No matter how delightful they are, if your forsythia bushes aren't working in your landscape, you may decide to remove it. Check it over first to make sure there are no likely cuttings that you should take.
Cut the Shrub Back
To save making a mess in the yard make sure you have some good strong bags to collect the shrub in as you cut it down. To save time, hire a wood chipper for the day to shred the bush. If that is not possible, cut the shrub down in short lengths that will fit comfortably in a strong bin bag. You will finish up with a couple of bags of wood chip mulch or several bags of sticks that can be dried and used as kindling. Start on one stem and cut it back a piece at a time. If you cut off too long a piece, it will not go into the bag. Once you start letting pieces lie outside the bag, your efforts at keeping the yard tidy will end, and you will waste more time at the end of the day picking up the pieces.
If any of your neighbors have ever admired your forsythia, you might be able to get them to accept cuttings so they can grow their own. It would certainly be neighborly to offer them the option before they discover what you are doing with it.
Forsythia has a very healthy and reasonably deep root system, so leave about 2 feet of the final stems above ground. Dig around to find the roots and cut them back. Dig down around the stems to get as much of the root out as possible. You might be able to worry the roots lose by heaving the stems backward and forward. Try to pull the stems up. If they are too well fixed for you to pull them up, try a little leverage. Thread a pry bar under a shoot and put a rock under the end of it. By lifting the other end of the pry bar you will increase your pulling power.
Cut the Roots
When you can no longer remove the roots, simply hack them to pieces in the ground. If you can trace any horizontal roots rip them out of the ground. The roots that you don’t remove will die and eventually rot away. Forsythia roots are not known for regeneration.
Destroy the Stems
Forsythia is probably one of the easiest shrubs to grow from cuttings. Make sure that when you clean the site up that you do not leave any twigs or green stems—they will probably start to root and grow. It is definitely a waste not to use the forsythia for mulch.
Prepare the Site
While the site has been broken up in your effort to remove the shrub, you have the ideal opportunity to prepare the site for the next plants that are going to be put there and benefit from the forsythia root compost.