Plant Species of the South Kaibab Trail:
rock goldenrod, Petradoria pumila

See Plant Guide to South Kaibab Trail for an introduction to this page, especially the Introduction To These Species Pages.

Characteristics

Identification status: 100% to the species. The Grand Canyon plants fit the descriptions of the species given in the Flora of North America, the Jepson Manual and the Utah Flora, and match the online pictures perfectly. The leafy stems, with wide, clearly-veined leaves, ending in separated flat-topped flower clusters, with whitish/yellowish phyllaries with darker yellow tips, make a fairly distinctive combination.

However, the variety is unclear. Either the Grand Canyon plants are intermediate between the two varieties, or the varieties do not exist. See the discussion below.

Keying from McDougall Asteraceae Family:

1'   Corolla not 2-lipped
3'   Non-woody plant
42c  Heads radiate
99b  Pappus of hairlike bristles
106' Ray flowers yellow
136' Pappus of bristles
138' Leaves alternate
141' Pappus entirely of hairlike bristles
144' Phyllaries unequal in more than 1 series
149  Heads small and numerous; phyllaries without green tips
150  Plant low; stems clustered from a branched base;
     heads cylindric in a more or less flat-topped cluster

The above keys it to one of two species in McDougall, which are separated by him as follows:

151  Leaves 3-nerved, the lower ones oblanceolate to nearly linear
                               ...  Solidago petradoria
151' Leaves 1-nerved, the lower ones narrowly linear 
                               ...  S. graminea

McDougall said that S. petradoria is very similar to S. graminea but the leaves are broader and 3-nerved.

Both of these taxa are now combined into a single species under a different genus, Petradoria pumila, with S. petradoria now being Petradoria pumila var. pumila and S. graminea now being Petradoria pumila var. graminea.

This is the only species in the Petradoria genus. It is now separated from Solidago by at least two characteristics. First, the phyllaries are in vertical ranks for Petradoria, and not so in Solidago. This characteristic is used in both the Jepson Manual and Utah Flora keys. Second, the Jepson Manual also uses the disk flowers being staminate in Petradoria and fruiting in Solidago.

The Flora of North America key to separate the varieties is:

Leaves usually 1(-3)-nerved, 1-2 mm wide; involucres 1.3-2 mm wide; ray florets usually 1, laminae 0.7-1.5 mm wide; disc florets 2-3 ... var. graminea
Leaves usually 3-5-nerved, 2-12 mm wide; involucres 1.9-3 mm wide; ray florets usually (1-)2-3, laminae 1-2.4 mm wide; disc florets 2-4 .... var. pumila

The plant I photographed on the S. Kaibab Trail (see pictures below) has leaves mostly 2 nerved (= 2 veined), sometimes with just 1 nerve and sometimes with a faint third nerve. The leaves are narrowly oblong and about 3.0 mm wide; the involucres are 1.7-1.8 mm wide; and the number of ray florets is typically one.

The plant I photographed and measured just south of the S. Kaibab Trailhead (see pictures below) has narrowly-oblong leaves that are mostly 1 nerved, sometimes with 2 nerves, and range from 20 mm long and 3.0 mm wide to 40 mm long and 4.5 mm wide. The heads typically have one ray flower and one disk flower.

Neither of these plants can be clearly determined to a single variety using either the McDougall or Flora of North America key. The number of nerves is indeterminate between the two varieties for the Trail plants and favor var. pumila for the near Trailhead plants; the width of the leaves clearly goes to var. pumila for both plants, but the width of the involucre and the number of ray florets goes to var. graminea. The number of disk flowers fits neither species.

Usually in such situations I've found that the varieties do not actually exist, and are just extreme forms of a single species. One way to check on the taxonomic validity of such varieties is to examine their geographic separation.

Plots of the distribution of the varieties from SEINet show that both varieties are present at the Grand Canyon, but without any geographic separation. In fact, there is one voucher of each variety from the South Kaibab Trail! The SEINet plots do show only var. graminea in the extreme northeastern portion of Arizona, so there appears to be some weak geographic distribution differences within Arizona.

The two varieties have an odd distribution in their entire range, with the range of var. graminea (AZ, UT, NM) contained entirely within the range of var. pumila (AZ, UT, NM, plus CA, NV, ID, WY, CO). The Utah Flora gives this taxon only to the species, but adds the following note: Most of our specimens belong to the broad-leaved var. pumila, but a few specimens from Emery and Garfield counties seem to be clearly allied to var. graminea ... known previously only from NM and AZ.

The case for the existence of two varieties seems quite weak:

Given the above, I'll follow the lead of the 1987 Grand Canyon Flora, and the current Park List, and determine these plants only to the species name.

From a SEINet search on 1 September 2008, there are 26 vouchers of this species from the Coconino County portion of the Grand Canyon. 13 of those vouchers are given just to the species, and may or may not refer to var. pumila; 9 of those vouchers are stated as being var. pumila, and 4 of those vouchers are var. graminea. Two are from this trail: South Canyon, Kaibab (of var. graminea); and Kaibab Trail (of var. pumila). Note that there may be additional vouchers at other herbaria not available through SEINet.

First occurrence on South Kaibab Trail: mile 0.27, elevation ~6950 feet (~2120 m).

Number of plants along Trail: This species was only found off-trail, in a fairly large mass at a single location, in August 2008.

Pictures

From 24 August 2008, from a plant just south of the S. Kaibab Trailhead:

From 23 and 24 August 2008, mile 0.27:

There are two veins on the leftmost leaf, and two veins plus a very faint third vein on the rightmost leaf:

There are two veins in the following leaf:

There seems to be only a single vein in the following leaf:

References

Flora of North America description and illustration.

SEINet pictures

Southwest Colorado Wildflowers. The mass of plants in the bottom pix here are very similar to the mass of plants below the S. Kaibab Trail at the only location of this species on that trail.


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Copyright © 2008 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:
http://tchester.org/gc/plants/species/petradoria_pumila.html
Comments and feedback: Tom Chester
Last Update: 1 September 2008