What Temperature is Your Ornamental Grass?

By Kerry Ann Mendez

Ornamental grasses are divided into two temperature categories: cool season and warm season. Do you know the differences between these grasses? Knowing this information will help with their care, placement and creating more beautiful landscapes.

Cool vs. Warm

Cool season grasses jump start fresh growth in early spring, despite cooler air and soil temperatures. Many are semi-evergreen and never truly go dormant. Cool season grasses mature quickly and typically begin "flowering" in June, unlike warm season grasses that lay dormant (evidenced by ugly brown, straw-like blades) until late spring and then begin flowering mid to late summer. Cool season grasses don't grow as tall as their warmer cousins, some of which can reach 15 feet. Feather Reed Grass (Calamagrostis) is the tallest cool grass, reaching between 4 to 5 feet in height. The majority of cool season grasses are much shorter and more similar to Blue Fescue 'Elijah Blue' (8-10").

When To Divide

Cool season grasses should be divided in spring while in active growth. Warm season grasses should never be divided when in flower, especially in fall. This is when they begin dormancy and are less able to handle transplant shock and new root development. Wait until they break dormancy in late spring and then do the deed. In general, warm season grasses don't need to be divided as often as cool season varieties.


Because cool season grasses are shorter in stature, they are best placed towards the front of a garden. These grasses make wonderful companion plants for spring blooming bulbs. When bulbs start to go dormant, the lush grass will screen ratty looking foliage. Warm season grasses look best viewed from a distance and are ideally sited where they are backlit by the sun.


Cut back warm season grasses in late winter or early spring, pruning them to within inches of the ground. Cool season grasses rarely need pruning � simply remove brown blades by running your fingers (or a small rake) through the clump. Feather Reed Grass is an exception; whack it to the ground in late winter.

Suggested Varieties

Some cool season grasses include Blue Fescue (Festuca) and Feather Reed Grass (Calamagrostis). Warm season beauties include Switch Grass (Panicum), Fountain Grass (Pennisetum), Maiden Grass (Miscanthus) and Hakone Grass (Hakonechloa).