THE TOPIC OF GRASS is vast — literally, 20% of our planet’s landmass is covered in grassland. Clearly, grasses are adaptable — maybe a bit too adaptable at times.
Shimmering with light and moving with the slightest breeze, ornamental grasses animate the landscape. Just remember to pay attention to a plant’s growth habit to find a good fit for your garden.
Grass growth is temperature-sensitive. Cool-season grasses flush new growth in spring and often remain semi-evergreen in winter. Blue fescue (Festuca glauca), purple moor grass (Molinia caerulea), feather reed grass (Calamagrostis acutiflora) and giant needle grass (Stipa gigantea) are all cool-season grasses that shine in Pacific Northwest gardens.
Warm-season grasses, like switch grass (Panicum virgatum), maiden grass (Miscanthus sinensis) and fountain grass (Pennisetum spp.), don’t initiate growth until temperatures climb and the soil warms up. Warm-season grasses typically go dormant and are cut back to the crown in late winter. Plant a mix of cool and warm season grasses to extend garden interest.
Grasses get around in various ways.
Running grasses spread by a rhizomatous root system (see world domination above). This growth habit is great if you’re using Elymus mollis to stabilize a sand dune, but less welcome when variegated ribbon grass (Phalaris arundinacea ‘Picta’) invades your entire planting bed. Mounding grasses, like purple moor grass and blue oat grass (Helictotrichon sempervirens), form clumps, gradually increasing in diameter over time, and play well with other perennials and ornamental plantings.
That is, unless they wantonly seed about.
Most warm-season grasses that bloom at the end of the growing season don’t have enough time to ripen seed in our relatively cool climate, basically neutering plants like pampas grass (Cortaderia selloana) and some fountain grasses that seed invasively in warmer regions. Where we get into invasive seeding problems is with cool-season grasses that flower early and set seed during the growing season. Yes, you, Mexican feather grass (Nassella tenuissima) and pheasant tail grass (Anemanthele lessoniana) — a couple of my favorites, but I try to be a good garden citizen and monitor seedlings closely.
Native to open spaces, most grasses require sunny conditions, although golden Japanese forest grass (Hakonechloa macra) is a lovely exception, with a politely spreading habit that brightens shady borders with cascading mounds of foliage to about 1 foot tall.
Flower forms vary from glittering oatlike awns to softly arching seedheads and bottlebrush plumes. Blue grama grass (Bouteloua gracilis ‘Blonde Ambition’) is a warm-season grass that quickly fills in gaps left by spring bulbs, with curious eyelashlike seedheads that persist for months.
Most ornamental grasses thrive with an annual top dressing of compost and little to no additional irrigation once the plants are established. Maintenance is minimal — usually a late-winter tidy is all that’s needed for a well-groomed plant. However, don’t underestimate the labor involved to cut back giant maiden grasses. On the other hand, Molinia caerulea subsp. arundinacea ‘Skyracer’, another favorite that elegantly shoots flowering stems to 8 feet tall and turns pure gold in fall, is a cinch to tidy in late winter, when the foliage neatly separates from the crown without cutting.