Perennials reach maturity in as little as two seasons. A well-chosen selection rapidly forms a fine tableau of textures and colors, both foliage and flowers providing visual interest for months. They can be featured as specimen plants, massed in a traditional herbaceous border, or interspersed with shrubs, annuals, and biennials in a mixed border. For variety, they can be cultivated in containers or windowboxes, or grown among fruit and vegetables in a kitchen garden.
What are perennials?
Precisely defined, perennials are plants that live for 2 years or longer and, once mature, flower annually. In gardens, however, the term perennial is commonly applied to herbaceous plants that form flowering stems each year before seeding, then die back in autumn to ground level, sending up new growth in spring. The term is also used to describe some non-woody, evergreen plants, such as Bergenia and Yucca, as well as subshrubs like Artemisia and Penstemon.
Ornamental features
Perennials are probably the most diverse plant group, providing a huge variety of shape, form, color, texture, and scent with which to design a planting. They range in height from low, creeping plants, useful as groundcovers, to feature plants, such as Rheum palmatum which is 8ft (2.5cm) or more high.
Colorful foliage plants can add contrast to a predominantly mid-green backdrop, accentuating each plant’s structural form. Possible choices include the glossy, dark gren leaves of Acanthus, the purple foliage of some Canna species, the variegated leaves of Hosta cultivars, and the silvery foliage of Onopordum.
Perennials with unusual shapes, arrangements, or textures of leaf can also produce eye-catching effects, especially when used as accent plants (usually taller feature plants used to accentuate contrasts).
Flowers of perennials are extremely vaired in color, size, and form, presenting the gardener with endless possibilities for contrasting, complementary, and single-color plantings. Well-chosen groupings can also provide form and structure, for example, the flat corymobs of Achillea, the tall spikes of lupines (Lupinus), or the tiered whorls of Phlomis russeliana. Popular fragrant perennials, attractive to bees and butterflies, include species of Asclepias, clematis, Echinacea, phlox, and verbenas.
Borders
The traditional herbaceous border dates back to the nineteenth century and earlier. It was usually a large rectangular plot, set into a lawn or atainst a hedge or wall, filled with summer- and autumn-flowering perennials banked according to height. Today, herbaceous borders are frequently more modest in scale, often incorportating a carefully planned color scheme and using foliage as well as flowers for color, texture, and structure. Some gardeners still prefer a banked effect. Others choose to rrange plant heights unevenly.
Borders can be planned as a series of units (groupings of 3 or more plants) or as a progression of subtle associations along the border. Swathes of massed plants, at an oblique angle to the front of the border, have strong visual impact. Experiment with merging informal drifts of several different species and cultivars, or create intricate patterns using regularly spaced groups of a more limited number of plants.
For groundovers, choose low, mat-forming, or creeping perennials displaying attractive foliage, such as Lamium, or flowering, clump-forming plants, such as Bergenia and some hellebores, can soften stark winter borders in cold area.
Mixed borders of herbaceous perennials, shrubs, bulbs, climbers, annuals, and biennials, are excellent for providing year-round interest. A careful selection of perennials and deciduous and evergreen shrubs that fill up to a third of the border will provide a balanced planting. Fill out the border with annuals until the perennials and shrubs mature. For extra interest in spring, many perennials, particularly those coming into growth in mid- or late spring, such as hostas can be underplanted with bulbs or other early-flowering plants, such as tulips, anemones, and scillas. Tender perennials, such as dahlias, can be added to enhance a late-summer and autumn display.
Container ideas
Some hardy perennials are ideal for use in containers; ornamental grasses, for example, offer excellent choices of shape, structure, and color. Less hardy plants, such as Agave, Cordyline, Melianthus major, and variegated Phormium cultivars, are also attractive, but may need overwintering indoors. Perennial subshrubs in decorative containers, for example, Argyranthemum and lavender (Lavandula), can serve as focal points in paved areas. For a long floral display, choose summer- and autumn-flowering perennials, such as Felicia and geraniums.
Cutting and drying
Perennials include some of the best garden plants for cutting. Blooms often last well if they are cut early in the morning. Remove leaves from the bases of the stalks. To encourage them to take up water, bruise or slit the base of each stem, or plunge stems briefly in very warm water, before immersing them in warm water to the necks (where the flowers begin).
The flowers and seed heads of some perennials are also good for drying, particularly those that hold their shape and color well, such as Achillea and Eryngium. Some less rigid flowers with a higher water content may be air-dried if this is done quickly, by suspending small bunches in a dark and airy place. Flowers with papery petals can be dried in a commercial desiccant. Pick blooms as they begin to open. If picked too soon, the stems will not be sufficiently stiff; if picked too late, the color of the flowers will have deteriorated or the petals (or seed, in the case of grasses) will fall.
Cultivation
When planning a herbaceous or mixed border, match the needs of plants to the exposure and conditions of the chosen site. Plant in carefully prepared ground, usually in spring or autumn. Keep the surrounding area free of weeds, and water young plants regularly until established. Mature perennials require little watering except during prolonged dry periods. Apply an annual top-dressing of bone meal or a balanced, slow-release fertilizer, preferably in early spring after rain.
Most herbaceous plants produce vigorous shoots in spring, but some may be spindly. When the plant is one-quarter to one-third of its final height, pinch out or cut back weak shoots; the remaining sturdy shoots will usually bear larger flowers. This particularly benefits plants such as asters, delphiniums, and phlox. Plants that require support should be stakes when young to ensure that lax stems remain upright. Deadhead regularly.
In autumn, cut shoots down to the base, and remove dead and faded growth and weeds, leaving the border tidy in winter. In spring, when the ground is moist, apply a mulch of organic matter, such as mushroom compost or bark chips. Where practical, perennials grown in a border should be divided not only for propagation, but also to maintain vigor, ideally 3 to 5 years. Take care when lifting and replanting not to damage the rots of surrounding trees and shrubs.
To appreciate grasses and other plants throughout the winter months, delay cutting and mulching until spring. In cold areas, leaving top-growth in place during winter will also offer some protection to the crown of a plant.
For container-grown perennials, use soil-based soil mix or lighter soilless mix missed with a slow-release, balanced fertilizer. Although the extra weight of the soil adds stability, use soilless mixes in plastic containers for roof gardeners or balconies wher3e heavy containers may be too great a load; lighter containers are also easier to move.
Ensure that soil mix in containers does not dry out when plants are in growth; water daily in hot, dry weather. Plants in larger containers require less frequent watering, especially if moisture-retentive polymer granules have been added to the soil mix. Mulching also helps to retain moisture; replace mulch when dividing or repotting plants.
Perennials can be propagated by seed, division, cuttings, or grafting (rarely used by amateur gardeners). Sowing seed is preferable for species where large numbers of plants are required. Hybrids and cultivars do not come true from seed; divide or take cuttings.
-Information accredited to The American Horticultural Society of A-Z Encyclopedia of Garden Plants. Page 38-39.
