Shade Gardening in Calgary

Many mature gardens in Calgary began with only a few shady areas but with time have become a haven for shade plants. Shady areas provide longer bloom time for flowers, more protection from hail and direct sun, and nearby trees often offer a natural leaf mulch. Unfortunately slugs love shade too! Two new books explore the potential of gardening in the shade: Larry Hodgson’s “Making the Most of Shade”, and “Shade Perennials” by W. George Schmid. Jane Taylor’s “The Shady Garden” makes good reading for design ideas. All are valuable books, but how do their recommendations apply in Calgary and do we have local native plants to use as alternatives? Are their zonal designations appropriate and how about their growing requirements or the authors’ definitions of shade? For example, the sun’s angle, and therefore intensity, is less here, but in the summer we get more hours of daylight than further south, and, typically eight hours of cloud-free skies. Our highest elevations, greater than1280m (4200 feet), have high sun intensity and increased probability of Chinooks, both of which affect plant growth.

Light:

Full or heavy shade can exist north of or adjacent to high walls, or under low-limbed, dense deciduous trees that allow a couple of hours of direct sun each day during the growing season. Few species grow well in full shade, and the drier and darker, the more difficult it is to garden. Partial or semi-shade has two to four hours of direct sun; open birch for example provides this dappled shade. Light shade exists where full sun reaches the ground for about four to about six hours per day. Precise local definitions for shade are easy to make but hard to apply. For your own garden, determine when the sun shines, both in terms of time of day, and the time of the year, and how complete the shade is. In Calgary there are hundreds of perennial varieties that will grow well in partial and light shade conditions. We need to determine the right combination for specific areas of our gardens: checking other gardens to see what does well in similar shade conditions at various times during the year can be very helpful. Morning sun is less harsh than late sun for the same number of hours.

Soil and water:

In areas shaded by trees, one difficulty is competition from tree roots, both for moisture and nutrition; shade trees also act as umbrellas. Poplars and spruces for instance have very shallow roots, but oaks and pines have tap roots and compete less with shallow-rooted plants. To amend the soil, you can safely add at least 5 to 10 cm of loose soil/compost mix without “drowning” the tree roots. Many shade plants evolved in deciduous forests where organic mulch is provided naturally. To mimic these conditions, add organic matter, neutralize and lighten the top 20 cm of our clay-rich alkaline soil. It’s good to use mycorrhizal fungi when planting, and afterwards add slow-release 18-6-12 fertilizer. Every two or three years, in the spring just before a shower, add a high nitrogen fertilizer (eg urea 46-0-0) for leaf growth. Shade plants typically grow slowly, so it’s worthwhile buying large plants; the larger the surface area of the leaves, the more energy can be collected from the sun.

Design, framework:

Focus on foliage colour, leaf and plant form, and leaf texture, and for impact highlight with occasional bright flowers: try a monochromatic colour scheme, lily of the valley and white wood anemones for instance, and annuals such as begonias in pots, then add the potting soil in the fall. Contrast solid green or variegated leaved hostas (under-planted with early, small bulbs such as scillas) with newly developed heucheras, semi-evergreen ferns, shade-tolerant grasses or sedges, and elegant structural features like ligularias. Try Clematis alpine ‘Willy’, and hostas with strongly contrasting heights, leaf form and colour. Provide a contrasting background consisting of attractive and effective mulch. Often plants benefit from early spring sun; these include bergenias, hepaticas, early bulbs, and various clematis, even though they may be in deep shade later in the year: some early bloomers like shooting stars become dormant later in the year … make sure you know where they are! Some evergreens such as Pfitzer Juniper can be are shade tolerant: ferns happily grow through them and provide a good contrast. Periwinkle is not only shade tolerant, but evergreen: highly valued attributes!

Winter:

We have many months of winter, so it’s important to use evergreens, including ground-hugging vines, and to leave the spires of attractive tall perennials for fall and winter interest. The biennial honesty can be used well for this purpose, as can the dogwoods. Berry-bearing plants too can add fall or winter interest.

For the more adventurous soul, how about pushing the limits with Japanese Toad Lily, Tricyrtis hirta, Yellow Evening Primrose, Oenothera biennis, Great Blue Lobelia, Lobelia siphilitica, or the pink double form of Liverleaf, Hepatica nobilis, contrasting with the great white trillium, Trillium grandiflorum?

Other local reference: Ken Girard’s November 2005 talk to the Calgary Horticultural Society.

Glynn Wright               

May 2, 2006

Prepared originally for the Calgary Horticultural Society