When Forsythia Grows Out of Control: A Guide to Pruning
In 2013, I planted four Forsythia (Forsythia x intermedia 'Spring Glory') along the line of my neighbor's chain link fence to add a privacy screen to my backyard. Other than lopping off the occasional nuisance branch, I did zero trimming or pruning. I like the natural look, and the bigger and thicker it was, the more privacy I got.
This spring I went to clip a few blooming branches to bring inside for a flower arrangement only to discover that the flowers were mostly toward the top of the branch. A few weeks later, I noticed very tall spikes - easily 12 to 14 feet - shooting up from the bushes. Both of these are telltale signs that the shrub wants to be thinned so it can live a healthier life. As one does, I still avoided trimming the plant until Memorial Day weekend because wow, what a pain.
How to Prune Forsythia
Forsythia are not hedges, and do not want to be clipped like one. The proper way to cut back a Forsythia is to practice "renewal pruning." Basically, you cut each branch where it comes out of the ground, removing the oldest branches and any dead ones. The oldest branches are generally the thickest, or branches that are losing their pretty color. Remove about 1/3 of the oldest branches and repeat this every year. Renewal pruning is also the proper way to trim red twig dogwoods, and other shrubs that branch out from the ground (not a trunk) and have a natural look.
You can also cut all the branches close the ground and start over.
What I Actually Did
My Forsythia were a total mess, so I ended up cutting more than half out of two, and maybe a third to a half out of the other two. It looks terrible, but I'm pretty sure it'll come back just fine.
This left me with a gigantic pile of branches that all had to be baled for trash collection. Read on for my baling hack.