Perennial Shade Gardening
By Dick Christensen, Master Gardener
Colorado State University Extension, Larimer County
August 11, 2007
Shade offers welcome relief from the intense Colorado sun, but cultivating shade gardens can be tricky. Most gardeners think of shade as a challenge for growing plants. While most plants do not grow well in low light, many thrive under these conditions. The goal is to discover which ones are adapted to your garden setting. Remember to use the "right plant, right place" rule. Another key to a successful shade garden is to combine and contrast the forms, textures and colors of the leaves.
TYPES OF SHADE
Full shade has little or no direct sunlight reaching the ground at any time of the day. Full shade describes light on the north or northeast side of structures during the growing season, where the ground remains in shade throughout the entire day.
Partial or medium shade occurs where sun rays are blocked for most of the day, and is cast by evergreens or dense, deciduous trees such as lindens or maples. Areas under decks or stairs, or in deeply recessed south-facing entries, can be classified as medium shade.
Light shade is an area that is shaded but bright, or an area completely shaded for several hours each day. Light shade prevails underneath the canopies of smaller ornamental or more open deciduous trees and in garden areas receiving only morning sun. The sun's rays may be blocked by a wall or building for several hours at midday, but the area is sunny the rest of the day. Edges of shady gardens or areas under the canopies of solitary, lightly branched trees are typical of filtered sunlight. Certain flower and foliage colors may be more brilliant when plants are shielded from intense midday sunlight.
WATER AND NUTRITION
Determining if your shaded areas are moist or dry will help in selecting appropriate shade plants. Root competition for moisture is another consideration in shade gardening. Some shade-tolerant plants adapt to low-moisture situations, while others require moist shade. Consider a plant's horticultural needs when selecting and watering plants, especially if placed near trees and shrubs. Shady garden spots may have soil depleted of nutrients or water from nearby structures or tree roots.
Compost, leaf mold or sphagnum peat moss help soil hold moisture, reducing water needs. Regularly adding three to four inches of amendments to the soil helps restore nutrients and texture. This is important because shade plants typically do best in soil rich in organic matter. In Colorado gardens, most shade is dry. However, if a shaded area receives runoff from rain or sprinklers, it may qualify as moist shade. Most Colorado soils are mineral soils and more alkaline than those in moist temperate forests. Old pine needles or sphagnum peat moss can help acidify the soil. Gently till organic matter into the soil to avoid damaging nearby roots.
SHORT LIST OF SHADE-TOLERANT PERENNIALS
Siberian Bugloss or Brunnera (Brunnera macrophylla), often confused with forget-me-nots, display sky-blue flowers in spring and reliable clumps of large heart-shaped leaves all season. These plants tolerate moist or dry conditions and often will self-seed in the garden. They make an attractive and versatile groundcover, particularly under shrubs, and grow 12 to 18 inches. Many new cultivars have variegated and patterned leaves. Clumps may be easily divided in early fall.
Columbine (Aquilegia spp.) are native North American perennials, which perform beautifully in a garden that gets about a half day of shade. They can grow up to three feet tall, depending on the cultivar, and produce elegant blooms in shades of white, blue, pink/red, and yellow. Columbines also self-sow readily. Hardy geranium (Geranium sanguineum) is very different from annual geraniums, known as pelargoniums. Hardy geraniums form low mounds eight to 18 inches tall and bloom from spring through fall in white, red, purple, pink, or blue. They will thrive in partial shade; some do well in full shade. Another bonus is that hardy geraniums tolerate dry soil.
Coral bells (Heuchera) provide unbeatable color in shade gardens. Recent breeding has produced plants with foliage in a variety of colors, like coppery orange, black, deep purple and chartreuse. Coral bells thrive in partial to full shade and form low mounds of foliage. They also produce flower spikes of delicate, bell-shaped blooms. Coral bells size and height vary depending on the cultivar.
Bleeding heart (Dicentra spectabilis) is a beautiful, old-fashioned plant for shade gardens. In spring and early summer, delicate, dangling flowers will appear, looking like rows of heart-shaped lockets. Bleeding heart blooms in shades of white, pink, purple, and red, and grows to 18 inches tall or more and up to three feet wide. It dislikes summer heat, however, and may go dormant if allowed to dry out. Not to worry--it will come back in the spring. Bleeding heart thrives in partial and full shade. It prefers morning sun over afternoon sun, which can be too hot. Primroses (Primula spp.) are showy spring flowers that grow to about six inches. The flowers can be yellow, red, or blue, blooming in April and May.
Sweet violets (Viola odorata) are delicate spring flowers that grow to about six inches. The flowers can be white or purple, blooming in April and May.
Monkshood (Aconitum napellus) grow up to three-to-four feet tall and are showy with attractive foliage blooming from early midsummer to late summer. Some cultivars are purple, white, and rose colored. Monkshood does well in dense shade. Be careful planting this in the garden with children or pets-all parts of the plant are poisonous.
Hosta (Hosta ventricosa), or plantain lilies, prefer partial shade or deep shade and are grown primarily for their foliage. Leaf colors range from lime green to dark bluish green; some have a green and white/cream variegation while others are two-tone green. They can range in size from two to 24 inches in height. As an added bonus, flower spikes shoot up briefly in midsummer.
Buckwheat, 'Kannah Creek' (Eriogonum umbellatum v. aureum), is a perennial that grows 12 to 15 inches high and 12 to 24 inches wide. It grows best in full to partial shade and tolerates most soil types. It blooms from May to July with masses of yellow flowers that turn orange as they age. The spreading green foliage changes to a vivid purple-red in winter and is considered a vigorous and adaptable Western native. This is a 2007 Plant Select from Colorado State University and Denver Botanic Gardens.
PERENNIAL GROUND COVERS
Plants that are low-growing (generally less than 12 inches) and that spread easily are suitable ground cover plants. Ideally, a ground cover should be dense enough to inhibit weeds.
Most yards have shady corners where grass doesn't prosper or bare earth under shrubs or trees that would benefit from a low-maintenance groundcover. There are many shade-loving groundcovers that thrive in Rocky Mountain gardens such as:
Periwinkle (Vinca minor) grows four to six inches and has green or variegated leaf forms. The flowers can be pink and white, as well as the classic bright violet-blue. Periwinkle has oval, leathery leaves about an inch long, which are beautiful most of the winter. Vinca can spread vigorously.
Dead Nettle (Lamium maculatum) has small spikes of flowers, principally in late spring and grows six inches wide. A sturdy, fast growing ground cover, it tolerates a wide range of soils and moisture levels. The leaves are white edged with a thin line of green; flowers are a bright white and appear in late spring. This plant lights up dark areas of the garden with its variegated foliage and can spread rapidly in the right conditions.
Bugleweed (Ajuga reptans) grows less than six inches tall and makes a fast-spreading mat of oblong leaves with rather formal spires of pink, lavender, or white flowers, depending on the variety. The foliage also varies, from showy, tricolor leaves to deep purple to solid green. It does best with a few hours of direct early morning or late afternoon sun.
Sweet woodruff (Galium odorata) grows six to eight inches with white starry blooms from late April well into June. Considered as an aggressive grower, sweet woodruff remains one of the best ground covers under shrubs.
Creeping Oregon grape holly (Mahonia repens) is a slightly taller ground cover that grows six to 12 inches with yellow flowers in the spring. Considered to be xeric, it has holly-like foliage and a low bushy form.
Kinnikinick (Arctostaphylos) is a low-growing, four to six inch evergreen shrub with red, edible berries. Kinnikinick is a popular choice for areas under established evergreens because it grows well in acidic soil.
Bishop's weed (Aegopodium podagraria 'variegatum') grows ten to 12 inches tall with variegated green and white foliage. It grows in shade but is very aggressive and can become a problem. Buyers beware.
Lily-of-the-valley (Campanula carpatica) has six- to ten-inch tall leaves with fragrant, white flowers in May and June and inedible, red berries later in the summer. This classic plant can spread aggressively.
Creeping or mat penstemon (Penstemon caespitosus) grow one to two inches tall in a prostrate mat of tiny narrow leaves, with purple flowers in May and June.
DESIGN CONSIDERATIONS
As trees and shrubs mature, the garden landscape receives greater shade. What once was a sunny garden usually slowly evolves into a shady space. Assess the degree of shade in your garden periodically to determine if changes in plants are needed. The degree of shade may also change with the season. Areas in full summer sun may be in partial shade in spring and fall, when the sun is at a lower angle in the sky. Bright, bold colors are less common in shade-tolerant plants than in sun-loving species, and flowers usually are produced less abundantly in the shade. For these reasons, shade gardens are often more subtle and restful than sunny ones. Plant textures, forms, and slight color differences become more important elements of a design.
Texture has many aspects. Large-leaved plants, such as hosta, have a coarse or deeply ribbed texture, while periwinkle creates fine, glossy texture. Strong contrasts in texture accentuate their differences. Use strong textural contrasts where emphasis is needed. Variegated or yellow-green foliage is evident in the shade more than solid green or blue-green foliage. Light colors-white, cream, yellow and pastel pink-stand out in the shade. Deep reds, blues and purples may fade into the shade unless set off by a contrasting lighter color. To emphasize plantings in the shade, concentrate on plants with light-colored flowers or foliage.
More information on these shade-loving plants and others are available from Colorado State University Extension Fact Sheet numbers 7.231, 7.230, 7.235, 7.400, 7.402, 7.405, which can be obtained by visiting the Larimer County Extension office or the Colorado State University Extension website at ext.colostate.edu.
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