Culture
Easily grown in loose, medium moisture, well-drained soils in full sun to part shade. Best flower production is in full sun. Tolerates relatively poor and clayey soils. Moderate drought tolerance once established. Shrubs are vegetatively winter hardy to USDA Zones 5-8, but may not always flower well in Zone 5 because of harsh winter temperatures or late winter freezes of unopened flowers. Flower buds are typically lost when winter temperatures fall below -5 degrees F. Development of unkempt growth often occurs if shrubs are not regularly pruned immediately after spring flowering (pruning done after mid-July will remove flower buds for the following spring). A wide range of pruning options exists for 6-10’ tall hybrids, one option being (a) an annual post-flowering removal of old wood combined with cosmetic shaping of the shrub, and (b) a major cut back of stems to almost ground level every 3-4 years for rejuvenation. Shrubs have good tolerance for urban conditions. Some cultivars may sucker. Cultivars are primarily propagated by rooted stem cuttings.
Noteworthy Characteristics
Forsythia × intermedia, commonly known as border forsythia, is a deciduous hybrid shrub with upright-arching to spreading, often square-stemmed branches clad with ovate to lanceolate medium to dark green leaves (to 3-5” long and to 1” wide) which have toothed margins in the upper 1/2. Most cultivars mature to 6-10’ tall spreading to as much as 12’ wide, but some compact cultivars rise to only 30” tall. Shrubs are primarily noted for their brilliant, 4-lobed, often abundant golden flowers (each to 1 1/2” long) which typically bloom in clusters of 2-6 along the branches in late winter to early spring (February-March in Atlanta but March-early April in St. Louis). Flowers bloom before, or in some cases simultaneous to, the emergence of the new foliage. This shrub is sometimes referred to as the harbinger of spring or the ultimate symbol of spring because the flowers brighten the landscape at a time when not much else is in bloom. These shrubs are hybrids between weeping forsythia (F. suspensa) and greenstem forsythia (F. viridissima), with × intermedia referring to the hybrid characteristics being intermediate between those of the parents.
Notwithstanding their excellent late winter-early spring bloom, however, these hybrid shrubs are often described as one-season wonders which somewhat fade into the landscape after bloom. Fruits (small brown capsules) are non-ornamental. Fall foliage color is typically an ordinary yellow-green, but sometimes purplish. Growth can be rampant, often requiring occasional rejuvenation pruning.
Genus name honors William Forsyth (1737-1804), Scottish superintendent of the Royal Gardens of Kensington Palace and author, among other works, of A Treatise on the Culture and Management of Fruit Trees which in its day was probably the most widely read work on the subject.
The hybrid name intermedia refers to the hybrid characteristics being intermediate between those of the parents.
‘Sunrise’ is a broad, dense, mounded, spreading deciduous shrub. Semi-compact habit to 5’ tall and as wide. This cultivar (an Iowa State University introduction) produces flower buds which are reportedly able to withstand -20 F temperatures in winter.
Problems
Some susceptibility to leaf spot, crown gall and dieback. Watch for spider mites, aphids, four-lined plant bug, Japanese weevil and northern root-knot nematode. Deer tend to avoid this plant.
This cultivar (an Iowa State University introduction) has flower buds which are reputedly able to withstand minus 20 F temperatures, however, thus providing excellent flower bud hardiness.
Uses
Group in borders. Mass on banks or slopes. Sunny areas of open woodland gardens. Cottage gardens. Hedges.