With some 10,000 different species in the grass family, botanists have devised a classification system to sort them into natural groups. Species and genera are sorted into sub-families on the basis of the flower, leaf structure and photosynthetic mechanism. Within the sub-families, those with similar flower structures are grouped into tribes. These groupings reflect how grasses have evolved and how closely they are related.

Botanists know the grass family as the Poaceae, although an alternative name widely used in the past was Gramineae; this name appears on many plant labels in Kew's Grass Garden.

Meadow grass sub-family (Pooideae)

This sub-family largely occurs in the northern temperate regions of the world, although some species grow in the cool zone of mountain tops within the tropics. Its members have the unmodified C3 photosynthetic pathway, common to most green plants. Various metabolic modifications have enabled them to adapt to extremes of heat or cold within the temperate environment. The annual species of Mediterranean regions flower and produce seed before the summer drought sets in, and then germinate in the autumn. Other species found in northern parts of Europe survive harsh winter conditions as perennial underground rhizomes, which produce new stems annually when conditions improve in the spring.

Included in this sub-family are many important pasture and lawn grasses such as fescues (Festuca), ryegrass (Lolium), meadow grass (Poa) together with wheat, barley, rye, oats and other cereal crops.

Teff sub-family

The Chloridoideae occupies the drier regions of the tropics and sub-tropics, often in stressful habitats. Its members possess the more complex C4 form of photosynthesis that enables them to survive in hot tropical conditions with high light intensity and limited water availability. Many of the grasses in the sub-family are tufted or produce runners (stolons) at the soil surface (eg Bermuda grass – Cynodon dactylon ). Included here are various grain crops such as teff (Eragrostis tef) and finger millet (Eleusine).

Millet sub-family (Panicoideae)

Found mainly in the tropics and sub-tropics, these grasses live in a wide variety of habitats ranging from forest shade to grassland, swamp and desert. Some members of the sub-family have the C3 photosynthetic pathway, while others possess the more efficient C4 pathway. Their distinctive two-flowered spikelets contain a fertile upper floret and a male or barren lower floret. The flowers are often complex and gathered into large inflorescences. The sub-family contains many useful grasses, amongst them sorghum (Sorghum bicolor), millet (Pennisetum), maize (Zea mays) and sugarcane (Saccharum officinarum) together with a number of tropical pasture (Andropogon), forage (Brachiaria) and lawn (Stenotaphrum) grasses.