Knowing when to prune is crucial for dependable flower blossoms

By Joel M. Lerner
Saturday, October 2, 2010

Why didn't your shrubs bloom when they should have this year? If they're healthy, planted in the proper location with good soil and everything else is in order, it could be because the plants were pruned at the wrong time.

Some shrubs set flower buds on the previous year's growth. Others will flower on the current year's growth or bloom from buds that formed last year and this year.

The time of year to cut back or open up the form of your shrubs is as the flowers fade. This means that some plants are best pruned in winter, some in spring, others in summer. Woody plants usually produce more buds if they are pruned right after flowering.

The following pruning suggestions offer the best chance for having blossoms every year.

Plants that flower on prior year's growth:

-- Evergreen azalea will blossom best in spring if it is pruned right after the flowers fade as new branches and flower buds form that will open the following spring. As blooms fade, selectively prune branches if they're too long. Use a hand pruner by reaching down into the shrub and cutting back the taller stems at a point inside the plant above a branching stem. After pruning two or three stems you'll notice that the plant can be reduced in size or opened up creating air circulation with several carefully made cuts. If azaleas are pruned in summer, fall or before they flower in spring, you will lose a year of flowers. However, sometimes this can help renew a plant by focusing its energy on new foliage rather than blossoms.

Another method of renewal pruning evergreen azalea is to cut it hard, leaving five to 10 branches about six to 12 inches tall on which the plant can re-grow. This method requires that you to have the courage to make big cuts -- sometimes called "the mighty whack" by growers -- pruning back to bare stems that will produce new leaves and stems through the wood of the plant. Evergreen azaleas will renew over a period of several months. Most importantly to note is that this type of hard renewal pruning should be done between Feb. 28 and March 7, before any chance of growth begins for azaleas. Use great care when pruning. Pay attention to where your hands and fingers are at all times, and wear gloves.

-- Florist, or big leaf hydrangea (H. macrophylla), is a favorite of gardeners. The clusters of blue blossoms in summer are showiest because their sterile flowers create big balls of color. This popular shrub grows about four feet high and wide, and it seldom needs pruning, except to remove dead canes or lightly shape the plant.

-- Hybrids of hydrangeas have been introduced in recent years offering longer seasons of bloom and wider varieties of color. One is endless summer (H. macrophylla 'Endless Summer(TM)'). Its unique quality is that it's the first blue or pink (which depends on the soil's pH) florist hydrangea that blooms on last year's bud set, and it will also set flower buds in spring and bloom on the current year's growth.

-- Annabelle hydrangea (H. arborescens 'Annabelle' ) is easy to care for and flowers dependably. It grows 3 to 5 feet high and wide in moist soil and partial shade and blooms white from June to September. It flowers on buds formed the previous year, but it also blooms on new growth. You can cut it back in late fall leaving 6- to 10-inch woody stubs, and it will return to flower the following year.

-- Lilac (Syringa vulgaris) should be pruned after flowering. Deadhead blossoms and bring them indoors for fragrance. Prune the oldest wood on the plant that is beginning to form a furrowed bark. Cut back to ground level to help ensure the absence of lilac borers. New shoots will grow from the roots, and they will develop into the next flowering stems. As the younger stems mature and flower, the height of the lilac will be at a level where you can enjoy the fragrant flowers in spring.

-- Forsythia (F. x intermedia) sets its flower buds the prior year for next year's early spring blooms. Cut this deciduous shrub back hard (1 to 2 feet) after it flowers in spring. If you are using the forsythia as a border or hedge planting, you can prune it to any size, but always prune just after flowering.

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