If you are new to using or understanding a dichotomous key, the Introduction (and links provided there) to Keys to the Aquatic Biota of the Hawaiian Islands will be useful. In particular, you should read the "Beginner's Orientation" section describing how our keys are set-up and how they are used. You may find a polychotomous key easier to use than a dichotomous key if you are just beginning to tackle grass/sedge indentifications. Try the grass identification key at CPIE Multiaccess Key (MAK). This grass MAK key is being modified to respond only with grasses found in the Marianas (Guam and the Commonwealth of the Northern Marianas).

Grasses and sedges present an additional challenge independent of your experience or familiarity in using a dichotomous key: botanists have developed a fairly specialized terminology for the structures of a grass and especially the grass flower (forget "petals" and "sepals"). The terminology applied to sedges is similar. You will need to gain an understanding of this terminology and A Key to the Grasses, Sedges, & Rushes of the Island of Guam is set up to make this (hopefully) as painless as possible.

This key is based upon the knowledge and personal experiences of the author, but also relies heavily on the references cited on Page x. All photographs and drawings are those of the author, unless credited otherwise. The keys are regularly updated; a significant advantage of a web-based presentation over a printed book. You can see when a particular page was last updated at the bottom of that page where the version number is given as a date. We very much welcome comments on the key and particularly concerning any couplet that is confusing or leads to an erroneous identification. Send comments to the author (guinther@aecos.com).

All of the pages in this web document are regularly reviewed and additional species incorporated as time allows—consider the entire document comprises over 45 pages, many considerably longer than a normal book page. In 2024, attention has been paid to improving the entire section on sedges, including expanding the integration of Guam sedges into the keys. A new feature is being added to the keys: in cases where it may be helpful to be aware of a similar appearing species described on a different page or is in a different genus, a link is provided in a "consider somewhat similar [link]..." statement placed below the species description.

The organization of a dichotomous key can be either diagnostic (natural) or synoptical, the latter reflecting scientific classification based on evolutionary relationships, the former more appropriate to a key intended for less rigorous field or "home" use and limited in scope (to commonly occuring grasses and sedges within a specified area, as here limited to Guam and the Hawaiian Islands flora). A standard organizational technique in a diagnostic key is to sort out the most obvious structural differences early in the key and then progress through requiring ever finer detailed observations of the specimen . This approach has been used on Page 1 in sorting out woody bamboos (clearly distinct from other grasses) from all other grasses at couplet [10] and then grasses with other than clearly linear leaves at couplet [12].

The keys presented here differ from more typical published keys not just in using a less technical language but in providing descriptive detail on the final couplet for each species. This additional information (and accompanying figure(s) in some cases) allows the user to confirm arrival at a correct identification without having to seek a description of the species elsewhere. Realize that the steps (couplets) passed through in working towards a final identification may be an important part of the species description not necessarily repeated in the final couplet. Working backwards several steps (using the couplet source link) can be used to review these other important characteristics.

A secondary goal of this guide is to move the user to an identification without requiring very close examination (using a magnifying lens, lupe, or microscope) of miniscule grass or sedge structures like glumes and ligules—a "secondary" goal because, for many species of grasses or sedges, confirmation of an identification is not possible without resorting to this level of anatomical detail. However, with experience, many of Guam's common grasses and sedges can be recognized on sight. To this end, lists of the most common grasses or sedges on Guam fitting a certain set of characteristics may be given at appropriate places in the key. These short lists allow the user to "jump" to a likely conclusion, where a brief description and photograph(s) may provide sufficient confirmation of an identification. Another option would be to start at the CPIE multi-access key (MAK) to herbaceous plants and reduce the possible identifications to a manageable number. The possibles list of species resulting from using the grasses MAK or sedges MAK will have links to descriptive couplets and figures in this dichotomous key. The MAK key to grasses has been adapted specifically for the Marianas Islands flora.

Although our keys follow the normally accepted layout for a dichotomous key, presentation as a webpage allows links to enable movement through the key. At each couplet, in parentheses next to the first choice, is a back link to the last couplet selected. In the far right column is a link to the next couplet based on the choice made. Using these links, the user can easily go backwards and forwards through their sequence of choices, and perhaps correct as necessary. Other conventions encountered are: 1) text following a curly brace "{" provides additional information that may be helpful but is not definitive of a species; 2) blue triangles () indicates that a link will leave the page (forwards or backwards).

Using the list of most likely grasses obtained from say the "multi-access key" described in the previous paragraph is a short-cut of sorts for those not really into keying things out, but just hoping to ID a common grass plant. For those with a good knowledge of grasses, jumping ahead to a recognized genus or inflorescence type is also a short-cut. To facilitate this, the Table of Contents following will prove useful; but be cautious—skipping early steps in a key can easily mislead. Confirmation of a correct identification may require far more information than is provided in a terminal few couplets. If an attempt to identify a grass or sedge in hand has led to an unsatisfactory result, it is generally best to start over at the very first couplet. Clicking back through the links and more carefully considering the choices can result in a better outcome. Realize that any key will work only for those species considered in constructing the key. Consult the bottom of a page for species NOT considered (genus pages primarily).