There are many benefits to pruning perennials. Pruning is typically defined as removing growth from an herbaceous or woody plant to maintain health and vigor. Generally people think of pruning as a technique that is only beneficial for woody plants, but it is also beneficial for perennials. This article focuses on the pruning of perennials and techniques that can be used to change the appearance of your perennials. Topiaries are a specialized form of pruning woody plants that many people are familiar with. This article will describe how herbaceous perennials can be designed into interesting forms as well.
Pruning is done to regulate the plants shape and size, and to control flowering and fruiting. Among the benefits are extending the blooming time, regenerating the plant, and encouraging new growth. Pruning can control the flower size and the number of flowers. Pruning is an excellent way to keep plants from invading the space of others. Pruning can control pests and disease. Pruning perennials can induce growth where and when it is wanted, and change the direction, quantity and vigor of the growth as desired. Pruning perennials can be an artistic venture that allows the gardener to change the shape of ordinary perennials. The only disadvantage of pruning perennials is that it can be a time consuming venture, but the rewards surely outweigh the drawbacks.
Tools that can be used for pruning are by-pass pruners, pruning scissors and hand-held shears. By-pass pruners are recommended for use in deadheading and cutting back perennials. By-pass pruners make a clean cut through the stem of the plant. Anvil type pruners can damage the stems of perennials by crushing them. Hedge shears are useful for pruning several stems at one time. Hedge shears are especially useful for cleanup in the spring and fall. Smaller pruners should be used when doing more delicate and detailed pruning. Fingers are another great tool that you can use to prune perennials, and you always have them with you.
Types of Pruning
The basic types of pruning are deadheading, cutting back, pinching, disbudding, and thinning.
Deadheading perennials refers to the removal of spent flowers. Removing the old flowers is beneficial for the plant because it promotes the growth of new flowers. If the flowers are left on the plant they will eventually go to seed. The production of seeds consumes a great deal of a plant's energy. When the plant's energy is not being used to produce seeds, the energy is focused on producing vegetative and root growth, which contribute to a stronger plant.
Methods of deadheading differ depending on species and the growth habit of the plant. The main thing to look for when deadheading is new buds and flowers. Remove only the dead flowers from the plant. Depending on the plant, you may need to deadhead to a lateral flower, bud or leaf.
Some plants need to be deadheaded to the ground or basal foliage such as Heuchera and Alchemilla. Most perennials should be pruned to the bud, lateral flower or leaf. When deadheading, prune off the spent flower stem to a new lateral flower or to a lateral bud. Pruning in this manner masks the cut that you have made and does not mar the attractiveness of the plant.
Some plants are best not deadheaded to promote self-seeding. Columbine (Aquilegia) will self-seed in the garden and look very beautiful coming up in unexpected places. Hydrangea flowers look beautiful as they age and die. Yarrow (Achillea millefolium), bishop's goutweed (Aegopodium podagraria 'Variegatum'), bugleweed (Ajuga reptans), lady's mantle (Alchemilla mollis), columbine (Aquilegia hybrids), aster (Aster novi-belgii), maiden pink (Dianthus deltoides), purple coneflower (Echinacea purpurea), bloodred geranium (Geranium sanguineum), dame's rocket (Hesperis matronalis), wild sweet william (Phlox divaricata), orange coneflower (Rudbeckia fulgida 'Goldsturm'), and sweet violet (Viola odorata) are some of the more common plants that self-seed. Deadheading is a useful method to keep plants confined to their designated space. However, some plants tend to be weedy and spread to unwanted areas of the garden by seed dispersal.
Perennials such as daylilies (Hermerocallis) have unattractive spent flowers and need frequent deadheading to look good. If you do not want to spend a lot of time pruning, this type of plant should be avoided in the design. Other plants should be left unpruned because they have beautiful seedheads such as hydrangea and false indigo (Baptisia australis).
You may have provided plants to attract wildlife or birds. The seeds of many plants provide a great food source. These plants should be left unpruned so they go to seed. Purple coneflower (Echinacea purpurea), joe-pye weed (Eupatorium maculatum), sunflower (Helianthus salicifolius), spike gayfeather (Liatris spicata), beebalm (Monarda didyma), black-eyed susan (Rudbeckia hirta), and hosta have attractive seed-heads that are a good food source for birds.
Cutting back may be considered a drastic form of pruning in the garden. Cutting back refers to pruning a plant to renew its appearance, encourage a new growth and flowering. Cutting back can be done either before or after flowering. Cutting a plant back to the ground is beneficial for certain spring blooming species (see next paragraph). Other perennials are cut back later in the growing season to renew the plants appearance or encourage new growth and flowering. As a general rule, plants that are cut back after they have flowered have regrowth that remains shorter than the normal mature height of the plant. Cutting back perennials can control the flowering time and height of the plant. When cutting a plant back, buds, flowers and leaves may all be removed. Approximately two inches of stem should be left of the plant when cutting to the ground. The reason for leaving the 2 inches is that new buds may be present slightly above ground. The best tool to use for cutting back is hedge shears. With some plants it may be necessary to cut the plant all the way down to the ground. Cutting back is generally done during the spring after growth has started. It should be done early in the season when the weather is cool in order to cause the least amount of stress to the plant. Special care should be taken with plants that are cut back to the ground. They should be well watered, and the soil around the plant should be aerated. When perennials are pruned back for maintenance or aesthetic reasons, they should be pruned after flowering.
Pinching perennials is similar to cutting back, however only a small amount of the plant is removed. When pinching, only the growing tips and first set of leaves are removed. The best tool for pinching is fingernails. It is best to pinch the stem just above a node. Pinching helps improve the growth habit of the plant. Pinching can also be used to stagger bloom time. Pinching is normally done is May or early to mid-June in the Midwest. The following is a list of plants that respond positively to pinching: Common yarrow (Achillea millefolium), Artemisia sp., Aster sp., pink turtlehead (Chelone iyonii), joe-pye weed (Eupatorium maculatum 'Gateway'), willowleaf sunflower (Helianthus salicifolius), cardinal flower (Lobelia cardinalis), beebalm (Monarda didyma), beardtongue (Penstemon barbatus), russian sage (Perovskia atriplicifolia), garden phlox (Phlox paniculata), Rudbeckia sp., Sedum 'Autumn Joy', goldenrod (Solidago hybrids), and spike speedwell (Veronica spicata).
Disbudding perennials refers to the removal of the plant's terminal or side buds. Removing the terminal bud of a plant will cause the side buds to produce more flowers. The flowers will be smaller, but more numerous than if the terminal bud was left intact. Removal of the side buds will cause the terminal bud to produce a larger flower on a longer stem. Disbudding is a pruning technique that is commonly used on dahlias, mums, carnations and peonies. Disbudding should be done before the buds are too large. Waiting too long to disbud can cause scars along the plant's stem.
Thinning perennials refers to the removal of stems from a plant. The benefits of thinning perennials are improved appearance, increased flower size, and disease prevention. To thin a plant, cut the stems to the ground in spring. A rule of thumb is to thin one in three stems. Aster, delphinium, monarda and phlox are all prone to mildew and thinning is a great method to help reduce the disease. The following perennials are prone to rot or mildew, and thinning improves the air circulation around the plant: Lady's mantle (Alchemilla mollis), bugleweed (Ajuga reptans), Bethlehem sage (Pulmonaria saccharata), beebalm (Monarda didyma), garden phlox (Phlox paniculata), and lamb's ear (Stachys byzantina).
Pruning Spring-Flowering Perennials
Pruning spring-flowering plants can be beneficial in several ways. Many of the low-growing rock garden and edging plants will benefit from being cut back to one-half the size of the plant after flowering. Pruning back by one-half the size of the plant will prevent it from opening up at the center, which looks unattractive in the garden. Rock garden plants that benefit from this type of pruning are evergreen candytuft (Iberis sempervirens), maiden pink (Dianthus deltoides), and moss phlox (Phlox subulata). Some plants such as catmint (Nepeta) will rebloom after the pruning. Many spring-flowering perennials can be cut back to one-half the size of the plant so that the garden looks well kept and green. The cutting of the plants promotes fresh new growth that makes the plant look more attractive. The following is a short list of spring-blooming perennials that benefit from this type of pruning: Columbine (Aquilegia hybrids), rock cress (Arabis caucasica), siberian bugloss (Brunnera macrophylla), ground clematis (Clematis recta), maiden pink (Dianthus deltoides), dame's rocket (Hesperis matronalis), evergreen candytuft (Iberis sempervirens), bearded iris (Iris hybrids), spotted deadnettle (Lamium maculatum), catmint (Nepeta mussinii), wild sweet william (Phlox divaricata), moss phlox (Phlox subulata), and wooly thyme (Thymus praecox).
Pruning Summer Flowering Perennials
The main difference between pruning spring-versus summer-flowering perennials is the amount of cutting back that is required after flowering. Depending on your objective some perennials should be pruned before flowering and others should be pruned after flowering. Pruning after flowering improves the aesthetics of the garden. When the weather has been very hot and dry, the plants usually look a little weather beaten. Depending on the species, some plants should be cut to new basal foliage, and others should be cut to the ground. Some plants look best if they are cut back by one-half or one-third of their mature height. When pruning perennials, cutting out the brown parts of the plant will stimulate growth of fresh green foliage. The following is a list of perennials that benefit from cutting back in the summer: Bishop's goutweed (Aegopodium podagraria 'Variegatum'), lady's mantle (Alchemilla mollis), southernwood (Artemisia abrotanum), wormwood (Artemisia schmidtiana 'Nana'), false indigo (Baptisia australis), tickseed (Coreopsis grandiflora), foxglove (Digitalis purpurea), queen-of -the-prairie (Filipendula rubra 'Venusta'), Geum hybrids, sunflower heliopsis (Heliopsis helianthoides), daylily (Hemerocallis sp.), small-flowered alumroot (Heuchera micrantha 'Palace Purple'), wild sweet william (Phlox maculata), garden phlox (Phlox paniculata), goldenrod (Solidago hybrids), and virginia spiderwort (Tradescantia x andersoniana).
Other perennials can be cut back before flowering for height control, and to stagger or delay bloom time. Staggering the bloom time may lengthen the bloom time of the plant. It is also a great way to control when a plant will bloom. There may be plants in your garden that you do not want to flower at the some time because of a bad color combination. Staggering the bloom time is a great way to avoid clashing colors.
As an experiment in late May of 2000, two species of perennials were pruned in the University of Minnesota's Display and Trial Garden. Each perennial was pruned in a different fashion. I watched the plants throughout the summer and recorded the height, flowering stage and overall appearance of the pruned plants. The plants were pruned on May 27 before the flowers or buds appeared. The plants that were pruned were Chelone iyonii and Monarda didyma. The Chelone iyonii was pruned into two tiers.
The front half of the plant was cut back 6 inches. The back half of the plant was left unpruned. The first change that I noticed in the plant was a distinct difference in the color of the pruned section. The pruned section was a pale green compared to the unpruned section. I noticed this change on July 4.
I checked the plant again on July 18, and the color of the plant was more uniform. The entire plant was a dark green color.
In the beginning of July the unpruned section of the chelone developed flower buds, while the pruned portion did not. By the end of July, buds had also formed on the pruned portion of the plant. By August 13 the unpruned section of the chelone had started to bloom. The pruned section had buds that were starting to open. The pruned section bloomed at the end of August. Pruning chelone proved to be a successful way of staggering the bloom time.
The other perennial that was pruned was a large clump of Monarda didyma 'Gardenview Scarlet'. The monarda was pruned into a mound shape by cutting the stems on the outer edge of the plant shorter than those towards the middle. The very center of the plant was not pruned, leaving flower heads intact.
By July 4, the flowers in the middle were completely open and developed. The flowers on the edges were not fully developed.
The plant was in full bloom when checked on July 18.
When I checked the monarda on July 28, it did not look very attractive because it was covered with powdery mildew. On August 13 the monarda had turned completely brown. The monarda had an attractive and interesting mound shape. It did not significantly stagger the bloom time of the plant.
Brickell, C., Joyce, D. 1996. Pruning and Training. DK Publishing, Inc. New York, NY.
DiSabato-Aust, Tracy. 1998. The Well-Tended Perennial Garden. Timber Press, Inc. Portland, OR.
Still, Steven M. 1980. Manual of Herbaceous Ornamental Plants. Stipes Publishing Company. Champaign, IL.
This maintenance report was developed by Dana Gardner, student, University of Minnesota Department of Horticulture Science.