Our front garden (and I use the term in the losest possible sense) has a 45 degree slope down from the porch down to the sidewalk, with steps leading up to the front door. Right slap bang in the middle of the left side of the garden was a large forsythia bush. Allegedly they can grow as large as 10 feet tall and 15 feet wide, ours was a “mere” 6 feet in all dimensions. It was an absolute bear to try to trim and prune - the rear was just about reachable but unless you play basketball in one of the professional leagues, there was no way you’d be able to prune the front of it at the top. Consquently the bush had a reasonable round shape, trimmed at the top at the back and an unsightly sprout of branches at the front. Ugly.
Somewhere online it said
Pruning Forsythia:Flowers form on prior year’s growth, not new growth. So, it’s important to prune them immediately after the flowers have bloomed. You can cut back old growth to about four inches from the ground.
Don’t hesitate to be aggressive in pruning these fast growing plants. They will grow back at a fast rate.
Hmmm, ours was blooming to about 3′ on all sides, on the outside of the tangled mess. Looked like old wood to me, so we went at it with wild abandon, pruning it to a random assortment of anything between 2 and 8 inches in length. You’ll note that I’ve used the past tense when speaking of the bush - one 6 foot forsythia has now been reduced to 4 (packed) yard waste sacks and about a dozen woody stumps. Strangely enoug the yard looks so much better without it. It’ll probably take a year or so to grow back but I believe that careful pruning of the new growth should leave us with something that looks better than ever.