To Prune or Not to Prune: Shrubs

Note: This article was supposed to be published two weeks ago, but ... well, you know what. As a storm-related addendum, the picture accompanying the article is of a hydrangea that took a hit primarily from the snow storm after Sandy. Shari assures me that its wilty, sad state is a result of frost/snow damage so, if you have shrubs that look similar, not to worry--they'll come back. Now, the article as it was originally written:

Shrubs add visual interest with their diverse colors, flowers and shapes. But knowing how best to maintain them can be a challenge. Deadheading? Shearing? Pruning? Even with the best intentions, it can be easy to make mistakes.

To help you out, Reeves-Reed Arboretum's Director of Horticulture, Shari Edelson, answers questions about deadheading and pruning shrubs:

Should you deadhead shrubs and when is the best time for that?

                “Deadheading” is the removal of old flowers from a plant after it has finished blooming – it’s commonly done to keep plants looking tidy, and in some cases, to prevent them from setting seed and spreading unwanted “babies” throughout the landscape. It takes a great deal of energy for a plant to produce fruit and seed, so by removing old flowers before these fruits and seeds are formed, you also encourage the plant to spend its energy on other things, such as the growth of healthy roots and foliage. However, there are also some reasons NOT to deadhead your shrubs – for example, if you like your plants to produce seed as food for wildlife, or if you enjoy the look of those dried hydrangea blossoms or bright orange rose hips in the garden. If you do decide to deadhead, the best time to do so is usually just after the plant has finished blooming. Use a pair of sharp pruners to remove the old flower and stalk, and try to make your cut just above a healthy leaf and bud.

When is the best time to cut back shrubs with the goal of shaping/sizing and promoting fuller growth?

                If you’re planning on cutting a shrub down to the ground in order to promote new, healthy growth – a practice referred to as “rejuvenation pruning” – the best time to do so is usually during the winter, when the plant is dormant. Just as many woody plants are dormant over the winter, so are many of the diseases and insect pests that attack them. Pruning in winter decreases the chances of disease – plus, when vigorous growth begins in the spring, the plant will be able to quickly seal any wounds, thereby thwarting pests and pathogens for good.

Another reason to prune in winter is that it avoids problems caused by weak new growth, which often results from late summer or autumn pruning. When plants are pruned in late summer or fall, they respond by producing tender juvenile growth, which is then easily injured by the onset of frosty weather. Not only does this lead to unsightly dieback, it can also weaken the plant as a whole, depleting energy the shrub needs to make it through the winter and emerge from dormancy the following spring.

 And finally, pruning in winter makes the job easier for you, the gardener! Without all those leaves in the way, you’ll have a much easier time seeing what you’re doing – and when you have a lot of pruning to do, this is very helpful indeed.

One word of caution, though – shrubs that bloom on old wood, such as azaleas, rhododendrons, and many species of hydrangea, will not bloom the following spring if cut back to the ground over the winter. For these shrubs, it’s often better to practice a more selective pruning technique, removing the oldest and gangliest-looking limbs each winter to encourage new growth, while leaving plenty of old wood intact to provide abundant spring blooms. If you must cut a spring-blooming shrub, such as forsythia, all the way to the ground, the best time to do so is immediately after flowering in the spring. This way, the plant has the entire growing season to develop new growth, making it more likely that you will have flowers the following spring.

What is the difference between thinning and shearing and which is better for the shrub?

                Shearing involves trimming a shrub’s outermost twigs and foliage, usually several times a year, to maintain a sculpted shape and dense appearance. Shearing imparts a formal look to the garden, and works well for maintaining hedges, topiaries, or other shrubs that you’d like to maintain as geometric accents to your landscape. Some types of shrubs respond particularly well to shearing – these include boxwood, privet, Japanese holly, and yew.

For other shrubs, however, shearing can cause stress, invite disease, reduce flowering, or simply undermine the beauty of the plant’s natural shape. Most shrubs grown for their flowering displays are poor candidates for shearing – these include lilac, forsythia, viburnum, azalea, rhododendron, hydrangea, and spiraea.

For these plants, thinning is often the better choice. In thinning, up to a third of a shrub’s branches are selectively cut all the way back to the ground, while the rest of the plant is left intact. Branches selected for removal are usually those that are old, diseased, or unsightly, or those that have grown too tall for their allotted space in the garden. Thinning encourages new growth from the base of the plant, giving tired old shrubs new life. In addition, it has the added benefit of increasing air circulation between branches, thereby combating diseases such as powdery mildew which thrive in damp, still conditions. Twiggy plants such as shrub dogwood, mock orange, and butterfly bush are perfect candidates for thinning.

Thanks again, Shari. Our community is fortunate to have Reeves-Reed Arboretum as a resource. Be sure to check out the Arboretum website. You can get more horticulture tips by reading "Cuttings," the Arboretum's monthly newsletter, available online.