If your bush is full of high, dead growth or outgrown limbs, cut the branches all the way back to the ground, and let new growth come up. Or you can cut out one-third of the worst now, another third next year and a third the following. You can make forsythia less straggly by pinching off the tips of strong-growing canes in late spring or early summer. The varieties F. viridissima and F. ovata are preferred if you don’t object to the straggling habit.
By way of an annual, partial rejuvenation, the best time to prune is right after the flowering season. These plants flower from branches developed during the preceding summer. If the bush is old and unsightly, you can cut it back in spring or summer. Don’t wait until fall. Weeping forsythia and the upright types should not be “bobbed,” that is sheared like a hedge, as you’ll ruin the floral display. You can cut the tips back in dormant period (before buds break in spring) or wait until after flowering is over, then prune back to desired height. If you don’t care about sacrificing blooms, you can prune almost any time. February is a good time to prune, then you can bring the blooming twigs indoors and have fresh flowers at no cost. Stick the twigs in a vase of water. The best way to handle forsythia is to cut out some old branches annually, making the cut at the base of the bush. Thinning out the bush is good practice.
Non-blooming is due to cold winters freezing buds. You see flowers around the bottom of the bush only – due to the heat of earth or snow protection. Higher up the buds were killed, hence no blooms. This happens to exposed bushes, for example, near a corner. A bush that’s protected 6 feet away will bloom. Buds drying up may be due to a dry previous summer or a severe winter. Forsythia sometimes bloom in fall, due to extra strength or a mild autumn. Sometimes the blossoms will start out yellow and turn green, or will come out green. No one knows what causes this, but it is believed that cold weather in spring – sudden changes in temperatures – will cause this color change. It may also be tied up with dry summers the year before, although temperature change is more likely the cause. Nothing can be done to correct this.
Diseases and insects
Bacteria, fungi or insects can cause Stem-gall, lumps on stems. Cut off and burn all canes and branches that bear galls.
Leaf spots are due to four types of fungi. Pick off and burn spotted leaves. Spray with a fungicide containing chlorothalonil or mancozeb.
Die-back is a fungus that invades blossoms and flower stalks, then twigs, and kills them back. Prune out dead twigs and stems.
If insects eat holes in leaves, spray with Sevin or an insecticide targeted at the specific pest.
Start new plants from cuttings rooted in sand. Or dig up young plants nearby. If the tip of a cane touches ground, it will root. Dig this up and plant it. Forsythia is ideal for covering slopes, and its branches make nice filler for floral vases.
Forsythia are vigorous growers and need very little plant food.
by Doc and Katy Abraham
Throughout 2008, “Gardening” will present material originally written by George “Doc” and Katy Abraham, the beloved husband-and-wife gardening team who offered advice to amateurs and experts for over 50 years. The two Cornell graduates, with double degrees in horticulture and journalism, opened a small greenhouse business after World War II. They had two children, Darryl and Leanna, and wrote a syndicated gardening column that at one time reached 5 million readers. In 1950, Doc and Katy began a call-in radio program on WHAM 1180-AM in Rochester. Their folksy “Green Thumb” program featured useful gardening advice, humor and inspiration. The complete archives of their life’s work can be found at the Kroch Library at Cornell.