The immense variety of plants commonly referred to as grass belongs to the family Poaceae, a group of monocotyledonous flowering plants. Defined by their simple leaf structure, hollow stems, and fibrous roots, grasses are found on every continent except Antarctica, dominating ecosystems like savannas and prairies that cover over 40% of the world’s land area. This family includes nearly 12,000 species that are foundational to human and animal life, providing the bulk of the world’s food supply and playing a major role in landscape management and soil health. Grasses are typically classified by their preferred climate for growth, their structural method of propagation, or their primary function.
Understanding Climate-Based Classifications
The most common way grasses are classified is based on the temperature range in which they actively grow, dividing them into cool-season and warm-season types. This distinction is rooted in their biological mechanism for photosynthesis (C3 for cool-season and C4 for warm-season), which determines how efficiently they fix carbon dioxide at different temperatures.
Cool-season grasses prefer temperatures between 60°F and 75°F, experiencing their most vigorous growth during the moderate weather of spring and fall. These grasses are typically found in the northern United States and regions with moderate summers, and they are more tolerant of frost. Common examples include Kentucky Bluegrass, various Fescues, and Ryegrass. These varieties will often go dormant and turn brown during the peak heat of summer or the deep cold of winter.
Warm-season grasses grow best when temperatures are consistently between 80°F and 95°F, with their peak growth occurring throughout the summer months. These varieties are prevalent in the southern U.S. and tropical climates, known for their superior heat and drought tolerance. Examples include Bermuda, Zoysia, St. Augustine, and Centipede grass. These grasses enter a state of dormancy, turning brown when cold weather arrives, and take longer to green up again in the spring.
Identifying Grass by Growth Habit
Grasses are also categorized by their distinct growth habits, which describe how the individual plants spread and fill in space. This classification is crucial for understanding how a lawn recovers from damage or how a pasture develops a dense cover. The two primary structural types are bunching grasses and spreading grasses, with the latter divided by the type of horizontal stem they produce.
Bunching grasses grow vertically from a central crown, forming dense tufts or clumps that expand only by producing new shoots (tillers) at the base of the plant. These grasses, such as Perennial Ryegrass and Orchardgrass, do not send out horizontal runners. Bare spots in the turf must be filled in through reseeding or the slow enlargement of the crown, which can result in a less uniform surface.
Spreading grasses, also known as sod-forming grasses, propagate horizontally by specialized modified stems to create a dense, interconnected mat. These creeping stems are either called stolons or rhizomes, or sometimes both, depending on their location relative to the soil surface.
Stolons
Stolons are horizontal stems that creep along the top of the ground, rooting at nodes to form new plants. This method of propagation is common in St. Augustine and Centipede grass.
Rhizomes
Rhizomes are similar horizontal stems, but they grow underground, allowing the grass to spread and store energy beneath the soil surface for better recovery. Kentucky Bluegrass and some varieties of Zoysia grass primarily use these subterranean runners to fill in damaged areas and form a tight sod. Grasses that utilize both stolons and rhizomes, such as Bermudagrass, are aggressive and persistent spreaders.
Major Categories of Grass Use
The Poaceae family serves three major functional roles globally, extending far beyond typical turf applications. These categories demonstrate the importance of grasses, ranging from supporting livestock to feeding the human population. The family’s diversity allows it to provide raw materials, food, and environmental stability.
Forage and Feed Grasses
These grasses are cultivated to feed livestock through grazing, hay, or silage. Examples like Tall Fescue, Ryegrass, and Sorghum are grown for their high biomass and nutritional content, forming the basis of beef, dairy, and other animal industries. These grasses are managed carefully, often being selected for rapid growth and resilience to heavy grazing and trampling.
Cereal and Grain Grasses
The most economically important group is the Cereal and Grain Grasses, which supply the majority of human dietary energy worldwide. Staple crops like Wheat, Rice, Maize (Corn), Barley, and Oats are all botanically grasses, with their seeds harvested and consumed as grains. Rice alone provides approximately 20% of the world’s dietary energy.
Ornamental and Specialty Grasses
This final category serves purposes beyond food or typical turf applications. This includes decorative varieties like Pampas grass or the massive woody stalks of Bamboo, which is used for construction and materials. Other specialty grasses are used for erosion control due to their dense root systems or for biofuel production.