A common problem faced in the growth of forsythia bushes occurs when all flowering on the bush occurs at its base. This damage is a result of winter temperatures - because flower buds of most forsythia are not very hardy, they are often killed by low winter temperatures (-15 F). Flower buds at the base of the shrub are often protected by snow, and avoid damage.
One remedy may be the selection of hardier cultivars, such as: Forsythia Intermedia `Karl Sax', Forsythia Ovata `Meadowlark', `Northern Gold', `Northern Sun', `Sunrise', `Ottawa', `Tetra-gold', and Forsythia Mandschurica `Vermont Sun'.
Planting methods may aid in the prevention of damage. The shrubs should be planted in protected areas with ample sunlight. These areas should not be in low areas, where frost may easily occur. The injury to flower buds usually occurs in the late winter months. One way to enjoy the flowers of these bushes through the winter is to cut some of the branches from the bush after January 1 and to force them indoors.
SOURCE: Hortopics, May 1984 By Lee Taylor and Clancy Lewis Department of Horticulture Michigan State University