The Pacific Northwest climate challenge
Much of the Pacific Northwest, especially west of the Cascades, has mild, wet winters and dry summers. Most of that area falls in about zones 7 to 9, so cold is seldom the main problem for grasses. Instead, the challenges are standing water and rot in winter, followed by several dry summer weeks. East of the Cascades, winters are colder and summers hotter and drier, so choose with your local hardiness zone in mind. For help with the zone ranges, our guides to zones 7 and 8 and zones 9 and 10 cover many of the same plants.
A few principles help across the region:
- Drainage matters most. Add compost when planting (one Portland nursery suggests roughly one-third compost to two-thirds garden soil) and avoid planting where water pools.
- Regular water for the first few years. After that, drought-tolerant grasses need little to no summer water.
- Match grasses to the rhythm of the climate. Some grasses grow through the cool, wet months and rest in summer; those suit the region well.
Native grasses and sedges
The Washington Native Plant Society points out that the Northwest has many attractive native grasses and sedges that nurseries rarely offer. Some highlighted by local gardeners:
- California fescue (Festuca californica): bold, gray-green, evergreen tussocks with upright flowering stems; works in dry sun or dry shade. It is native just short of Washington, reaching the Columbia River.
- Alaska onion-grass (Melica subulata): early-emerging, flowers in spring, and spreads by seed in shady spots.
- Poverty oatgrass (Danthonia spicata): small, tolerant of shade and drought, but does not like crowding.
- Pacific reed grass (Calamagrostis nutkaensis): a large, evergreen grass of about 3 feet from southern Alaska to northern California that tolerates salt spray, poor soil and a range of light conditions. Xera Plants lists it as an Oregon native.
- Tufted hairgrass (Deschampsia cespitosa): a cool-season native that suits sun to part shade.
- Sedges: sand sedge (Carex pansa), thick-headed sedge (Carex pachystachya) and fox sedge (Carex vulpinoidea) are described as tough and adaptable; some tolerate mowing or foot traffic.
Native grasses vary in availability, so you may need a native plant sale or specialist nursery. Seed can be collected only with permission, and some seed needs cold stratification.
Reliable ornamental picks
Regional nurseries commonly recommend these:
- ‘Karl Foerster’ feather reed grass (Calamagrostis × acutiflora): about 5 feet tall, with green blades and pink plumes. It tolerates moist conditions and clay soil, and is deciduous.
- Hameln fountain grass (Pennisetum alopecuroides ‘Hameln’): about 3 feet, with creamy plumes and moderate drought tolerance.
- Blue oat grass (Helictotrichon sempervirens): about 2 feet, steel blue and drought tolerant.
- Elijah Blue fescue (Festuca glauca): neat, evergreen, blue 8-inch mounds.
- Japanese forest grass (Hakonechloa macra): cascading and happy in the region’s cool shade, with moist soil. See growing Japanese forest grass.
- Tufted hair grass cultivars like ‘Northern Lights’, with cream, pink and green variegation.
- Sedges like Carex testacea (orange New Zealand sedge), Carex ‘Ice Dance’ and ‘Evergold’: evergreen and often tolerant of both sun and part shade. See the best Carex varieties for shade gardens.
- Purple moor grass (Molinia): a deciduous clump with good form.
- Miscanthus cultivars: from roughly 4 to 9 feet, can handle moisture, but see the invasive note below.
Evergreen and winter-growing grasses
Because winters are mild and wet, evergreen and winter-active grasses suit the region. Xera Plants, a Portland-area nursery, notes that western Oregon “turns green with the first rains,” and suggests favoring plants that grow in that season, such as Mediterranean Sesleria species, rather than grasses that stay golden and brown through winter. Evergreen sedges and fescues keep their color through the dark months; see evergreen ornamental grasses that stay green all year. Grasses that go fully dormant in winter, such as Miscanthus and switchgrass, give a different look, with standing straw colors, and then regrow in spring.
Grasses to avoid
The Washington Native Plant Society warns that grasses are among the world’s worst invasive plants, and some are still sold in the Northwest, including:
- Cogon grass (Imperata cylindrica) and Japanese blood grass varieties
- Common reed (Phragmites australis)
- Mexican feather grass (Stipa tenuissima / Nassella tenuissima)
- New Zealand wind grass (Anemanthele lessoniana), also called pheasant tail grass
- Maiden grass (Miscanthus species)
Nurseries still sell some of these, and Xera Plants notes that pheasant tail grass seeds around, advising gardeners to remove seed heads before they mature. Check your state’s noxious weed list and your county’s guidance, and prefer clumping grasses that do not seed freely. Portland Nursery notes that most ornamental grasses clump and are not a threat, but running types spread by rhizomes.
Planting, drainage and watering
Plant in fall or early spring in the Northwest, when rain helps establishment. Most grasses prefer sun, with at least six hours a day; shade grasses prefer morning sun or light dappled shade. Raise beds slightly or add organic matter if drainage is poor. During the first few dry summers, water regularly. After that, drought-tolerant grasses (fescues, blue oat grass, muhlies, many natives) usually need little to no water, while moisture lovers such as Japanese forest grass, sweet flag and many sedges benefit from occasional summer irrigation.
Cutting back in February
Portland Nursery advises that in the Pacific Northwest it is best to cut back dormant grasses in February to avoid rot, since leaving old foliage in place in a wet winter is generally better than cutting early. When new growth appears, trim last year’s foliage to just above the new shoots. Evergreen grasses need only a spring clean-up to remove dead leaves and old flower stalks. For timing in other climates, see cutting back ornamental grasses.