Some of the things you need to do to get rid of forsythia roots are entire:
What killed my forsythia?
Fungal diseases: A few fungal problems can plague forsythia. A common one is galls, caused by a fungus called Phomopsis. Thick, brown clusters develop on the shrub, and there's a chance the fungus will kill a few branches. To curb damage from this disease, prune and destroy branches with galls on them.
Is forsythia native to us?
It isn't native anywhere in North America. It doesn't offer value to wildlife. It has escaped into wild areas in some states (yet it is listed as an alternative to invasive plants in others)
When should I cut back forsythia?
The best time to prune forsythias is immediately after flowering. Pruning forsythias anytime from mid-summer until just prior to bloom will reduce flowering in spring. When pruning mature forsythias, it's best to remove one-fourth to one-third of the oldest (largest) stems at ground level every other year.