Grasses come into their peak in autumn and winter, giving the garden a second wind. Changing from a quiet green backdrop for spring and summers flowers, to the main attraction in September and October, ornamental grasses belong in almost every Colorado garden. Autumn is a perfect time to plan where to add them to the garden next spring.
Lucky for us, Plant Select — a cooperative program administered by Denver Botanic Gardens and Colorado State University in concert with horticulturists and nurseries — has done much of the hard work of sorting through the hundreds of species and cultivars of ornamental grasses available. All Plant Select recommendations perform beautifully in most Denver gardens.
I especially enjoy the native selections of ornamental grasses promoted by the program. Big bluestem (Andropogon gerardii), blue grama (Bouteloua gracilis), little bluestem (Schizachyrium scoparium) and Indian grass (Sorghastrum nutans) are all native within the borders of Colorado. Ruby muhly (Muhlenbergia reverchonii) and giant sacaton (Sporobolus wrightii) are native to neighboring states and do very well in Denver. Using native grasses has several benefits — they are well adapted to the weather extremes of Denver, use less water than most non-native ornamental grasses and provide habitat for birds and insects.
The smallest native section from Plant Select is Bouteloua gracilis — “Blonde Ambition.” Blue grama also happens to be Colorado’s state grass and large expanses of it were present in the shortgrass prairie that once dominated the Front Range. Growing 18-24 inches tall and about as wide, this grass loves sun and heat. Plant it in full sun and you will rarely have to water it once established. Its main season of glory is autumn and winter when the beautiful delicate eyelash-like seed heads turn to a beautiful blond color that last well into early springs.
The Undaunted strain of ruby muhly grass (Muhlenbergia revercnoii) is about the same height as “Blonde Ambition,” but grows up to three feet wide. It loves sun and a good deep soak every week or two seems to keep it looking best. This is one of the last grasses to come into bloom and doesn’t really shine until early September.
Next up in size is Schizocaprum scoparium, “Standing Ovation,” a form of little bluestem that stays upright where other forms flop from too much water. Like the “Blonde Ambition,” this grass is perfect for sunny, dry sites. Too much shade and water will make it flop. We also recommend “Blue Heaven,” which is bluer and taller.
Two larger grasses are Andropogon gerardii “Windwalker” and Sorghastrum nutans “Thinman.” These grow four to six feet tall and make excellent backdrop plants. They do best in full sun, with little water (deep soakings once a week or less should suffice). Both have beautiful bluish foliage that is a wonderful contrast to other plants in the garden.
The largest native grass offered by Plant Select is giant sacaton grass (Spropbolus wrightii) that reaches five to eight feet tall and wide. Give this grass plenty room and sun. It can be a good substitute for the ravenna grass or hardy pampas grass (Sacharum ravennae) which has shown invasive tendencies.
Additional wonderful non-native grasses from Plant Select include the Undaunted strain of alpine spike grass (Acanantherum calamagrostis) and Korean feather reed grass (Calamagrostis brachytrycha). Both require more water and shade then the native grasses, but they still need at least six hours of sun to truly be at their best.
Lastly, do not rush cutting your grasses down in the fall. Leave them for winter interest and shelter for native insects and wildlife. You can cut your grasses back in late February or March, once a heavy snow has flattened them.
Mike Kintgen is the curator of Rock Alpine Collections with the Denver Botanic Gardens. He can be reached at horticulture@denverbotanicgardens.org.
