Answer: Forsythia normally grows with rampant abandon, responding to that type of pruning by replacing the cut branches within the season. It also usually grows very large: 8 to 10 feet high and round, so pruning for size is somewhat of a losing battle.
The dieback could be attributed to a number of causes, including improper soil preparation, improper planting techniqe (such as leaving encircling roots or failure to remove synthetic burlap wrappings), poor drainage so the soil is too wet, insufficient moisture, or even disease. Your county extension office can help determine the cause of the poor growth and dieback. I'm sorry I can't be more specific.
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