Understanding Garden Grasses 

By Bryan Fischer, Curator of Plant Collections and Horticulturist

Thoughtfully placed grasses—whether in clumps, bands or just sprinkled throughout a planting—are one of the surest ways to elevate a design without increasing maintenance. In fact, perennial grasses are one of the most reliable and easiest to care for groups of perennials. They are also one of the most overlooked and among the most poorly understood by gardeners. To use grasses effectively, the gardener must understand a few basic aspects of their biology and lifecycle. 

Ornamental grasses have entered horticulture from around the world. Such a broad origin has resulted in an almost overwhelming diversity of grasses, all of which are adapted to slightly different conditions. Of this broad array, however, grasses typically fall into one of two categories of value to the gardener: cool-season growing grasses (sometimes called C3 grasses) and warm-season growing grasses (sometimes called C4 grasses).  

Cool-season grasses are those adapted to grow actively at temperatures above 50 F but below 75 F or so. This category includes garden fescues, tufted hairgrass (Deschampsia cespitosa), most grasses in the genus Poa (including natives and Kentucky bluegrass lawns), and others. These grasses green up in spring and are flowering in the garden by late June or July. As a result, such grasses make great complements for blousy perennial beds that suffer from “green blob syndrome,” resulting from too many leafy plants or “chaotic gardens” due to so many floral elements at play. In both cases, adding bands or clusters of grasses can break up the business by providing a structured visual break without creating a formal aire. It’s worth knowing, too, that cool-season grasses enter a state of semi-dormancy through summer’s heat and then grow actively again in fall as temperatures drop. 

Warm-season grasses (C4), on the other hand, are adapted to grow most actively in temperatures from the mid-70s and up. Included here are most of our “classic” prairie natives, like blue grama (Bouteloua gracilis), little bluestem (Schizachyrium scoparium) and golden prairie grass (Sorghastrum nutans), among others. These grasses are sleepers, forming green tufts in spring and then rocketing out of the ground as things heat up. They also require a bit more even soil moisture during July—ideally a monsoon or some form of summer irrigation in our region—to flower well. Many of these grasses, too, struggle in mountain gardens that don’t sustain the heat needed for them to grow with vigor, though they excel in gardens on the Front Range and High Plains. Expect the show from these grasses to be particularly emphatic and to peak early Aug. through late Sept. or even the first week of Oct., depending on species and summer/fall heat. So, such grasses can be great late-season interest as perennials start to look worn-out, but they can’t be counted on for a mid-season display. 

Incorporating both warm and cool-season grasses into your gardens can reduce weed pressure since the garden will then have plants active April through Oct., rather than leaving a window in the season with exposed dirt (free real estate if you’re a weed). Growing both also provides excellent shelter and forage for wildlife if left standing. When it comes to cutbacks, remember that since cool-season grasses can be actively growing as early as April, pruning must be done sooner than with warm-season grasses to avoid cutting growing foliage along with last year’s dried material. Warm-season grasses can wait a bit longer, should you prefer it, or if you are pinched for time, as many of us are in spring.