The short answer
Cool-season grasses grow mainly in spring and fall, when temperatures are mild. Warm-season grasses grow mainly in summer heat, then turn tan or brown in winter. That single difference decides when you plant them, when you divide them and when you cut them back.
What makes a grass cool-season or warm-season
The two groups use different photosynthesis pathways. Cool-season grasses are called C3 grasses and warm-season grasses are called C4 grasses. In practical terms:
- Cool-season grasses do best at roughly 60 to 75°F. Growth slows in summer heat and picks up again when it cools.
- Warm-season grasses prefer roughly 80 to 95°F. They wake up late in spring, grow fastest in the heat, and slow down or go dormant as fall arrives.
Warm-season grasses also tend to use water efficiently, which is part of why many of them handle heat and dry spells well once established. See drought-tolerant ornamental grasses.
Cool-season ornamental grasses
Cool-season ornamental grasses often look fresh in spring and fall and may stay attractive, or even green, through much of the winter. Common examples:
- Feather reed grass (Calamagrostis × acutiflora), including ‘Karl Foerster’
- Blue fescue (Festuca glauca)
- Blue oat grass (Helictotrichon sempervirens)
- Tufted hair grass (Deschampsia cespitosa)
- Purple moor grass (Molinia caerulea)
- Autumn moor grass (Sesleria autumnalis)
- Bowles’ golden grass (Milium effusum ‘Aureum’)
Many sedges (Carex) behave like cool-season plants as well, though they are not true grasses. See grasses vs sedges for the difference.
Warm-season ornamental grasses
Warm-season ornamental grasses do their best work from midsummer through fall, when plumes appear and foliage colors up. Common examples:
- Maiden grass or Japanese silver grass (Miscanthus sinensis)
- Fountain grass (Pennisetum alopecuroides)
- Switchgrass (Panicum virgatum)
- Little bluestem (Schizachyrium scoparium)
- Big bluestem (Andropogon gerardii) and Indian grass (Sorghastrum nutans)
- Pink muhly grass (Muhlenbergia capillaris)
- Prairie dropseed (Sporobolus heterolepis)
- Blue grama (Bouteloua gracilis)
If you are choosing between two of the best-known warm-season genera, our comparison of Miscanthus vs Pennisetum may help.
A few plants blur the line. Some gardeners classify northern sea oats (Chasmanthium) or Japanese forest grass (Hakonechloa) as warm-season but note that they start growing earlier in spring than the rest of the group. Treat the lists as a guide and watch how a plant behaves in your garden.
How the two types differ in the garden
| Cool-season | Warm-season | |
|---|---|---|
| Main growth | Spring and fall | Summer |
| Spring | Greens up early | Slow to start |
| Winter | Often semi-evergreen or tidy-looking | Usually brown or tan, standing or flattened |
| Best planting time | Spring or early fall | Late spring to early summer, or spring |
| Cutting back | Very early spring, leaving some foliage | Late winter to mid or late spring, close to the ground |
| Dividing | Spring or early fall | Late spring to midsummer |
Cutting back is the task that most often goes wrong, so read whether all ornamental grasses should be cut back in spring before you pick up the shears. For the full yearly routine, see how to care for ornamental grasses.
Lawn grasses: a short list
Many searches for these terms are about lawns, which are a separate topic from ornamental grasses. For reference:
- Cool-season lawn grasses: Kentucky bluegrass, tall fescue, perennial ryegrass, fine fescues, bentgrass. These suit northern regions.
- Warm-season lawn grasses: Bermudagrass, zoysiagrass, St. Augustinegrass, centipedegrass, bahiagrass, buffalograss. These suit southern regions.
Between the two regions sits a transition zone where both types struggle at times, which is why some lawns there use a mix. Lawn advice does not always carry over to ornamental grasses.
How to tell which type you have
- Check the plant tag or nursery listing. Many labels say “cool-season” or “warm-season”.
- Look up the botanical name. The genera listed above are a good starting point.
- Watch the timing. If a clump is already green in early spring and blooming by early summer, it is likely cool-season. If it only starts to grow once the weather warms, and plumes appear in late summer or fall, it is likely warm-season.
Choosing between them
In a mixed bed you can happily use both. Cool-season grasses give early-season color and sometimes winter greenery, while warm-season grasses carry the garden from late summer into winter. If you are wondering about combining them, see can I mix cool and warm-season grasses. If you are planting during a hot spell, read how hot is too hot to plant cool-season grass. For a wider overview of what is out there, return to the guide on types of ornamental grasses.