Hi. Photo is of a mature forsythia after 1 year having been moved to this south facing, full sun location in Newmarket after all above ground removed due to fire.
The goal us to have a plant 8 feet tall and 6 feet diameter. How should I prune it to meet that end? The branches are all about 6 feet in length.
Since the Forsythia was cut down to it’s base, I would let it grow this year without doing any further pruning but would make sure that it gets about 2″ of compost around the base of the plant and some higher phosphorus fertilizer to encourage healthy root growth to help it recover from it’s recent fire fiasco. It will also need to be watered during dry spells until it builds up it’s root system again and establishes itself in the new environment. In times of drought a couple inches of water in a week would be helpful, otherwise natural rainfall should be enough for this plant. Next year it may not have flowers because it often only flowers on wood that is 2 years old. Make sure to cut out any of the dead, diseased, damaged or crossing branches to help it avoid any diseases or pests but don’t do any further pruning at this time.
The following year, the shrub should be flowering again and will follow more of the yearly pruning techniques for this bush:
Forsythia is always pruned just after it finishes flowering generally to encourage abundant flowers, avoid overgrowth, and to maintain it’s natual weeping shape. Regardless of the size you want to end up with it also needs to be pruned to cut out any dead, diseased, damaged and crossing branches. This will help to ensure your shrub does not get diseases or pests. Then 1/3 of the oldest stems need to be cut back to the base each year to keep the plant rejuvenated and productive. It will also give adequate airflow and sunlight for all the branches to help the shrub maintain it’s overall health. For Forsythia to grow well and to it’s maximum capacity, it also prefers full sun and somewhat fertile, moist and well draining soil. Adding 2″ of compost around the bottem of the plant in the spring or fertilizing in the early spring (February/March) with a higher phosphorus fertilizer would help the plant recover from the winter and also produce brighter blooms. In your south facing environment, the Forsythia should eventually grow to it’s natural height (approximately 4 years) which is about 8 to 10 feet tall and be healthy and productive if the above conditions are met.