Q. I have a number of shrubs that I need to prune and was wondering when is the best time to prune them, spring or fall? They include azaleas, oakleaf hydrangea, burning bush, weigela, bridal wreath spirea and red twig dogwood.

A. The best time to prune flowering shrubs depends on when they produce their flower buds. Some plants, including azaleas, bridal wreath spirea, and oakleaf hydrangeas, produce their flower buds during the previous growing season. They are said to "bloom on old wood." Other plants, such as weigela and many hollies, produce their flower buds on the current season's growth. They are said to "bloom on new wood."

Shrubs that bloom on old wood begin to produce next year's flower buds soon after they finish blooming. They should be pruned as soon as they are finished flowering so that you do not remove too many of next year¹s flowers. Those that bloom on new wood should be pruned in winter or early spring before new growth starts.

Burning bush is really grown for its colorful fall foliage rather than its inconspicuous flowers, so you can prune it whenever it is most convenient for you. Red twig dogwoods bloom, but they are grown more for their colorful winter stems than their flowers. Because they produce the best red stem color on young stems, they are usually pruned in the spring.

You can cut all of the stems down to within a few inches of the ground. This will force new stems to sucker up from the roots. Not only does this treatment produce the most colorful stems, but also it is a great way to keep these vigorous plants in bounds.

If any of these plants are very large and overgrown, consider sacrificing the flowers and pruning them when they are dormant. Early to mid-March is ideal for the plants and the person doing the pruning because we can have very nice weather at that time of year. Severe pruning is not as stressful for plants when they are dormant. If the shrubs are healthy and vigorous, it is fine to prune them back into shape in one session. However, very old plants or those that are not healthy and vigorous should have severe pruning spread over a three-year period to reduce the impact of removing so much wood.

It is best to have all pruning finished by mid-July. Pruning forces new growth on actively growing plants, and if that happens too late in the season, the new growth can be killed by cold winter weather. Dead branches or stems should be removed whenever you notice them, regardless of the time of year.