I remember being introduced to the concept of summer hardiness when, for a couple of years, I lived and gardened in central Texas. As a Yankee transplant, I was accustomed to thinking of winter as the testing season for plants, but in Texas, it was summer with its intense heat and drought that winnowed out the less-than-hardy plants. Our northeastern summers are not nearly as intense as those of Texas, but they have become distinctly hotter and drier over the last decade, a harsher test for plants. That’s why I’ve developed a new appreciation for warm season grasses. While most of my garden standbys struggle to survive the dog days, the warm season grasses revel in them.
The familiar turf grasses of the Northeast such as Kentucky bluegrass (which is actually a native to northern Europe) and perennial ryegrasses (another northern European native) are cool season grasses. They grow best in the cooler, moister weather of spring and fall. They typically go dormant as the temperature rises into the high 80’s. Most of our garden plants share a similar pattern of growth. Among other things, they struggle to photosynthesize — to manufacture food using the energy of the sun — when temperatures are this high. There is a subset of so-called warm season plants, however, which include many native North American grasses, that have evolved a different photosynthetic pathway, one that enables them to function and continue growth when temperatures are higher. Warm season grasses in particular also utilize water more efficiently, using just a third as much water to complete each unit of photosynthesis as cool season plants.
This difference in photosynthetic pathways is why lawns composed of cool season grasses require so much water to be kept green over the summer: Not only do they require more water for photosynthesis, but they also require cooler temperatures, which are furnished by the cooling effect of water evaporating off their leaf surfaces. This is why lawn manuals recommend applying one inch of water to a lawn, an amount roughly equivalent to 13,500 gallons per half acre, each week in summertime. In the Northeast, at least, warm season grasses typically thrive with just the natural precipitation.
Examples of warm season grasses include many prairie natives such as big bluestem (Andropogon gerardii) and little bluestem (Schizachyrium scoparium), Indian grass (Sorghastrum nutans) and switchgrass (Panicum virgatum). A trio of non-native warm season grasses that were favorite plantings a generation ago are Japanese silver grass (Miscanthus sinensis), hardy pampas grass (Saccharum ravennae) and perennial fountain grass (Pennisetum alopecuroides), but these have all proven invasive in North America and should be avoided.
The discriminating gardener will surely wish to investigate the selected types of the native warm season grasses that have begun to appear on the market. Big bluestem ‘Blackhawks,’ for example, sports burgundy foliage, while switchgrass ‘Dallas Blues’ has striking bluish leaves.
The warm season grasses tend to be statuesque; Indian grass, for example, makes clumps of leaves and stems up to 8 feet tall, although the little bluestems content themselves with a height of just 2-3 feet. Depending on the size, these grasses can be ideal for screening and defining a space in the garden, or (as in the case of the little bluestems) for providing a vertical contrast in a perennial border.
Many warm season grasses provide plume-like flower heads as well, generally in late summer or early fall. These are often attractively colored, as in the case of the golden flowers of Indian grass and the purple-red ones of switchgrass. The pink-tinted flowers of prairie dropseed (Sporobolis heterolepis), a 2-3-foot-tall clump-forming grass, are even fragrant, smelling (depending on whom you consult) like coriander or buttered popcorn. The seeds that succeed the grasses’ flowers feed songbirds and small mammals. Although most of the warm season grasses age to shades of tan or russet in the fall, some have striking autumnal foliage colors, such as little bluestem ‘Blaze,’ whose leaf-blades turn red, pink and orange.
If your garden is just surviving at this season, think about mixing it up with warm season grasses. Then late summer will become a time for special beauty.
Be-a-Better-Gardener is a community service of Berkshire Botanical Garden, located in Stockbridge, Mass. Thomas Christopher is a volunteer at Berkshire Botanical Garden and is the author or co-author of more than a dozen books.