When to Trim a Forsythia Bush and How
I trimmed back my Forsythia late last year and I hardly got any flowers this spring. This was the first year I didn't get a lot of flowers. I do believe I did something incorrect. Did I trim back too much, or too late, or what? Any hint (like your article on Lilacs) would be of great help.
Certified GKH Gardening Expert
Pruning is usually limited to spring after flowering; however, a light trim in the fall will not harm the shrub. It will simply inhibit the flowering of next season's blooms. Here is an article that you may find helpful: https://www.gardeningknowhow.com/ornamental/shrubs/forsythia/forsythia-pruning.htm