Warm season magic
Timing isn’t everything, but it matters. Especially for warm-season grasses.
Wherever you are, spring bloomers are strutting their stuff and getting most of the gardening love. They broke bud back when temperatures were much colder, and they were a welcome sign of the growing season. But nestled among these showy performances, other plants are biding their time. They’re awakening and gathering themselves for the boost they’ll get from warmer weather.
We’re talking about warm-season grasses. Many of the most familiar native grasses like little bluestem, switchgrass, and hairawn muhly are warm-season growers. They break dormancy in mid- to late spring when soil and air temperatures are warmer, and they bloom later in the season.
This is the group to keep an eye on. Why? Because late spring is a good time to start adding them to your garden.
When the Heat is On
In the horticultural world, we typically divide true grasses (in Poaceae) into cool-season and warm-season species. That’s because these groups have different growth cycles and cultural requirements. Eons ago, members of the grass family developed adaptations that set them apart and allowed them to thrive under extraordinary conditions. They worked warm-season magic.
Most land-based plant species use C3 photosynthesis, which works well in cooler, wetter climates. However, planetary changes millions of years ago made it less advantageous. Much of the Earth became hotter, drier, and sunnier, and levels of atmospheric carbon dioxide dropped. The C3 process is far less efficient under these conditions. Grasses evolved a solution: the C4 photosynthetic pathway. It operates much more efficiently under hot, dry conditions than the C3 pathway does.
The grass family was the first to evolve this new photosynthetic approach, and more than half the species that use the C4 pathway are grasses. These species are more common in warm, sunny regions where their structural and chemical adaptations impart greater growth potential and greater efficiency with sunlight, nutrients, and water. How much more efficient? C3 grasses use only 15% to 30% of available sunlight, while C4 grasses can use up to 80%. With one-half the nitrogen, C4 grasses can photosynthesize just as well as C3 grasses and can produce the same amount of biomass using one-third to one-half the water. C4 grasses can also reduce water loss in dry environments by closing their stomata (tiny pores on the leaves that allow gasses in and out of the plant) during the day. They’ve nailed the efficiency thing.
On a basic level, grasses that use C4 photosynthesis are typically referred to as warm season, and those that use C3, cool season. In actuality, it gets a little more complicated, but this translates well enough for horticultural purposes.
Connect the Warm-Season Dots
Knowing what’s happening inside warm-season grasses, let’s translate that to gardening with them.
Many of the most popular native grasses available commercially are warm-season species. They include widespread grassland stalwarts like Panicum virgatum (switchgrass), Schizachyrium scoparium (little bluestem), Andropogon gerardii (big bluestem), and Sorghastrum nutans (yellow prairie grass). Shortgrass prairie species Bouteloua gracilis (blue grama) and Bouteloua dactyloides (buffalo grass) are part of the club, as is the short and showy Eragrostis spectabilis. One of my favorite grasses, Sporobolus heterolepis, (prairie dropseed) is warm season, along with the well-known Muhlenbergia capillaris (hairawn muhly).
If you’re familiar with these grasses, you’ll see a pattern. They love full sun, grow well in the heat of summer, and tolerate or even prefer dry conditions. They also bloom later in the growing season—in summer or fall.
The C4 process is most efficient when soil and air temperatures are high (Soil: 70°F-90°F Air: 80°F-95°F) and sun is abundant. So, their period of greatest growth is midsummer when they can take full advantage of heat and sunlight. We’ve already noted that warm-season grasses break dormancy later, and now that makes more sense, doesn’t it? Their growth is slower in spring because temperatures are cooler, and their optimal photosynthetic conditions haven’t kicked in yet. They start out slowly, putting their initial energy into root growth. Then they get a burst of vegetative growth above ground once soil and air temperatures rise in summer.
If you plant warm-season grasses early in spring, they aren’t going to do much. Better to plant them when they’ve broken dormancy and are starting a flush of growth with warmer temperatures. Here in the Piedmont of North Carolina, the Panicum virgatum and Schizachyrium scoparium in my garden started showing new shoots in April, and they’re now several inches tall in the first days of May. Now is when I could add new ones to keep them company. In fact, planting them any time from late spring through the summer takes advantage of their growth potential. They may need a little help with supplemental water if there are extended dry periods before their roots establish. But generally, they grow best in the heat of summer.
On the flip side, cool-season grasses are growing most actively when soil and air temperatures are cooler (Soil: 50°F-65°F Air: 60°F-75°F). They emerge earlier in spring and put on vegetative growth quickly while it’s still cool. As temperatures rise, their growth slows, and some even go partially dormant. With C3 photosynthesis, they have higher water and nutrient needs and are more susceptible to heat stress. This pattern fits cool-season grasses you may know, like Deschampsia cespitosa, Deschampsia flexuosa, and Calamagrostis canadensis. They look lush and wonderful in spring, bloom early, and are happy in fertile soils with consistent moisture. While they can take full sun, they often do best with afternoon shade during the heat of summer. This pattern also fits Carex (sedges), grasslike plants that are in a different family than true grasses. All the Carex species use the C3 pathway and are considered cool season. They may be grasslike, but sedges are a lot of attention on their own (see our posts on the sedge trials at Mt. Cuba Center and great subs for Liriope).
No Pampering Needed
The uber efficiency of the C4 pathway has implications for where warm-season grasses do best. Not only are they well-adapted to dry conditions, they’re also able to use available water efficiently when it’s present. The same goes for nitrogen—warm-season grasses grow just fine with what they get naturally, but they’ll grow vigorously with additional nitrogen (i.e., in fertilizer). A gardener’s urge to provide “optimal” growing conditions with rich, fertile, irrigated planting beds will backfire here. Abundant water, rich soils, or heavy fertilization can produce lush growth that flops. The grasses are happy to grow when given all those resources, but it’s probably not the look you want.
Instead, remember that warm-season grasses thrive on heat, don’t need fertilization, and don’t need supplemental water once they’ve established. That helps you use fewer resources in the landscape. They’re also problem-solvers for tough spots. I like to put warm-season grasses “on the edges,” in the hell strip between the sidewalk and the road, at the edges of driveways, or in rocky, poor, urban soils.
Warm-season grasses do well in less difficult spots, too, and bring in essential design elements. Their linear foliage contrasts beautifully with broad leaves and softens the stark edges of hardscape. The tall, upright species create a focal point; placing several strategically in a mixed planting provides emphasis and visual rhythm that tie the composition together. Low-growing species serve as groundcovers and a foundation for a diverse, layered plant community. And they all provide support for wildlife, whether it’s with food, nesting materials, or shelter.
Supply and Demand
With the first signs of spring, most gardeners get excited about planting. The rude reality hits when you can’t find the plants you want. One big factor in availability is when plants can be produced by growers. Few temperate plants can be propagated and grown successfully year-round, so production is usually limited to certain times of year (i.e., production windows). The production window for warm-season plants often limits their availability in early spring.
Panicum virgatum is a textbook example. Growers usually stop production in late summer or early fall because the plants are heading into dormancy and don’t root in as well. They’ll keep those last crops through winter to have them available in spring. The overwintered crops ship out to customers in spring, but if all the plants are spoken for, they’re sold out until the next production cycle is complete. And that cycle usually begins in late spring when the plants are actively growing (and once the spring production rush for other perennials is done). That pushes the next available crops out to summer at the earliest. Basically, the availability of Panicum virgatum in spring is subject to whether enough plants are produced late in the previous year’s growing season to meet the demand the following spring. Simple, right? And that’s just one crop!
The timing is also complicated by whether nurseries are growing crops in controlled environments (greenhouses), in coldframes (unheated greenhouses), or outside in normal weather conditions. Crops grown in controlled environments may break dormancy earlier, as will those in coldframes. If the plants are shipped to an environment that’s colder, they may need to be held in similar conditions until they acclimate. (see our Wake Me Up post about receiving plugs in early spring).
At retail gardens centers, devoting valuable space in spring to just-emerging plants can be a hard sell. The plants may need extra care if they’re coming out of a controlled environment, and buyers know consumers generally go for the showy flowers and “retail-ready” plants rather than a pot of soil with a few sprigs of green. We experienced gardeners know the roots are more important, but limited shelf space makes for hard choices.
Bring on the Magic
There are a lot of moving parts with plant production, timing, and the rush of spring. Warm-season grasses don’t necessarily have shorter or fewer production windows than other perennials; they’re just not as aligned with our early spring fever to plant. Fortunately, spring progresses quickly. Summer is around the corner, and warm-season grasses are becoming available now. Start planting in late spring when temperatures are rising, and your patience will be rewarded as that fabulous photosynthetic process works its magic.
All images © Izel Native Plants unless otherwise noted.
Shannon Currey is a horticultural educator with Izel Native Plants. She’s worked in the nursery trade since 2006, and has established herself as an expert on graminoids, including native grasses and sedges. She serves on the North Carolina Plant Conservation Program Scientific Committee and the board of the Perennial Plant Association. In 2022 she joined our team to expand our outreach efforts. Shannon lives in Durham, North Carolina and loves exploring the incredible plant diversity around her.
