Forsythia benefit from rejuvenation pruning. The younger thinner stems produce the most flowers and resist disease and pests, so cut the thickest oldest stems down to the base. If your shrub is overgrown, you can completely cut the entire plant down to the ground and the root system will regrow. It probably won’t bloom for several years, but it will be healthy and beautiful. Forsythia is not native to North America, so it would be even better if you replaced this plant with native vernal, witch