Gerald L. Klingaman
FORSYTHIA x intermedia 'Karl Sax'
Plant Common Name
Border Forsythia, Karl Sax Border Forsythia
Named for a famous botanist and plant geneticist, this vigorous and tall forsythia is an older cultivar noted for its very early bloom time, good bud hardiness and dense bushy habit.
This tried and true spring-flowering deciduous shrub is known by all and loved by many. Its cheerful bright yellow blooms appear before the foliage unfolds and are some of the first to appear. In fact, the early blooms often open too early and are damaged by frost. Forsythia x intermedia is the most popular hybrid in commerce and is a cross between the two Asian species Forsythia suspensa and F. viridissima.
When mature, this large deciduous shrub develops a dense, bushy habit. It bears bright golden yellow flowers with four prominent petals in late winter or early spring, depending on the temperature. The flowers are quick to appear in warm weather, which typically makes them vulnerable to damage during untimely warm spells. Its medium-green leaves are thin, plain and unassuming. They can develop purple hues before they drop in the fall but otherwise offer little in terms of visual interest.
Forsythia is very easy to grow. It grows best in full to partial sun and will tolerate a wide range of well-drained soil types. It is hardy to USDA zone 4, but the flower buds are only reliably hardy to zone 6. Prune judiciously to preserve its lovely arching habit. Forsythia has many landscape applications. It works well as a specimen plant, in informal hedges or massed in swaths across large open landscapes. Late in the winter, while still in bud, its branches may be cut, placed in a vase of water and forced indoors. The buds usually take a week or so to open.
AHS Heat Zone
9 - 6
USDA Hardiness Zone
6 - 9
A2, A3, 2b, 3a, 3b, 4, 5, 6, 7, 8, 9, 10, 11, 14, 15, 16, 18, 19
Full Sun, Partial Sun
7'-8' / 2.1m - 2.4m
6'-7' / 1.8m - 2.1m
Early Spring, Late Winter
Cutflower, Foundation, Hedges, Mixed Border, Screening / Wind Break
Sharp or Has Thorns