Keep perennials in shape all year.
When and how you do these chores depends on when a plant blooms and how it grows. These guidelines are some of the most popular perennials.
SHEAR LOW-GROWING PERENNIALS
These ground-hugging types provide a flush of bloom in spring (or later). They include basket-of-gold, dead nettle, diascia, evergreen candytuft, most geraniums, nepeta, rockcress, Santa Barbara daisy, and Swan River daisy.
All these plants benefit from being sheared back after 80 percent of the blooms have faded. Shearing them helps initiate another bloom cycle.
The best way to shear low plants is with a long-bladed tool, such as hedge clippers. For most plants, cut off flower heads and an inch or so of foliage, then apply fertilizer or compost. After spring bloom, cut back old foliage completely to the ground; new growth emerges from the base.
In fall or winter, plants like catmint, dead nettle, and diascia may look ragged. Clean out dead foliage and cut back plants to keep them looking neat.
CUT BACK WOODY PERENNIALS
These fast-growing plants develop woody stems. Examples include Artemisia 'Powis Castle', Erysimum 'Bowles Mauve', Mexican bush sage (Salvia leucantha), and Russian sage (Perovskia 'Blue Spire').
To encourage lush new growth, periodically cut plants back hard. Mexican bush sage needs pruning every year. Artemisia 'Powis Castle' also looks best if pruned every year. Others may need it only every two years. Besides severe pruning, trim off spent flowers during the growing season to encourage new flower production.
The best time for pruning varies. In mild climates, cut old stems of Mexican bush sage to the ground when they look ratty in mid- to late winter. But in colder climates, wait until after the last frost, or cut them back and then cover with mulch.
Cut back A. 'Powis Castle' and Russian sage in late winter or early spring, just when new growth is emerging. It's best to cut back Erysimum 'Bowles Mauve' in late spring after the major flush of bloom (however, since E. 'Bowles Mauve' is short-lived, the plant may not regrow after cutting back).
Handle lavender differently from other woody perennials; most large lavenders don't respond well to severe shearing. Start pruning when they're 2 years old. Shear off old flowers and 2 to 4 inches of growth after bloom (don't cut into wood, only green growth). Long-blooming types such as English lavender can be pruned anytime they seem to need it except during hot weather. Maintain dwarf kinds by shearing off flowers.
CUT BACK SOME WHOLE FLOWER STEMS
To encourage additional bloom during the growing' season and to prevent floppy growth, some plants should have their entire bloom stalk cut to the ground after it has faded. The classic example is garden penstemon. Other examples include delphinium, Salvia superba varieties, S. uliginosa, and tall yarrows.
These same perennials should be completely cut back during the dormant season (late fall or late winter) or when they look scraggly.
To encourage bushy new growth on plants that bloom only once (euphorbias, hellebores, and Sedum spectabile), cut off their bloom stalks after flowering and before they set seed.
DEADHEAD FLOWER STEMS
Many long-blooming plants require deadheading (pinching off old flowers) through the growing season to encourage formation of new flowers and to keep plants looking neat.
Examples of plants whose flower stems need deadheading include armeria, coreopsis (shear off flowers when there are too many to pick off by hand), dianthus, gaillardia, geum, purple coneflower, red-hot poker, rudbeckia, scabiosa, Shasta daisy, and veronica. At the end of the flowering season (or fall to late winter), shear the entire plant (except for armeria) to encourage bushy new growth.