Discussion of this question also here.

Just how many forsythias did she put in there? One would do the job, two might be constructive overkill, in that the gap would fill in faster. More than that, and there just may be too much competition.

Unless you are going to plant something small that can survive total darkness, or wage continual war against forsythia that are out to eat your side of that bed, I'm not sure you can plant anything there.