Gardening Blog

Living Mulches: Two Great Groundcovers for Shade

One of my favourite groundcovers for shade is sweet woodruff (Galium odoratum, Zone 3) which spreads slowly but surely via short underground rhizomes. It bears fragrant cymes of star-shaped white flowers for several weeks in early summer, and while its spread may be indefinite, it rarely grows taller than 10 centimetres. Even when not in flower, sweet woodruff remains attractive with its circular whorls of leaves that hug the ground and provide the perfect backdrop for larger plants.

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Seen here in my own garden at the base of a ‘Diane’ witchhazel (Hamamelis ×intermedia ‘Diane,’ Zone 4), the growth habit of sweet woodruff is open enough to allow a self-seeded Japanese painted fern (Athyrium niponicum, Zone 3) to insinuate itself into the shade-loving mix. Bloodroot (Sanguinaria canadensis, Zone 2), golden Japanese forest grass (Hakonechloa macra ‘Aureola’, Zone 5) and ‘Robustissima’ Japanese anemone (Zone 4) fill out the background. Sweet woodruff will tolerate dry shade once established, but grows faster in moist, well-drained soil that’s high in organic matter such as shredded leaves, leaf mould, compost and/or composted manure.

Formerly known as Asperula odorata, sweet woodruff is native to European forests and records show that it was a common plant in English gardens before 1450. The dried leaves give off a sweet scent reminiscent of new-mown hay, and it was widely used as a “strewing herb” to freshen bedrooms. Galium odoratum was also mixed with eiderdown to stuff mattresses, quilts and pillows—so much so that one of its other common names is “bedstraw.”

Elizabethan herbalist John Gerard (1545-1612) advises us that sweet woodruff “being made up into garlands or bundles, and hanged up in houses in the heate of sommer, doth very well attemper the aire, coole and make fresh the place, to the delight and comfort of such as are therein.” A sound suggestion for hot Canadian summers, too!

The chemical constituent that gives sweet woodruff its delightful fragrance is called “coumarin,” and since its discovery it 1820, it has become a staple of the perfume industry. Other plants that contain coumarin include Eurasian sweet vernal grass (Anthoxanthum odoratum), the South American tonka bean tree (Dipteryx odorata), sweet clover (Melilotus spp.) and indigenous sweet grass (Hierochloe odorata), native to every Canadian province and territory.

Unlike sweet woodruff, ‘Oregon Trail’ tiarella (Tiarella cordifolia, Zone 4) is a brand-spanking new cultivar, and it’s such a good performer that I’ve no doubt that it’s well on its way to becoming a garden mainstay.

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It’s seen here in a shady section of my perennial border, next to the emerging foliage of Astilboides tabularis (Zone 4); each leaf of which will eventually grow about 75 centimetres wide—and with ‘Oregon Trail’ carpeting the ground below—both species will be shown off to maximum advantage.

‘Oregon Trail’ produces 20-centimetre-tall racemes of white flowers in early summer, but most gardeners consider the blooms a bonus: like hostas, tiarellas are grown primarily for their striking, colourful foliage.

‘Oregon Trail’ was introduced by Terra Nova Nurseries (Oregon) as part of their American Trails Series: I also have ‘Happy Trails’ (green leaves with black venation and white flowers) and ‘Sunset Ridge’ (crinkled glossy green leaves with intricate contrasting venation and white flowers) from the same series, and both are thriving in other shady sections of the garden.

While all of the cultivars in the American Trails Series can be employed as groundcovers par excellence, they were also intended to be used as “spillers” (or trailing plants) in containers, largely to replace English ivy (Hedera helix cvs.) which can escape and become invasive in milder regions of Canada and the U.S. (Zone 7 and warmer). Given the choice—and where all other considerations are equal—I always prefer to grow native species rather than introduced ones.

And speaking of native plants: What do you get when you cross a Tiarella with a Heuchera? Why a ×Heucherella of course! And here’s one of my favourites, ‘Sweet Tea,’ the result of a cross between two indigenous North American genera (plant crosses are usually between species, not genera).

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In spring, ×Heucherella ‘Sweet Tea’ (Zone 4) produces rosy orange leaves with maroon veins which gradually darken as summer progresses; then in autumn, their palmate foliage brightens up again. Introduced by Terra Nova in 2010, it’s seen here surrounded by various ferns which enjoy the same growing conditions: part-shade in moist, well-drained soil with plenty of organic matter. On the heuchera side, ‘Sweet Tea’ is principally derived from Heuchera villosa, hence it’s amazing tolerance to high heat and humidity—a perfect perennial for steamy Canadian summers!

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