that would happen with a bare strobe head.
The comparison in the shed was done with a Duncan Hines frosting can to disperse the light from the Canon 600.