Answer: Forsythia tends to make a thicket, rooting where branches touch the ground. If you can allocate it the square footage it has now, simply cut the whole thing back to the ground next spring right after it blooms. This is the fastest method of rejuvenating the plant. (It will grow back quickly and bloom just fine the next year.)
If on the other hand you need to reclaim some of the space, it will be a big job. In some ways it would be easier to kill the existing patch with herbicide containing glyphosate and start over, even possibly with a new forsythia. You could also selectively kill out some of the self rooted sections, but you will need to sever them from the section you wish to keep and apply the herbicide carefully so as not to contact the portions you want to save. Keep in mind that it may take several applications to complete the job -- be sure to read and follow the label instructions carefully for best results, including allowing adequate time for the herbicide to translocate to the roots where it does the job.
Some plant experts consider forsythia to be a "common" plant, but it has many good features -- long lived, self sufficient, cheerful spring bloom, good screening ability as it leafs out early and holds its foliage late into the fall, and so on. Spirea is another old fashioned shrub that can grow very large and needs regular maintenance to keep it looking good. Cotoneaster is a slower growing plant and there are many types of it, the lower ones are mainly used as a groundcover effect.
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