Warm Season vs. Cool Season Grasses

Understanding the distinction can improve your designs

Confessional moment: I am a grass freak.  Of the vast universe of plants I adore, ornamental grasses are among my favorite plants to design with.  No other plant expresses the ephemeral and sensory beauty of a garden like grasses.  They catch light like a stain glass window, rustle with the slightest breeze, and glisten with the morning dew.  Grasses are a wonderful and sustainable addition to any border, yard, or planting.  But there is one pitfall to designing with grasses that almost no one mentions: understanding the difference between warm season grasses and cool season grasses.
Before a garden book seduces with you photos of a grasses glowing in the sun, you really should understand how to use warm season and cool season grasses in a designed setting.  I’ve learned the hard way.  Some of my biggest planting fiascos resulted when I failed to pay attention to this distinction.  Here’s what you need to know. 
Gardeners frequently call anything that looks grassy a “grass.”  True grasses are members of the Poacaea family.  Other grass-like plants include the popular  Carex genus (Cyperaceae), rushes (Juncaceae), and cattails (Typhaceae). But none of these are true grasses.  Horticulturists divide true grasses into two general categories that describe their growth cycle through a year: cool season and warm season grasses. 
Cool season grass Nasella tenuissima "browns out" in the heat and creates a lovely effect.
Cool season grasses start their growth early in spring and continue that growth while cool temperatures and rain prevails.  When summer gets hot, these grasses typically go dormant, often “browning out.”  Some cool season grasses even die back in the summer, leaving seeds to germinate during the next cool season.  If you’ve ever seen your lawn covered in Annual Bluegrass (Poa annua) in May only to see it disappear by June, it’s because this is an evolutionary strategy devised by this cool season grass. Cool season grasses are best planted/seeded in early spring or late summer/early fall.  They tend to germinate and establish quickly.  Cool season grasses foliage color looks best during late spring and early summer.
Warm season grasses, on the other hand, focus their energy on establishing deep roots during their first and second year of life, and then growing during their second and third year.  Warm season grasses are slow to establish, often frustrating gardeners during their first year.  This is especially true when seeding a meadow—weeds and other aggressive plants can take hold while WSG establish.  These grasses are best planted in late spring.  Fall planting is not recommended unless you plant them as plugs, quarts, or gallons before mid-September. If you seed warm season grasses in the fall, they will stay dormant and emerge in the spring.  Warm season grasses also tend to stay low during the cooler parts of summer, and wait till the heat of summer to put on their full height.

My mistake: Little Bluestem hugs the ground like a little clump until
August when it shoots up.  This left a hole in the planting bed most of the summer.
Here’s where you need to be careful.  Twice I planted a large grass massing next to a client’s house without paying attention to whether it was warm or cool season.  The result was jarring.  Both times, I planted warm season grasses (Little Bluestem and Pink Muhly Grass) that emerged as a little fuzzy clump only 4-6 inches off the ground and stayed that way until August.  All of the perennial and shrub planting around those grasses was lush and tall, while the warm season grasses left a hole in the border.  By August, the grass finally grew to height and the composition was beautiful.  But the client had to look at a gap in the border most of the summer.
My recommendation is to balance both warm and cool season grasses in a design.  Cool season grasses give you immediate volume and impact.  With cool season grasses, you have to be careful of them “browning out”.  This can be a good thing as some species like Tufted Hair Grass (Deschampsia caespitosa) look stunning dried in July. But other cool season grasses like Quaking Grass (Briza media) disappear almost to the ground by mid-summer.  Warm season grasses are wonderful in the landscape, but be sure to locate them strategically so their low height during the first half of summer is visually appealing. 
So which grasses are which?  Here is a list of some of the most popular ornamental grasses categorized for you:
Cool Season Grasses:
Briza media, Bromus spp., Calamagrostis x acutiflora ‘Karl Foerster’, Calamagrostis canadensis Chasmanthium latifolium, Deschampsia caespitosa, Deschampsia flexuosa, Elymus spp., All Fescues, Helictotron sempervirens, Koeleria spp, Melica altissima and ciliata, Molinia, Nasella tenuissima, Sesleria spp., Stipa spp.
Warm Season Grasses:
Andropogon spp, Bouteloua spp, Buchloe, Calamagrostis brachytricha, Eragrostis spp, Hakonechloa spp, Miscanthus spp,  Muhlenbergia spp, Panicum spp, Pennisetum spp. Sporobolus heterolepsis, Schizachyrium spp, Sorghastrum nutans

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