Regular maintenance will help
Annual and perennial flowers get most of the attention, while the shrubs gradually grow older and bushier. Those that expand by sending up new shoots at ground level develop into wide clumps of increasingly crowded stems. This preponderance of thick, woody stems causes the shrubs to lose vitality and become vulnerable to disease and insect attack.
You can save the cost and effort of replacing a valued, otherwise healthy overgrown shrub by using good pruning techniques to revitalize it. Renovation involves cutting the oldest and thickest of the stems down to ground level and removing them. Because most are likely to be pruning candidates, the basic rule of thumb is to cut out no more than 1/3 at one time. This way, the shrub continues to have a presence in the yard. Meanwhile, the pruning stimulates its root system to send up supple, vigorous replacement stems. If done gradually over a period of six years, this process completely renews an old shrub.
Some shrubs that respond well to renovation include: lilac, mock orange, quince, deutzia, weigela, privet, barberry, forsythia, bottlebrush buckeye, shrub dogwoods, beautyberry, spirea, bluebeard, rose of Sharon, Japanese kerria, and staghorn sumac.
Spring is the period of greatest plant growth. When shrubs emerge from dormancy, all systems are go. If stems are cut back to the ground, the new surge of energy will produce replacement shoots that vibrantly arise from the crown of the shrub and give older shrubs a new lease on life.
Keeping Up Appearances
Give shrubs routine maintenance every year, and they will not reach the point when they need thorough renovation. Begin by choosing the correct plant for each site, so that each has sufficient space to grow to its mature size.
Don't overfertilize shrubs. If their soil is decent and they are mulched, they won't need granular, slow-acting fertilizer after the first three or four years. Overfertilization encourages excessive growth and bushiness, which restricts their access to light and air. This stresses plants and makes them vulnerable to pest and disease attack.
Water shrubs well whenever rainfall is sparse. Most importantly, prune them lightly and regularly as they grow to establish their structure and shape. Thin their foliage branches if necessary. To prevent overcrowding, annually remove obviously aging stems before they become thick and woody. This helps a shrub constantly and gradually regenerate without undergoing a radical renovation pruning. It's particularly important in preserving the uniform look of a hedge each year.