Perennials: Selection, Planting, and Care

The main advantage to growing perennials is that they do not have to be replanted each year, unlike annuals, although some perennials must be replaced or divided every few years to keep them vigorous or flowering well.

Perennials also provide the opportunity to vary how a garden looks from one season to another. Many species have a limited flowering period, and different flower colors or shapes can be featured at different times of the year as plants come into and go out of bloom. For more season-long or nonstop color, grow several species of perennials that have different bloom times, or add annuals that flower more continuously from late spring to early autumn.

Perennials as a group are generally low-maintenance, but they can require some pruning and grooming to keep them looking their best. With some species, removing spent flowers (called deadheading) encourages the production of additional blooms, since the plant isn't spending energy ripening seed. Biennials (plants that live up to two years) or short-lived perennials can sometimes be tricked into another year of flowering if developing seed heads are removed immediately after flowering.

Over the course of the growing season, perennial foliage may become ragged and tired-looking from accumulated damage, or stems can get leggy (more bare-bottomed, having lost their lowermost leaves). Cutting damaged leaves off or pruning leggy stems down to the base of the plant can stimulate new growth in some cases.

Generally, you can plant perennials any time they are available for purchase and the ground is workable (not frozen and not too wet). In some cases, perennials that flower in late summer or autumn may transplant with less interruption in growth if planted in the spring, and similarly, spring-flowering perennials may acclimate more smoothly if planted in late summer or early autumn.

Due to limits on inventory space and customer shopping habits, area nurseries and their suppliers tend to offer perennials more seasonally, closer to the time of year they will be actively growing and coming into bloom. In that case, a perennial that naturally doesn't start to regrow until late spring (such as hibiscus) may not be stocked in early spring, and a perennial popular for autumn flowers (such as toad lily) might be stocked starting in summer as opposed to spring.

Regardless of the planting season, allow perennials sufficient time to establish themselves before they bloom or need to endure cold weather. It can take around three years after planting for perennials to become well-established and have a more lush, full appearance.

• Always start by assessing your site conditions:
  • Light: How much direct summer sun does the planting area get?
  • Soil: How well does the soil drain? Does it skew dry, or tend to stay wet a while after heavy rain?
  • Wildlife challenges: Do deer or rabbits regularly eat plants in the yard?
  • Space: What size of plant can be space accommodate? Do not rely on trimming to keep a large-maturing plant smaller, which may affect its health, stunt growth, or reduce blooming.
• Decide if you have a primary use or aesthetic feature for the plants, such as:
  • supporting wildlife
  • attracting bees, butterflies, hummingbirds, or songbirds
  • providing color, fragrance, or blooms at a certain time of year
  • managing stormwater, such as in a rain garden
  • screening for privacy
  • serving as a living mulch or groundcover
• For plant selection or combination ideas, observe the flowering times of perennials in your neighborhood or in a public garden. You may find plants that will flower together, or which will be showy (from flower or foliage color) when little else is in bloom. To learn specifics about how to grow particular plants or where to find uncommon species or varieties, consult local plant societies/clubs, specialty books, garden centers, and local botanical gardens.
• Incorporate a range of plants for a variety of leaf textures, growth habits, and flower traits (color, shape, season).
• Grow your own plants or purchase them from a local nursery.
  • Choosing plants while they are in bloom lets you be particular as to flower color.
  • Avoid buying plants with sparse or heavily-damaged leaves. Leaf color is not always a reliable indicator of plant health, since some varieties naturally have yellower or darker leaves, or the leaves will turn yellow if a plant is going dormant naturally. Gently remove the plant from its pot to inspect the roots, which should be firm and ideally numerous enough to hold the root ball together. Root color isn't always an indicator of health, although most roots are off-white; fern roots, however, are naturally dark.
  • If consistent traits (flower color, mature plant size, heat/cold hardiness, degree of disease resistance, etc.) are important to you, consider buying named varieties (cultivars) of plants, since their genetics are usually the same from one individual to another with the same cultivar name.

It is very common for container-grown plants to become root-bound while growing in the plant nursery and as they wait to be sold. As the roots reach the sides of the pot, they begin to circle and may become matted and tangled. Take the time to trim this impenetrable mass of roots before planting. It may look drastic, but in the long term, the plants will develop healthier root systems that spread out laterally into the soil instead of continuing to circle in a small area.

Pruning terms

• Pinching is removing the top few inches of soft growth on a stem with either a sharp, clean pair of shears or by pinching it off between your thumbnail and index finger.
• Disbudding is the removal of small, immature flower buds.
  • Removing of the top (terminal) bud causes the plant to produce more numerous, but smaller, flowers.
  • Removal of the side (lateral) buds will produce a larger flower on a longer stem. Dahlias, carnations, chrysanthemums, and peonies are commonly disbudded.
• Deadheading is the removal of spent, faded flowers. Flowers that go to seed consume large amounts of a plant’s energy, diverting it from vegetative growth or more blooms. When practical, plants can be deadheaded to a lateral leaf, flower or bud. In the case of a single flower stem that has few or no leaves, the flower stem should be cut off close to the base.

Reasons for pruning

Removing spent flowers

In general, perennials should be deadheaded after blooming, although there are exceptions when:

• self-seeding is desired, such as allowing a groundcover to spread
• the seed heads are attractive
• the seeds are a food source for wildlife

Many gardeners allow short-lived perennials like foxglove (Digitalis), and native wildflowers like Eastern columbine (Aquilegia canadensis), to go to seed to generate new plants. The seed heads of coneflower (Echinacea), Joe-pye weed (Eutrochium), beebalm (Monarda), mountain-mint (Pycnanthemum), and black-eyed Susan (Rudbeckia) are considered both attractive in winter and a good food source for birds.

Promoting plant health and aesthetics

Some perennials are thinned to reduce the risk of disease from crowding, to increase flower size, or to improve the plant's appearance. Thin by cutting some stems at the base of the plant. This improves air circulation around the plant, reducing the risk of disease outbreaks. Phlox, aster, beebalm, and delphinium benefit from being thinned because they are prone to mildew or rot.

Influencing growth

Pinching promotes bushy growth for many species. To avoid staking and to produce more compact plants with stronger stems, pinch back perennials such as chrysanthemums and asters several times before mid-July. Generally, the first pinching is done as soon as there are several sets of leaves on each stem.

Pinching delays flowering. Pinching only some of the stems results in an extended blooming season, since the pinched portions will bloom later, and the rest of the stems will bloom "on time." To stagger plant heights or bloom times, perennials such as garden phlox (Phlox paniculata) and 'Autumn Joy' sedum can be cut back once or twice in the summer before mid-July.

Perennials with one leafy or unbranched stalk, such as a lily, and those with leafless flowering stalks, such as iris, should not be pinched back before bloom.

End-of-season clean-up

Traditionally, cutting non-evergreen perennials to the ground in autumn would tidy-up the garden for aesthetics. Removing old plant debris can also reduce the risk of harboring a pest insect or infectious disease for the following year.

However, this clean-up can also remove the winter weather protection that broken, bent stalks provide to tender basal foliage and crowns. Perennials (including ornamental grasses) that are not cut back in autumn provide shelter from weather and predators, as well as food like seeds, for birds and other wildlife in winter. It also provides overwintering sites for pollinators and natural enemies (beneficial organisms) that work in our favor to control garden pests.

It is a personal preference whether to leave the plant debris or cut it back. Around plants that contracted severe disease or a significant pest outbreak, it may be best to remove debris once autumn frosts have killed stems and foliage. For all other plants, especially native species, leaving that material in place provides ecological benefits that can outweigh the risk of harboring a pest or pathogen.

If you need to transplant a perennial plant, do it on a cloudy day to reduce sun and/or heat stress. If you are careful, perennials can be transplanted even when they are in bloom; but it’s best to do it when they are dormant or just starting growth. Early spring or fall are the best times to transplant them.

Tips:

• Dig the new hole first, amending the soil with organic matter if possible.
• Fill it with water and let it drain.
• Dig the perennial up carefully, saving as large a root ball as possible.
• Settle the transplant into the hole, filling in with soil.
• Water the new transplant, mulch it, and protect it, if necessary, with a cardboard screen.

In the fall, the stems and seed heads of flowering herbaceous perennials and ornamental grasses can be left standing to provide food for birds (seeds) and overwintering habitat for pollinators and other wildlife that are harmed by habitat loss. There are a couple of exceptions to this:

1. Remove any plant material that had obvious symptoms of a disease (e.g., powdery mildew or other fungal leaf spots). In such cases, it is a good practice to rake out dead leaves and prune stems down to about two inches above the root crown. Put infected plant material out for your municipal yard waste collection or in the trash; do not compost it unless you can ensure that all of the material is exposed to a temperature of over 131°F for three consecutive days. Some plant pathogens can survive through the winter on dead plant material or in the soil. Removing infected plant material reduces the chances for pathogens to survive and reinfect plants in the next growing season.
2. Prune off the seed heads of invasive plants (such as fountain grass, Pennisetum) to control their spread. If you can, replace invasive plants with species that are native or non-invasive. (Use the Mid-Atlantic Invaders Tool to find out if a plant is considered invasive.)

If plants are healthy and not pruned back in the fall, prune them in early spring before bees start flying and seeking nesting sites. (Insect activity starts when the daytime temperature is consistently above 50°F so they can warm up their flight muscles.) Stems can be cut to various heights from 8 to 24 inches above the ground. The hollow tubes of the stems provide nesting sites for some types of non-stinging native bees (important pollinators). The top portions of pruned stems can be composted or put in a pile or bundle in an out-of-the-way area of your yard to create additional nesting and shelter spaces. Refer to the Xerces Society resource (PDF) Nesting and Overwintering Habitat for Pollinators and Other Beneficial Insects for more information.

Fall is a good time to transplant and separate perennials to give them more space. Increasing air movement between plants is another important method of preventing disease problems. If you are concerned about excessive bare space until the divided sections become full again, plan to fill in with annuals for the first two or so years.

After frost arrives in the fall, you can apply a 1-inch layer of mulch around perennial plants. Do not pile wood mulch on top of the plant crowns, which encourages rot or root development into the mulch instead of into the soil. Fallen leaves can be raked and placed around perennial plants as mulch; the leaves decompose and provide nutrients for the soil as free, natural fertilizer.

Seed  

Seed propagation has several advantages over vegetative division. Diseases are less likely to be carried over with seed. Less common varieties are more often available as seeds. Seeds of species plants will come true to type, whereas seed of cultivars may not come true. Plant perennials from seed either in mid-to-late summer, at the same time they would naturally germinate, or sow them in the spring.

Division 

Most perennials left in the same place for more than three years are likely to be overcrowded, with dead unsightly centers. The center of the clump will grow poorly, if at all, and the flowers will be sparse. The clump will have a “doughnut” appearance. The perennials need to be divided. In Maryland, divide spring and summer bloomers after they bloom, either in late summer or fall as the foliage dies down. Fall-blooming herbaceous perennials are usually divided in early spring, giving the plants an entire growing season to recover.

Tips:

• Water well prior to digging to help maintain the soil ball and keep the roots damp.
• Keep only vigorous side shoots from the outer part of the clump. Discard the center of the clump.
• Divide the plant into clumps of three to five shoots each. Be careful not to over divide; too small a clump will not give much color the first year after replanting.
• Divide perennials when the plants are dormant, just before a new season of growth, or in the fall so they can become established before the ground freezes.
• Don’t replant all of the divisions in the same space that contained the original plant. Give some to friends, plant them elsewhere in the yard, or compost them.

Cuttings

Many plants can be propagated from either tip or root cuttings. Generally, tip cuttings are easier to propagate than root cuttings.

Select new growth for cuttings. Make tip cuttings three to six inches long. Treat the base of the cutting with a root stimulant. Leave all foliage on the cutting except the part that will be below the soil line. Insert one cutting in a pot filled with damp sand. Place pots of tip cuttings in a lightly shaded place. Cover with a sheet of tented clear plastic. Check regularly to make sure the cuttings do not dry out.

When cuttings do not pull easily out of the soil, they have begun to root. Make holes in the plastic sheet or raise the plastic cover to increase air circulation. This will harden the cuttings. Make root cuttings of such plants as phlox, baby’s breath, and Oriental poppy. Dig the plants in spring. Select pencil-size roots; cut them into 4-inch sections. Put each piece in a peat pot. Prepare a tray of pots as for seeds, except the soil mix should be two parts sand, one part soil, and one part peat moss. Water thoroughly.