Grasses and sedges can be found just about everywhere on Guam: some thrive in wetlands, others in the shade of the forest, but most prefer open (unshaded) ground, the grasses and sedges dominating fields, lawns, and recently disturbed sites across the island. Distribution information for Guam grasses is from Raulerson (2016) and Clayton and Snow (2009). Raulerson lists 60 native and 72 introduced species of grasses (Family Poaceae) on Guam alone of the Mariana Islands and 41 species of sedges (30 indigenous species). Although no true rushes are listed in these two sources, at least one species is believed to be found on Guam.

The identification keys presented here differentiate 90 taxa of grasses, 35 taxa of sedges, and 1 taxon of true rushes found on Guam. These species have the coding "[GUM]" added to the species descriptions if the species occurs on Guam (and "[HAW]" if found in the Hawaiian Islands). A species description lacking the designation GUM but having HAW is a species not presently known from Guam. [?GUM] indicates a grass possibly occuring on Guam as the species has been recorded from other islands in the Mariana Archipelago.

Because this web document integrates keys to Guam grasses and sedges and Hawai‘i grasses and sedges, the start of the keys are here: Couplet 10 (start of grass key) and Couplet 9 (start of sedge key). The start of the key to the Family Juncaceae is at Couplet 30. Note that at these points, text and links are in the Grasses, Sedges, & Rushes of Hawai‘i and Guam; only the introduction pages are dedicated solely to users on Guam (page links at the bottom of shared pages may offer the option of returning to the Guam introductory pages). Because the sedge flora of Guam differs considerably from that of Hawai‘i, the keys for sedges collected on Guam is still being expanded. Sedge pages list, for each genus, the Guam species not yet covered in the key.

Another approach to identifying a Guam grass is the CPIE MultiAccess Key: Herbaceous Plants of Hawai‘i and Guam — GRASSES. This "MAK" key has been adapted to identifying grasses specifically from Guam and the Northern Marianas. A MAK key can be easier to use for persons just starting out in grass taxonomy, although a less precise result is usually a list of similar species rather than a single specific identification. The CPIE MAK key to grasses has links back to species descriptions in this dichotomous key, providing a way of narrowing down (and confirming) one of the species if more than one is provided.

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 most flowering plants are divided into Eudicots (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 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 and are very common in open areas on Guam. 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 and may be as prominent as grasses on disturbed ground.

Rushes of the Family Juncaceae are little reported from Guam; 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 of Hawai‘i where you can choose a plant family from a listing on that web page OR start at the first couplet [70].

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