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Identification: Characteristics of Grasses

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Grass flower heads

The flowering stem (flower head) is useful in the identifications of grasses. Flower heads of grasses are described according to the arrangement of a special floral unit called a spikelet rather that of the individual flowers.

A spikelet consists of one or more flowers (florets) plus two basal membranes (glumes). The spikelet is attached to an un-branched axis called the rachilla (see illustrations below).

Spikelet with one floret Spikelet with several florets
Illustration of membranous ligule Illustration of ligule as a fringe of hairs

Different flowering head structures can be distinguished by the presence or absence of branching along the flower stem (main axis) and the presence or absence of stalks beneath the spikelets. The three basic arrangements in grasses are spike, raceme, and panicle flower heads.

Spike

Main axis does not branch.
Spikelets are stalkless.

Raceme

Main axis does not branch.
Spikelets are stalked.

Panicle

Main axis branches.
Spikelets are stalked.
Main axis does not branch. Main axis does not branch Main axis branches.

Below are examples of grasses featured in the Weed Gallery:

Examples of spike flower heads
Quackgrass. Quackgrass. Quackgrass.
Quackgrass Hare barley Foxtail barley

Examples of raceme flower heads
Barnyardgrass Barnyardgrass
Smooth crabgrass Dallisgrass

Examples of panicle flower heads
Fall panicum. Fall panicum. Fall panicum.
Fall panicum German velvetgrass Wild oats
 
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