Alternative evergreen groundcovers exist for the Pacific Northwest

Weed management can be accomplished many ways, but one of the most effective is using evergreen groundcovers. A groundcover shades the soil surface in the same way as a mulch, but offers flowers and foliar interest through the year.

Using plants for this purpose has the additional advantage that they -- at least in theory -- don’t require regular replenishment. Groundcovers are especially useful in parking lots or other industrial or commercial situations where low maintenance is a requirement.

Many plants have been used as groundcovers in Northwest landscapes, with varying degrees of success. A casual look at new landscapes shows several common species. These include various Cotoneaster species and cultivars, Arctostaphylos uva-ursi (kinnikinnick or bearberry), Vinca minor and V. major (periwinkle) and Genista lydia and G. pilosa (carpet broom). Taller options include Viburnum davidii.

No perfect plant

There is no perfect plant, and all of these have their deficiencies. Cotoneasters in general have caused suspicions lately because of potential weediness, a problem they share with periwinkle. Kinnikinnick suffers from galls caused by aphids and fungi, as well as leaf spots. Viburnum davidii is often used in full sun where it often struggles with heat stress. The carpet brooms have been effective although they, too, occasionally struggle with disease problems as well as the negative perception that comes with being labeled “broom.”

The range of plants that could be used as groundcovers, large or small, is considerably larger than those I’ve listed, and includes a number of species that have seen little or no use in the Pacific Northwest.

Here are a few of those that have been used elsewhere with success, and some that have been used even in the Pacific Northwest on a limited scale and are deserving of consideration.

Many manzanitas

One genus that potentially has much to offer is Arctostaphylos. This is a wide-ranging genus in western North America, particularly in California but occurring widely up through Oregon and Washington. The common name, manzanita, is the diminutive of the Spanish word for apple, “manzana,” which refers to the resemblance of the reddish fruit to little apples.

All are evergreen shrubs that may be found in a variety of habitats, from seashore to the mountains. Plant size varies as well. Some achieve treelike size, but there are a multitude of forms suitable as groundcovers. The foliage is generally leathery and tough, and flower color is white or shades or pink. The ranges of some species overlap, and where this occur hybrids may be found.

A. uva-ursi is well-known, particularly ‘ Massachusetts,’ a cultivar selected by Robert Ticknor at Oregon State University from seed collected in the Northeast. Also grown locally is ‘Vancouver Jade,’ a cultivar selected in Victoria, British Columbia, and released by the University of British Columbia Botanic Garden. In the Pacific Northwest the species ranges widely along the coast from north to south and many selections from California populations have been named, including ‘Pt. Reyes,’ ‘Radiant,’ ‘San Bruno Mountain’ and ‘Pt. St. George.’ In California these seem best-adapted in cool coastal areas and struggle in interior locations. Whether these cultivars from warmer locations in California might tolerate interior locations in the Pacific Northwest is open to question.

Like other manzanitas, A. uva-ursi hybridizes. One available in the Pacific Northwest is ‘Emerald Carpet.’ This is a hybrid with A. mendocinensis from the Mendocino coast that was originally selected at Rancho Santa Ana Botanic Garden. It forms a mound of shiny, green leaves with white flowers. A. uva-ursi also hybridizes with A. columbiana, a shrubby species to 6 feet found both in the Coast range and on the west side of the Cascades, to form A. x media. The result is a mounding shrub that ranges in height from 1-3 feet tall with dull, green leaves. Selections of this hybrid may prove effective in warmer regions, especially where a taller groundcover is required.

A. uva-ursi also hybridizes with A. nevadensis, pinemat manzanita. This species ranges through the Cascades and Siskiyou Mountains, and though reputedly not of easy culture itself, does form dense mats in montane areas. Perhaps individuals selected at lower elevations from hybrid populations would prove better adapted to garden culture and yet more tolerant of heat and drought than A. uva-ursi.

One species of manzanita that has supplied several garden-worthy groundcovers is A. edmundsii, Little Sur manzanita. The two most common selections are ‘Bert Johnson’ and ‘ Carmel Sur.’ These dense, mounding plants reach 2-3 feet tall and 4-6 feet wide.

Monterey manzanita (A. hookeri) is another species from which an array of groundcover selections has been made. The species is native to a restricted area on Monterey Bay and is quite variable in growth habit. ‘Monterey Carpet’ is perhaps the most common groundcover selection in California, forming a mounding, wide-spreading carpet. A taller, but still dense selection, to almost 3 feet, is ‘Ken Taylor,’ which has proven to grow well in the Pacific Northwest.

Finally, a hybrid manzanita that is worth a try is ‘Pacific Mist.’ This Rancho Santa Ana introduction grows 3 feet tall and spreads upwards of 6 feet. The foliage is attractive gray-green and is borne on unusual upturned branches, which gives the plant an unusual look. In California it has performed well in a wide range of habitats.

Ceanothus gives blue flowers

Another group of plants that has seen sparing use in the Pacific Northwest is Ceanothus. The most commonly used groundcover is C. gloriosus, Pt. Reyes ceanothus. It forms a 2-foot-high and 6- to 8-foot-wide mat of stiff branches and leathery leaves. Pale-blue flowers appear in May. Like other Ceanothus, this plant is best used where space is available for it, but also in well-drained soils, preferably where summer irrigation is minimized or absent.

Several other C. gloriosus selections are worth consideration. C. gloriosus ‘ Anchor Bay’ was introduced by the Saratoga Horticultural Foundation. It was originally grown from seed collected in 1970 on the southern Mendocino coast near its namesake. The individual branches have a somewhat arching habit, and combined with the thick, glossy leaves, form a dense mat of foliage. Medium-blue flowers are produced, like the species, in May.

If there is space for it, a taller plant is C. gloriosus var. exaltatus ‘Emily Brown.’ This selection is the inland form of C. gloriosus, known as Navarro ceanothus. It’s taller, to 3 feet with an 8-foot spread, which makes for a more effective weed-smothering mat. Flowers are darker blue than Pt. Reyes ceanothus.

A much more diminutive selection of C. gloriosus is ‘Heart’s Desire,’ introduced by Phil Van Soelen of California Flora Nursery in Fulton. This grows to less than 1 foot tall and perhaps 4 feet wide, with relatively small, glossy leaves. Unlike other C. gloriosus, it would be suitable for small-scale groundcover or perhaps trailing out of large containers.

Rock roses are good for dry sites

Similar to Ceanothus, Cistus, or the rockroses, make effective groundcovers in dry sites, at least where there is space. The most common are C. x hybridus or C. x purpureus, both of which have a mounding habit. Both of these grow very strongly and often exceed the space allotted them in the landscape, which results in excessive and debilitating pruning. Two selections, which are just as effective but far smaller, are C. x obtusifolius or C. ‘Grayswood Pink.’

C. x obtusifolius could be thought of as simply a smaller version of C. x hybridus. It forms a near-perfect dome of gray-green foliage to 3 feet tall and wide, perhaps half the size of C. x hybridus. It bears single, white flowers in profusion in May. ‘Grayswood Pink’ grows 18 inches tall and 3 feet across with gray foliage and pink flowers in May. Both of these grow and flower well but are far smaller than commonly used rock roses if space is an issue.

Look at lower hebes

Hebes are typically thought of as specimen plants, but some are low and wide-spreading that can be used as groundcovers. The cultivar most often used this way is ‘Youngii,’ or as it is usually (incorrectly) sold, ‘Carl Teschner.’ There are three quite similar cultivars that do not exceed 18 inches tall, which makes them decent small-scale groundcovers: ‘Inverey,’ ‘Wingletye’ and ‘Bracken Hill.’ They are mentioned together because they are so similar.

All have small, blue leaves, a dense, matting habit and blue flowers in June. If a groundcover in a restricted space is required, and some irrigation is provided, these would work.

Shady situations

There is always a need for groundcovers that work well in shade, and maybe even dry shade. Choices for these landscape situations tend to be limited. Not often considered for groundcovers are vines, especially evergreen vines, but one that works well is star jasmine (Trachelospermum jasminoides). This plant suffers from a reputation for tenderness, despite a hardiness rating of USDA Hardiness Zone 7b, making it plenty hardy for the maritime Pacific Northwest.

There are robust examples found throughout the region, grown both as a vine and as a groundcover. What makes the plant so compelling as a groundcover is its large, glossy leaves and dense habit. Better still are the sweetly fragrant flowers, which are borne over a three-month period in summer and whose scent can be detected far from the plant. Even plants in partial to full shade flower heavily. It’s this characteristic that makes it such a desirable plant. Next to Sarcococca, the list of fragrant evergreen shrubs for shade is short indeed.

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- Neil Bell

March 2008