This species of forsythia are named after Prof. Karl Sax (November 2, 1892 – October 8, 1973), an American botanist and geneticist. A report put out in 1960 by Harvard University states that in 1944, Prof. Karl Sax crossed the forsythia named ‘Arnold Giant' with nearby diploids, including F.ovata and F. intermedia spectabilis in order to obtain triploids. Several progeny were obtained from this cross and two were selected as superior.

One was named in honor of Mrs. Beatrix Farrand. The second selection was different and became a new cultivar, (hybrid origin), named after the person responsible for originating the plant, Prof. Karl Sax. He used F. suspensa X F. viridissima. (courtesy Harvard University, 1960 report)

The Karl Sax forsythia is a deciduous shrub which is one of six varieties of this forsythia. It tends to be a moderate shrub that averages about 6 ft. in height with a spread of 10 – 12 ft. It is a vigorous grower and a very hardy plant. The branches are robust but not rigidly erect, which gives the shrub a more graceful habit. The shoots go upright as well as having some that gracefully arch. The summer foliage is a dull, dark green above with a lighter under-belly, displaying as ovate to lance-shaped toward the tips with toothed margins and measuring 3-5 inches long by 0.5-1 inch wide. The stems are somewhat squared with prominent lenticels. The autumn foliage is green to yellowish-green with the best plants having a purple or burgundy tinge over the yellow base color.

It flowers profusely with blooms that are a dark, deep yellow and golden, yet brilliant. They are borne in groups or clusters of 2-6 with four lobes, beginning in March and into April, with the blooms lasting up to three weeks. The fruit appears as a small brown capsule and has no ornamental importance. It is one of the earliest bloomers and is considered a sure sign of spring. The flowers precede the appearance of the leaves which make this shrub so uniquely beautiful.

The Karl Sax forsythia is low maintenance and adapts well to different soil types. It tolerates partial shade, urban conditions, drought, heat, humidity, seashore and slopes, however it does prefer to be kept moist. This shrub likes full sun with dappled shade and is easily transplanted and established. It is a rampant grower and will likely require pruning or shearing to keep the landscape looking tidy. Old, damaged and dead branches should be removed to increase airflow, decrease the chance of disease and to encourage new growth increases in the flower production. Therefore, it is advised that the shrub receives an occasional rejuvenation pruning. The flowers are only cold hardy from -10 to -15 degrees, but the plant itself is hard from -20 to -25 degrees.

It is critical for the shrub to receive a deep, but not soggy watering once a week during the first year. Propagating this shrub is somewhat difficult but tested results have shown that the softwood cuttings in cold frames do excellent, while the hardwood cutting in open fields does poorly. If you are not an experienced gardener, you would do well to seek the advice of a reputable horticulturist.

Now is the time to go to a nursery and pick up the lovely Karl Max forsythia and enjoy the brilliant flowers gracing your landscape in the spring!