Pruning is an important part of plant maintenance. There are four basic objectives for pruning a plant: (1) to influence flower or fruit production; (2) to direct the growth and the form of the plant; (3) to change the appearance of a plant; (4) to promote plant health. Plants should not be pruned unnecessarily as repeated pruning can negatively affect the form of a plant, create a stressed plant and leave open wounds that may attract insects and disease.
When pruning a plant, it is important to understand how the plant is being affected by the pruning technique and frequency. Removing more than one-third of the foliage will reduce the plant's photosynthetic abilities significantly, decreasing its ability to manufacture enough food to support itself. As a rule, no more than one-third of a plant should be removed per growing season. In addition, it is important to consider the mature size of a plant and choose a location where it will not require frequent pruning to prevent it from overgrowing its site.
It's important to understand how each plant should be pruned. Perennials are pruned differently than vines, ground covers, landscape grasses, etc. Some plants are pruned to encourage flowering, such as bee balm (Monarda). Others, including landscape grasses, are pruned to remove old, dead foliage and allow new growth to flourish. Removing some stems in the center of a plant can improve air circulation and allow sunlight to reach the base of the plant, reducing the chance of disease from organisms that thrive in moist conditions. Cutting back stems to just above a node can encourage leaf and bud production at that point and prevent "leggy" plants with long stems and few leaves. This improves the form of the plant.
Using a clean, sharp tool to remove diseased plant material or stems that have been damaged or broken will also improve plant health. Removing broken stems by making a clean cut helps the plant to heal more efficiently, protecting the plant from infection. Timing is also an important factor for proper pruning. Some plants, such as Geranium, should be pruned after flowering to encourage a second bloom. Herbs like basil (Ocimum), oregano (Origanum) and mint (Mentha) are continually cut and pinched back to encourage repeated foliage production and thus a continual supply of spices.
Cutting Back Plants
How much a plant should be cut back depends on the type of plant, its growth cycle, and the purpose of pruning. For example, some ground covers, such as Japanese spurge, (Pachysandra terminalis) should be pruned just after new growth has emerged in the spring to encourage a compact, bushy habit. Others should be cut back after blooming to promote a second bloom, as in the case of leadwort (Ceratostigma plumbaginoides) and candytuft (Iberis sempervirens).
Pruning will also encourage new growth on many ground covers by forcing buds to break along the stem or at the crown of the plant. This can easily be achieved in various ways, depending on the size of the area. Hand pruners or a mower with a sharp blade set at its highest mowing height will work well to prune ground covers. Pruning will not only improve rejuvenation, but also maintain size control of such ground covers as bugleweed (Ajuga reptans) and wintercreeper (Euonymous fortunei). A second flowering on such ground covers as cottage pinks (Dianthus gratianopolitanus) can be induced by cutting back plants after flowering. Plants that have long stems should be cut back about halfway to encourage branching. Insect problems and diseases can be deterred by pruning to increase air circulation, pruning out dead or diseased plant parts, and by enabling sunlight to reach the center of the plant.
Landscape grasses should be cut back to the ground each spring to encourage light and moisture to reach the new growth at the base of the plant. Do not cut them back in the winter as the old foliage acts as insulation for the root system and provides valuable winter interest. Choice of tools will depend on the size of the plant and how many plants require pruning. A hand pruner may be fine for a few plants, but for a mass of plants, electric hedge trimmers, a string trimmer or a hedge pruner is recommended. Burning is also an option for large areas and will require a permit from your municipality.
Pruning for Plant Form
Many people think pruning woody plants, not realizing that pruning can also help maintain the vigor and form of perennials. Pruning Perennials provides a basic lesson in pruning perennials along with photographs that detail the results.
Vines can sometimes be challenging to prune as they are often unwieldy and firmly attached to their support structure. These websites provide guidelines and tips for pruning vines:
Training / Pruning Vines
Cochise County Cooperative Extension Home Horticulture, "To Prune?", Arizona Master Gardener Program, previously featured in Sierra Vista Herald / Bisbee Daily Review. February 1999.
DiSabato-Aust, Tracy, The Well-Tended Perennial Garden. Timberland Press, Portland, Oregon. 1998
North Carolina State University, "Training / Pruning Vines", Erv Evans. 2000. http://www.ces/ncsu.edu/depts/hort/consumer/factsheets/vines/text_prunin.html
University of Minnesota Morris Media Services, "Pretty as a Petunia", Prairie Yard & Garden Tip of the Week, June 16, 2000.
University of Minnesota Extension, "Ornamental Grasses for Minnesota", FS-6422-GO, M. Meyer, D.B. White, and H. Pellett, 1998.
University of Nebraska Cooperative Extension, "Ground Covers: Their Establishment and Maintenance", NebGuide G84-697-A, Donald H. Steinegger and Luann Finke. Revised June 1992. http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=2049&context=extensionhist
University of Virginia Cooperative Extension, Virginia Tech and Virginia State Universities, "Pruning Vines", The Virginia Gardener Newsletter, Diane Relf. March 31, 1998.
Washington State University Extension Snohomish County, "Plant Sanitation", Rick Reisinger, February 1995. http://www.cahe.wsu.edu/~hp31/qa24.htm