Only the lower branches on my forsythia bloomed this spring. Why?
When selecting a forsythia, choose a variety (cultivar) that blooms reliably in Iowa. 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 -10 degrees Fahrenheit. Since most parts of Iowa experience winter temperatures below -10 degrees Fahrenheit, these cultivars often don't bloom well in the state. Most likely you have a variety that is not reliably cold hardy in Iowa. The lower branches bloomed because they were covered with snow and protected from the cold winter temperatures.
Excellent forsythia varieties for Iowa include 'Meadowlark' (8 to 10 foot tall), 'Northern Sun' (8 to 10 foot tall), and 'Sunrise' (5 to 6 foot tall). All three varieties possess excellent flower bud hardiness and bloom reliably in Iowa.