My forsythia shrubs are vigorous and healthy, but don't bloom well. Why?
Forsythias bloom on old wood. Unfortunately, the flower buds on some varieties are not reliably cold hardy in Iowa. For example, the flower buds on 'Lynwood Gold' and 'Spring Glory' are hardy to minus 10 degrees Fahrenheit. Since most areas in Iowa experience winter temperatures below minus 10 degrees Fahrenheit, these cultivars often don't bloom well in the state.
Improper pruning is another possible cause. Flower buds on forsythias begin to develop by early summer. Pruning the shrubs anytime from mid-summer until just prior to bloom will drastically reduce flowering. To achieve the best floral display, forsythias should be pruned immediately after flowering.