Answer: Forsythia is a very vigorous grower and will ultimately be in the eight to ten foot range for both height and width, so you are looking at baby plants which have not yet had a chance to grow and develop their natural thick branching pattern and overall shape. At this point, it is a good idea to simply allow them to grow for a few years without pruning. Eventually, the shrubs might need a bit of thinning to allow light to reach into them. To do this, remove some of the oldest branches by cutting them off at the base near the ground. Since forsythia flowers on "old wood" any pruning should be done in spring right after flowering.
If on the other hand, you are expecting a formal clipped hedge, you will need to trim them quite often as they are not only fast growing large shrubs but they are also apt to send up unruly branches as part of their normal growth pattern.
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