October 28, 2021 4 min read

Ornamental grasses are some of the true stars of the fall garden — and the garden in general! There's perhaps no better way to bring structure and movement to your landscape and container plantings than adding a grass or two, and that is especially true this time of year, when many grasses boast beautiful plumes of flower heads. Our resident horticulturalist Josie is here to demystify some of the different terminology that often gets thrown around when referring to these lush mounds of narrow-bladed foliage, as well as walk us through a few of her favorites in the video below.

"Coming to Terms" with Grasses

It's easy to get lost in the weeds (literally!) with the botanists who geek out over the nuances of plant taxonomy, but for the average gardener, the biggest question isn't so much "What makes a true grass?" as "Where and how can I grow this grassy thing?" For those of us who value specificity of language, though, Josie has an easy mnemonic device to help us remember the difference between grasses and a couple of similar-looking plants commonly encountered at garden centers:

"Sedges have edges, rushes are round, and grasses have knees that bend to the ground."

True grasses with their "knees" are plants within the Poaceae family, whose members range from the turf grass in your lawn to the tallest bamboo you can imagine. But when gardeners talk about ornamental grasses in the general sense, we often exclude plenty of true grasses like bamboos and include plants from other plant families that have a certain "look" we expect from grasses in the garden: vertical-to-arching, strap-like foliage that rises between six inches and eight feet off the ground. This includes the sedges and rushes, as well as many members of plant lineages as diverse as the iris and asparagus families (the Iridaceae and Asparagaceae, respectively).

Deciduous Grasses

An important consideration when gardening with grasses is how you can create different seasons of emphasis. Most true grasses are deciduous, meaning they enter a period of dormancy — typically during the winter months. These are generally thought of as three season plants, but even among winter deciduous grasses, there is often some ornamental value to their dried leaves and flower heads in the winter. Still, for the tidiest appearance, you will likely need to shear them back before they re-sprout in the spring, so it's important to plan for these negative spaces accordingly. Tall deciduous grasses make a great seasonal backdrop for shorter, evergreen perennials and shrubs, and medium-sized specimens can be weaved through a bed of early-emerging plants like bulbs that will take up the mantle of ornamental value in the early spring.

Cool Season vs. Warm Season Grasses

As Josie explains, when designing with true grasses especially, it is important to know what season your grass grows best in. Cool season grasses are earlier to sprout in the spring (around March in the Pacific Northwest) and will flourish in the cooler temperatures of spring and fall (between 60 and 75°F). These include varieties of Blue Oat Grass (Helictotrichon sempervirens), Tufted Hair Grass (Deschampsia sp.), and Autumn Moor Grass (Sesleria autumnalis). In the heat of the summer, their growth will slow down, and they may even "brown out" during periods of drought. But when rains resume and temperatures moderate, they are good about bouncing back.

In contrast, warm season grasses like Northern Sea Oats (Chasmanthium latifolium), Fountain Grass (Pennisetum sp.), and Maiden Grass (Miscanthus sp.) thrive in the same summertime temperatures (between 80 and 95°F) that slow down their cool season counterparts. Because they like it warm, they are slower to emerge in the spring, but once their temperature threshhold is met, they fill in quickly.

Generally, it is a good idea to make use of both cool and warm season grasses in your garden so that you have something at or near its peak at any given time.

Adding Evergreen Interest

If you want a taller focal point year-round, it's hard to beat the rainbow of New Zealand Flax (Phormium sp.) that we are fortunate enough to be able to grow here in the Pacific Northwest. Some can even reach a statuesque eight feet! For the front of beds and borders where you might prefer a shorter, year-round emphasis of grassy texture to draw your eye, there are several other stunning evergreen options among the grass lookalikes. Many of these also happen to accept more shade than true grasses, which is important to note if you are gardening in the shadow of a building or under trees. Many true sedges in the genus Carex (a blog post for another day!) fall into this category, as do varieties of Dwarf Lilyturf like the dramatically dark Black Mondo (Ophiopogon planiscapus). For a bright, bold accent, you might also try Golden Sweet Flag (Acorus gramineus 'Ogon') or EverColor® Sedge (Carex 'Everillo'). With their abundance of colorful forms and year-round presence, you can see why evergreen grasses also make for great focal points in containers

There is such a diversity of grasses in the world, it would be a shame not to garden with at least one. No matter your gardening situation, Cornell Farm has a fabulous selection of grasses to enliven your landscape this fall — more than Josie could ever possibly cover in a short video — and we have many mature specimens growing in our display beds for your inspiration. Be sure to check them out on your next visit to the Farm!