Structural details concerning grass identification will be introduced at various places throughout the key. To begin using the key, however, it is important to acquire an understanding of several general terms used to describe a grass plant, such as growth form and size.

The size achieved by an individual grass plant can (like most plants) vary considerably depending upon conditions under which it grows, so realize that the size categories described here (small, medium, large, and very large) are not absolutes but intended as guides to help confirm an identification. As a rule, a particular grass species will not be larger than the size given, but may be smaller if growing under adverse conditions or regularly cropped by ungulates or lawn mowers. It would be handy to say that once a grass is in flower, that plant is as large as it is going to get, but some species will flower when relatively young, small plants and continue to produce larger and larger flowering culms through the season. The terms used in the key for grass plant size are defined as follows:

Growth form and persistence (or duration) are useful characteristic in describing a grass species. Grasses are usually distinguishable as one of two basic growth forms: running (spreading by rhizomes or stolons or both) or tufted (bunching or clumping). A few tufted species "run" by arched-over (decumbant) culms rooting at the nodes.

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Figure E. This photo (click to enlarge), taken in an Ulupalakua Ranch pasture at about the 2000-ft (600-m) elevation, shows a medium-size, perennial tufted grass (Cymbopogon refractus) with a small running grass (probably Kikuyu) covering ground between the tufts.

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Another consideration related to growth form is whether a grass is an annual or a perennial. Although at first, this characteristic would seem impossible to apply in the field, various clues can be used to indicate duration. Running grasses are nearly all perennial, as, typically, are the larger tufted grasses. The latter may die back to crowded basal stems in the dry season, but the tufts put out new growth in subsequent years. Small, tufted grasses tend to be annuals, their dried remains not regenerating with the onset of the wet season and the population continuing only through germination of seeds dropped by the previous generation. In these annuals, not much besides roots can be found below the ground surface. Because of our mild climate, some annuals may persist beyond a year in favorable locations (for example, see Fig. F, below). In the photo above (Fig. E), both grasses would be judged to be perennial: the tufted species because of the density of leaves (both dried and fresh leaves are present after completion of this year's flowering); the spreading grass because annuals simply do not form extensive mats, the result of spreading by persisting rhizomes or stolons.

Plant Status—By plant status is meant the origin of a species (or subspecies) relative to the Hawaiian Archipelago or Mariana Archipelago. That is, a species is either native or not native (= alien, introduced) to Hawai‘i or native or not to Guam or other islands in the Marianas. This information is provided in the key in brackets at the identification couplet. Coding in the key appears as follows:

Each bracket includes the pertaining location; [HAW - ] for Hawai‘i or [GUM - ] for Guam. The two island groups are separted by some 3,700 mi (6000 km) of the vast Pacific Ocean and human population contacts were very unlikely before the modern era. Consider that Spaniards were sailing between Mexico and the Philippines (making stops in the Marianas) some 400 years before Hawai‘i was known to the western world, an indication of just how isolated the Hawaiian Islands are in the Pacific Basin.

Of the at least 286 grass species found in the Hawaiian Islands, 8 species are indigenous and 39 are endemic (Wagner, Herbst, & Sohmer, 1990). Two species are presumed Polynesian introductions: kō or sugar cane (Saccharum sp.) and ‘ohe ("native" bamboo, Schizostachyum glaucifolium). Unfortunately, the endemics are difficult to locate as most are absent or rare in the lowlands around the Islands. Several indigenous species are easy to find, if you know where to look. For example, ‘aki‘aki (seashore rushgrass, Sporobolis virginicus) can be found in dunes and soil behind the active beach all along our shores. Pili (Heteropogon contortus; indigenous or a very early Polynesian introduction) appears sporadically in the lowlands, but if you desire to see this grass, visit the state park reserve known as Na Pohaku o Hauwahine on windward O‘ahu.

For Guam and the Marianas generally, we rely on Raulerson (2016) for a compilation of native vs. non-native (introduced) grasses found in that island group: 74 native (indigenous and endemic) taxa and 79 introduced taxa.

Distribution of Grasses — Public knowledge on the actual distribution of grass species across the landscape is pretty much limited to the descriptions of environments, elevation ranges, and specific islands in the archipelago where collected for each species as presented in Wagner, Herbst, and Sohmer (1990). However, Keven Faccenda at the University of Hawai‘i at Manoa has started a mapping project "designed to be used to help understand the elevation ranges at which each [grass] species occurs and the approximate distribution of each species across the landscape". The project maps are available as the Grass atlas of Hawaii version 2.0. Eventually, this resource could prove useful in confirming an identification made using our key. If the species you have identified is not on your island, or not reasonably at the elevation you collected it at, you will want to be extra careful in coming to the conclusion that you made a correct identification.

Grasses as Invasive Plants — Many species of grasses can be described as invasive. Because the term "invasive" lacks a practical definition, highly problematical species are labeled herein as "noxious weeds". Noxious weeds are defined by Hawai‘i state law as “any plant species which is, or which may likely to become, injurious, harmful, or deleterious to the agricultural, horticultural, aquacultural, or livestock industry of the state and to forest and recreational areas and conservation districts of the state, as determined and designated by the Department [of Agriculture] from time to time.” Unfortunately, it is not clear that the list has been updated in this century. The list provided on the HDA website is dated June 18, 1992 and contains 10 grasses and 1 sedge. However, the list states that all islands are "Free or Relatively Free" for at least 6 of these species, in place of providing a sensible presentation stating which islands might be experiencing a problem. One has to conclude that the list is one of species that might be a problem, if and where they should appear in the Hawaiian Islands. A slightly more professional list, albeit identical in content, is provided by the federal government (at a USDA website). The USDA has its own "Federal Noxious Weeds" list designating 112 plant species. For grasses, this list is quite different from the state list, as one might expect. Although Guam has extensive problems with respect to invasive species, no noxious weeds list exists. For now, "noxious weeds" appearing on either the federal or state of Hawai‘i lists have been so indicated in the keys.

Wetland Status— The U.S. Army Corps of Engineers maintains a list (National Wetland Plant List) relating the indicator status (liklihood) that a plant species will occur in a wetland. Wetland status codes are provided for each species of grass in the key based on the latest publication (USACE, 2023). Wetland status appears in brackets following the plant status code; if missing, USACE (2023) has not assigned a code and the species is likely to be UPL. The codes are explained here: USACE Wetland Status Codes. For Hawai‘i, the published list is reasonably complete. This is not presently the case for Guam plants (included under the Pacific Islands Subregion) and users may apply the status given for Hawai‘i in the absence of any other source.

Text (grass flower basics) continues on Page v

Perhaps useful to students just beginning to identify grasses—although clearly an artificial distinction—is to differentiate lawn grasses from other grasses. Because many lawn grasses will occur in the "wild" outside of lawns–for examples, Hilo grass and pitted-beard grass are both common lawn grasses and wild grasses—the distinction is simply a practical one. Eventually, all common grass species can be reached by selecting the second lead (9b) in the couplet following, "lawn grasses" serving as a convenience for those curious about... well the grasses in their lawn (and the key includes weedy grasses and sedges commonly seen in local lawns).

Grass not as above; either larger and creeping, OR clump-forming (bunching, tufted, cespitose), OR location is not a lawn ("wild" grasses and large, ornamental grasses). start of grass key .. .