Introduction to warm and cool season grasses

Laurence Gale MScFeb 20, 2004in Consultancy

By Laurence Gale

This article will explain the difference between warm and cool season grasses and how they are combined to provide all year round turf performance in the United States.

Why warm and cool season grasses?

Climatic changes and seasonal periods vary so much in the United States that specific grass species will only grow and survive in certain climates. Thus, cool season grasses perform and grow well in moderately cool and cold climates normally found in the northern States but perform poorly during extreme hot and humid conditions. Likewise, warm season grasses perform very well in the hot and humid climates found in the southern States and do not perform well in cool climates.

However, there are transitional zones in the States where both hot and cool climatic conditions prevail. Turf grass managers have to combine and grow both warm and cool season grasses during their season to maintain colour and performance. Warm season grasses tend to go dormant during the cool periods of autumn and winter.

Cool season grasses

The main variety of grasses that fall into the category of cool season grasses are bluegrasses, fescues, ryegrasses and bent grasses that grow actively during the Spring and Autumn (Fall) when air and soil temperatures are cool, with optimum grass growth occurring when soil temperatures are between 50° and 65°F (10°-20°C), and air temperatures are between 60° and 75°F (15°-25°C).Cool season turf grasses are easily established by seeding with some varieties, notably Kentucky bluegrasses. Creeping bent grasses and tall fescues can be established using sods warm-1.cool-map-US-zones.jpg (turf). These grasses tend to be shade and winter tolerant.

Warm season grasses:

Bermudagrass, zoysiagrass and buffalograss are the main warm season grasses used in the southern States and are tolerant of high temperatures, humid and drought conditions. Optimum growth of these grasses occurs when soil temperatures are between 70° and 90°F (22°-30°C) and air temperatures are between 80° and 95 °F. (30°-35°C).

Warm season grasses are commonly established using vegetative propagation methods, such as sods (turf), sprigs (Rhizomes & stolons), with some able to be established from seeding.

However, these warm season grasses perform very well during the hottest weather but, similar to British grass species, slow down and stop growing during the winter period, which is usually during the playing season for most sports.

Bermuda grasses do not tolerate shade; grass growth will slow down and even die back during periods of shade.


Bermuda Grass	Zoysiagrass	Seashore paspalum

Managing grasses in the transitional zone:

Warm season turf grasses, unfortunately, are not able to maintain their green colour all season, these grasses begin to die down and become dormant during the winter period when temperatures are cooler, (middle of Autumn through to Spring) resulting in the sward looking brown and unattractive. Equally, cool season grasses are not able to perform well in hot humid climates.

Turf grass managers who work and manage facilities in the transition zones have to understand and manage both warm and cool season grasses, resulting in managing two seasonal renovation programmes. One in the Autumn when the warm season grasses become dormant and the other in the Spring when the warm season grasses begin to establish again when temperatures begin to rise.

Timing of the Autumn renovation is critical. Once temperatures begin to drop a decision must be made to shave off all the warm season grasses and over sow with cool season grasses, usually Kentucky blue grass (Poa trivalis) sown at double rates to get quick establishment. However, if a sudden rise in temperature occurs, this may stimulate a flush of Bermuda grass, which will affect the germination of the Kentucky blue grass (Poa trivalis) resulting in non-uniformity of the sward. It's a situation that all the turf grass managers have to deal with the best way they can.

Again, in the Spring when temperatures begin to rise, turf grass managers have to renovate the sward by shaving out the Kentucky blue grass (Poa trivalis) allowing the Bermuda to grow through.

Cool season Grasses
Grass species	Texture	Shade tolerant	Wear	Usage	Maintenance	Comments
Creeping bent grass Agrostis palustris	Fine	Mod	Low	Golf Bowling	High	Fine leaved, stoloniferous perennial which is adapted to cool and humid regions. Tolerates low temperatures and relatively poor, wet soils.
Bluegrass Kentucky Poa trivalis	Medium to Fine	Mod	Mod	Lawn	Low	Fine-textured shallow rooted perennial grass that spreads by rhizomes. Widely adapted, grown in all states, but seldom found in the Gulf states
Fescue Tall Festuca arundinacea	Medium	High	High	Lawns - Sports	Medium	A medium textured bunchgrass, more widespread because of greater head and drought tolerance than other species of turf grasses.
Fescue Fine Festuca rubra rubra Festuca stolonifera	Fine	High	Mod	Lawns	Medium	Fine textured perennial grass. Well adapted to cool, humid areas. Needs reasonably good drainage, grows on poorer and drier soils and those with low pH
Ryegrass Perennial Lolium perenne	Medium	Mod	High	Lawns- Sports	Low	Likes full sun, but will moderate some shade. Grows on a wide variety of soil types including heavy soils. Mixes well with fine fescues and bluegrasses.
Warm season Grasses
Grass species	Texture	Shade tolerant	Wear	Usage	Maintenance	Comments
Bahiagrass Paspalum notatum	Coarse to Medium	Low	Low	Lawns - Erosion & Wear areas	Low	A deep rooted perennial native to South America, bahiagrass is grown from east Texas to Florida, and north to the Carolinas. It is adapted to a wide range of soils growing best on sandy soils in a pH range of 5.5 to 6.5.
Bermuda Cynodon dactylon	Fine	Low	Medium	Golf & Sports	Medium	Drought resistant, grows on most reasonably well drained soil as long as it has adequate moisture and plant nutrients. Bermudagrass is more often propagated by sprigs than by seed.
Carpetgrass Axonopus affinis	Medium	Medium - High	Medium	Lawns	Low	Growth habit is prostrate, and the rooting of the surface runners forms a dense sward. Useful for lawns and for golf courses.
Centipede Eremochloa ophiuroides	Medium	Medium - High	Medium	Wet areas	Low	A low growing, creeping perennial grass which spreads by stolons. Will grow on both clay and sandy soils with adequate soil moisture and fertility.
St. Augustine Stenotaphrum secundatum	Coarse	High	Medium	Lawns	Medium	Coarse perennial found along the Gulf and southern Atlantic Coast from Texas to south Carolina. Seashore plant, will withstand salt spray
Zoysiagrass Zoysia japonica	Med - Fine	Medium - High	Medium	Lawns	High	Thrives in the shade. This grass usually crowds out other grasses and weeds due to the dense growth habit. Used on golf courses, parks and athletic fields.

The main reason why these different grasses are able to tolerate these extreme temperatures changes is due to the chemical and physiological make up of the plant, namely in the way it is able to photosynthesise.

All grass plants carry on photosynthesis by adding carbon dioxide (CO₂) to a 5-carbon sugar resulting in an enzyme creating a 6 carbon compound which breaks down into two molecules of 3-phosphoglyceric acid (PGA). These 3-carbon molecules serve as the starting material for the synthesis of glucose and other food. This process is called a C-3 Calvin cycle (C-3 Pathway). These are known as C-3 plants.

The reason warm season grasses can survive in hot climates is because they have evolved and modified the efficiency of the photosynthesis process, enabling the plant to conserve transpiration losses. It has achieved this my modifying and supplementing the C-3 Calvin cycle (C-3 pathway) to form a 4-carbon molecule instead (C-4 plants).

Warm and cool season grasses are not only found in America but are also seen in Asia and southern Europe.

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