Plant Species of the Bright Angel Trail: *knotweed, Polygonum arenastrum

See Plant Guide to Bright Angel Trail for an introduction to this page.

This is a non-native species.


Identification status: high confidence.

At least in California, specimens of Polygonum arenastrum have historically been determined as P. aviculare for reasons unknown to me. Before the Jepson Manual for California in 1993, southern California floras all used the name P. aviculare. In 1993, James C. Hickman, the author of the Polygonaceae treatment in the Jepson Manual called these plants P. arenastrum. He referenced Ronse Decraene & Ackeroyd 1988 Bot J Linn Soc 98:321-371, and gave the lengths of the perianth as 2-3.2 mm and the fruit as 2-3 mm. He noted: P. aviculare L. (perianth 3.1-4.9 mm, fr 2.8-3.8 mm); Styles 1962 Watsonia 5:177-214) is very closely related, apparently undocumented in CA.

The Grand Canyon flora only lists P. aviculare, so it was of some interest to me to see this taxon, even though it is a non-native species.

While researching these two species, I learned that P. aviculare was heterophyllous, meaning that not all leaves are the same size, whereas P. arenastrum is homophyllous. For P. aviculare, the earliest leaves are ~2x larger than the later leaves! See this illustration.

To my surprise, the plants I saw at the Bright Angel Trailhead, and on the grounds of the Maswik Lodge, are clearly P. arenastrum, not P. aviculare. Every perianth I measured was ~2.0 mm, and every plant I saw was homophyllous.

Of course, my observations do not imply that all Grand Canyon voucher specimens of P. aviculare are actually P. arenastrum. Both species may be present here, and P. aviculare may have been recently introduced.

Interestingly, the Flora of North America treatment made P. arenastrum a subspecies of P. aviculare, ssp. depressum, and says that P. aviculare ssp. aviculare is also now documented in California. In fact, there are now almost as many California vouchers for P. aviculare ssp. aviculare, 180, as there are vouchers of P. arenastrum, 188.

The Flora of North America separates these subspecies by the ratio of the perianth tube to the perianth length, being 40-57% for P. arenastrum and 20-40% for P. aviculare. It is difficult to reliably measure this from my pictures, but the tubes do look like they are probably on the order of half of the perianth length, further confirming the determination of P. arenastrum.

From a SEINet search, there are no vouchers of P. arenastrum from anyplace in Arizona, even though the Flora of North America says this species is in Arizona. There are 14 vouchers of P. aviculare from the Coconino County portion of the Grand Canyon, but I suspect some of these may be misdetermined.

First occurrence on Bright Angel Trail: mile 0.00, elevation 6845 feet (2086 m).

Number of plants along Trail: Only a single plant was found in a single location in September 2006, and no plants were found on the trail in September 2007. Annuals were much less abundant in 2007 than in 2006 due to the lack of rainfall in 2007.


From 14 September 2006, mile 0.00:

A black ellipse, just to the lower right of the middle of this photograph, surrounds two leaves, a nearly-dried-up older leaf, and a young leaf. Note that they are approximately the same size, since they appear to be the same size in the picture and are both near the focus of the camera.

From 5 September 2007, at the Maswik Lodge:

Note that there is no systematic variation in leaf size between older leaves and younger leaves. The leaf sizes do vary by almost a factor of two, but larger leaves are not found only at the base of the stems; they appear randomly placed along the stems.


See Resources for Grand Canyon Flora for further information on most of these references. Entries in the second column are either the name used in that source or a page reference. The name is linked to online pages when available. If a given reference does not contain this taxon, the entry is either left blank or contains a hyphen.

Scientific NamePolygonum arenastrum
1987 Grand Canyon Flora NamePolygonum aviculare
1987 Grand Canyon Flora page51
SEINet Image PagePolygonum aviculare
USDA PlantsPolygonum arenastrum
Flora of North AmericaPolygonum aviculare subsp. depressum
Jepson Manual for California treatmentPolygonum arenastrum
Jepson Manual illustration page893
Kearney and Peebles NamePolygonum aviculare
Kearney and Peebles Page #247, 1046
Vascular Plants of AZ name
Vascular Plants of AZ volume: page
Huisinga et al 2006 name-
Huisinga et al 2006 page numbers-
Epple Name-
Epple description page #-
Epple pix #-
McDougall 1964 namePolygonum aviculare
McDougall 1964 page #80
Brian 2000 Name
Phillips 1979 name-
Phillips 1979 page #-
Stockert 1967 name-
Stockert 1967 page #-

Go to:

Copyright © 2007 by Tom Chester.
Permission is freely granted to reproduce any or all of this page as long as credit is given to me at this source:
Comments and feedback: Tom Chester
Last Update: 24 September 2007