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Discovering Perennial Flowers  Tolerant of "Hot Shade"

By Carl Wilson, Colorado State University Cooperative Extension agent, horticulture, Denver County

East-facing flower beds backed by masonry walls set up a tough challenge for the perennial flower gardener: What will grow there?

The walls not only absorb and reflect morning heat back on the plants, they also limit light for bloom production by shading the bed during the afternoon.

Here’s a case in point: A honey locust tree is growing in a yard, east of a masonry wall. Logically, the shade cast by the tree should reduce heat accumulation, but it doesn’t. Most of the shade occurs in early morning. The masonry walls absorb plenty of heat during late morning which then is reflected back at the plants through the afternoon and evening. The tree does limit the light available for plants to produce flowers.

While Xeriscape flower lists reveal what plants grow with little water in some shade, none discuss reflected heat situations. What follows is a description of "hot shade" perennials that have survived in a Denver-area garden after five years of trials and many failures. Beware that plant size will be smaller and bloom period shorter, because the shade limits the usual production of energy that comes from leaves.

If you want a cool look, try plants in the purple-to-lavender-to-blue group. A large plant that can serve as a focal pivot point in the bed is Echinops ritro "Taplow Purple," globe thistle. This plant produces a bold, rounded mass of spruce green-toothed leaves and, from late June through July, is topped by interesting two to four-inch flower balls of blue to purple.

Another tough, large perennial is Echinacea purpurea, purple coneflower. This is a stiff, coarse native species with dark-green toothed leaves. The brilliant three to four-inch flowers are purple, with a hint of cerise, and they bear a large, orange-brown central cone. Bloom begins in July and lasts through August.

Flank these two bold plants with the softening influence of the tall, June-blooming white baby’s breath, Gypsophila paniculata. Include a few medium tall plants with lavender blooms, such as the mid-summer blooming showy fleabane daisy, Erigeron speciosus, and fall-blooming Aster x Professor Kippenburg. Add a stonecrop, such as Sedum telephium "Autumn Joy" for greater foliage variety. Front the plants with the six to 12-inch tall Plumbago larpentae that bears intense blue flowers from July until frost when the foliage assumes an attractive reddish hue. White blooming creeping baby’s breath, Gypsophila repens, is a nice addition for early spring bloom. Penstemons suffer leaf scorch in reflected heat, especially the newer red-leafed variety, `Husker Red.’ Locating them nearer the front of the bed can avoid this.

The yellow-to-orange bloomers such as Hemerocallis or day lilies, create a warm, bright look. Add a May-to-June blooming orange oriental poppy, Papaver orientale, but beware it easily gets spider mites in the reflected heat. Simply cut the foliage off to the ground after bloom which limits the annoying spread of this too-vigorous plant. Achillea filipendulina ‘Moonshine,’ with its pale lemon yellow flowers and silvery foliage, blends well with other flowers. This fernleaf yarrow doesn’t spread like some yarrows and takes reflected heat unlike common yarrow. Common yarrow, Achillea millefollium, located nearer the front of the bed may be worth trying, particularly the variety "Paprika." The strikingly erect, white-blooming spike gayfeather, Liatrus spicata "Alba" adds nice foliage contrast. Consider fronting the bed with the strange yellow-green hues of sulfur flower, Eriogonum umbellatum, and intense orange flowers of pine leaf penstemon, Penstemon pinifolius. Separate these with low-growing, oyster-white-flowered Greek yarrow, Achillea ageratifolia.

The secrets to growing perennials in "hot shade" are first to crowd them close together so they shade the soil. The reflected heat bakes moisture out of the ground, so use an organic mulch, such as wood grindings, to conserve moisture. Water regularly and water more than you think drought-tolerant plants such as these require. This likely will be about an inch of water a week.

You can create an interesting perennial flower border with varied leaf texture and color as well as bloom if you play it right in "hot shade."

Photograph courtesy of Judy Sedbrook.

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Date last revised: 01/05/2010