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A CPIE Notebook Project – Grasses and Sedges of Hawai‘i Page i

A Key to the Grasses and Sedges
of the Hawaiian Islands

Eric B. Guinther, senior ecologist

Grasses on a kona lava slope

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True grasses are classified in the plant family, Poaceae (or Gramineae in older sources) and are among the most successful plants on the planet earth with perhaps 8,000 or more species described worldwide. Sedges, while they generally resemble grasses, are separately classified in the Family Cyperaceae, with some 5,500 species worldwide. This CPIE "notebook" project is an attempt to provide a simplified identification guide (as a "dichotomous key") to grasses and sedges of the Hawaiian Islands, where more than 150 species of grasses and 70 species of sedges are found in nature, grown as crops, or used in landscaping.

Grasses and sedges can be found just about everywhere in Hawai‘i: some thrive in wetlands, others in the shade of the forest, but most prefer open (unshaded) ground, the grasses particularly dominating fields, lawns, and recently disturbed sites. Most places you find grasses, you will also find sedges, although typically in much lesser abundances. The identification key presented here—a work in progress—differentiates 126 taxa of grasses and 34 taxa of sedges found in the main Hawaiian Islands. A listing of these taxa is provided on the Index page. These included grasses and sedges are species most commonly encountered at lower elevations around the Islands.

Separating Grasses, Sedges, Rushes and other Monocots

Most grasses are easily recognizable as such, although members of several other plant families do resemble grasses, and in the absence of flowering structures, could be mistaken for grasses. Recall that flowering plants are divided into dicots (germinating seed produces two seed leaves or cotyledons) and monocots (germinating seeds produce a single seed leaf). Grasses, sedges, and rushes are monocots, and their basic structural characteristics are typical of the majority of monocotyledonous plants: leaves with parallel veins, fibrous roots, and other consistent floral and internal structures that differ from those of dicots (see Monocots vs. Dicots or Monocots and Dicots Chart).

Plants that are not grasses—but closely resemble grasses—are very likely other moncots. Sedges (Family Cyperaceae) are the most common grass-like plants encountered in nature. Sedges differ from grasses in a number of respects. Generally, sedges are coarser, the leaves concentrated around the base of a stem (or culm) or occuring as bracts (one or more stem or leaf-like appendages below the flowering heads), or so reduced in size as to appear absent. The culm may be triangular ("sedges have edges") in cross-section and solid, not hollow (although there are hollow culms in the family. Bulrush (Schoenoplectus spp.) is a sedge with a hollow (and mostly rounded) culm. Sedges are typically more abundant than grasses in wetlands and wet areas.

Rushes of the Family Juncaceae are rather uncommon in Hawai‘i, and likely to be mistaken for sedges if not flowering. Other rushes are included in the Family Cyperaceae. A few monocots are named, in common parlance, "grasses", but are not. Examples are: nut grass, a common lawn weed (it is a sedge), menehune grass (another sedge), and mondo grass (Ophiopogon spp., Family Liliaceae). A number of fully aquatic grass-like plants resemble (but are not) grasses, for examples: widgeon grass (Ruppia maritima, Family Ruppiaceae) and tapegrass (Vallisneria spiralis, Family Hydrocharitaceae). These aquatic plants can be keyed out at [CPIE] Vascular Plants where you can choose a plant family from a listing on that web page OR start at the first couplet [70].

The following couplets of our dichotomous key are modified from Keys to the Aquatic Biota of the Hawaiian Islands to separate sedges and grasses found in wetlands and along streams and should serve adequately here. Presently, selecting a choice leading to plants other than sedges or grasses will go to the CPIE aquatic biota key. Use link at bottom of page if you are uncertain about using a dichotomous key.

With Links to A Key to Aquatic Plants in Hawai‘i


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1a Plant generally grass-like: herbaceous, with leaves linear, very many times longer than wide, leaf veins parallel; if blades not exactly linear, veins are still parallel; OR plant a green stem without obvious leaf-like structures, or these small and sessile (leaf lacking a petiole or leaf-stem), clasping the stem. Flower heads may be conspicuous, but individual flowers are small and usually mature to some shade of brown or yellow (usually green when immature).
    ~ Class LILIOPSIDA (MONOCOTS, in part)

1b Plant somewhat "grass-like" with tall, woody, hollow (jointed) stems or culms; leaves not clasping but attached to side branches by pseudo-petioles. Rarely producing flowers in Hawai‘i. Bamboos
    ~ Class LILIOPSIDA, Family POACEAE, Subfamily Bambusoideae

1c Plant not grass-like: may be herbaceous or may be woody, but leaves at most only 5 or 6 times longer than broad; leaf veins arising from a central axis or radiating from a central point. Flowers variable, but many species with conspicuously colored or otherwise showy petals. Other aquatic monocots [107]
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2a (1) Plant mostly a soft, green, vertical stem (called a culm), without leaves, or leaves present only as basal sheaths without blades, or blades inconspicuous. Flower head or heads at or near tip of stem, in some cases, with a conspicuous bract subtending (found directly below) the flower head. Certain rushes
    ~ Family CYPERACEAE

Stem, if soft, green, and upright, then clasped by one or more long, narrow leaves or with a basal rosette of narrow leaves; OR stem otherwise (creeping, branching)

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3a (2) Leaves in two ranks (distichous: looking down on the culm, leaves come off on two sides). Stems usually hollow except at nodes. Clasping part of leaf below blade open along a vertical seam [4]

Leaves in three ranks: looking down on the culm, leaves come off on three sides; sheathing part of leaf closed. Stem usually solid, usually trigonous (three-sided), but exceptions exist. Sedges

    ~ Family CYPERACEAE
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4a (3) cattails 

Flowers arranged in a dense(and solitary) spike (1.5 - 3 cm diameter) arising above flattened, spongy, pale green leaves, lacking a midrib. Cattails

     ~ Family TYPHACEAE


Flowers arranged in various ways, usually in several to many spikelets. Leaves linear, pale to dark green, but not fleshy or spongy, most often with a midrib that is prominent on upper or lower leaf surface. Grasses

    ~ Family POACEAE


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