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    Petradoria pumila (Rock Goldenrod), Gutierrezia sarothrae (Snake Weed), and the various species of Chrysothamnus and Ericameria (Rabbitbrushes) are often difficult to tell apart.  Following are some general and then more specific points to help with these look-alike genera:

Comparing the appearance of the most common form of the three plants from ten feet away:

Petradoria is in a compact, cylindrical form about 4" wide and 9" tall with a posture of numerous erect stems. Leaves are bright green. Leaves are typically much wider (4-7 millimeters) and longer (7-12 millimeters) than those of Gutierrezia, Chrysothamnus, and Ericameria. Last year's dried leaves surround the bottom of the plant. Plants are often numerous, but well-spaced. In our area Petradoria almost always has all of these characteristics.

Gutierrezia, when a few years old, is in a rounded shape about 8" wide and tall with straight stems, some growing upward and many leaning outward. Leaves are a bright green and are only about 1-2 millimeters wide and 7 or so millimeters long. As the plant ages, it becomes very obviously woody, intricately branched, and nearly 3' high and wide. In our area, Gutierrezia most often has a rounded shape -- except when very young. Then it has just one to few upright stems, is only 3-6" tall, and looks very fern-like.

In their first years Chrysothamnus and Ericameria are a straggling, twisting, very quickly growing few stems. Leaves are dull green and often about the same width and shape as those of Gutierrezia although they are more commonly wider and longer than Gutierrezia leaves but narrower and shorter than Petradoria leaves. As the plants age, they produce many more stems which become woody and in a mass that can be, depending on the species, from a few feet tall and wide to 7 feet tall and wide. Some species have an upright posture; some are more intricately branched and leaning and arching. As the plants reach maturity, they are clearly different from the almost always much shorter Petradoria and Gutierrezia.

When the three genera are in flower, you will note the following details if you use a 10x hand lens:

Petradoria has pappus of capillary bristles and it has glabrous seeds.

Gutierrezia has pappus of scales and it has hairy seeds.

Chrysothamnus and Ericameria have pappus of capillary bristles and they most often have hairy seeds.

The three plants also have distinct smells, but I don't have the words to describe the differences. Squeeze them and see if you can use smell to help separate the species.

When you squeeze the plants, you will also notice that some, such as, Chrysothamnus viscidiflorus, feel sticky; some do not.

Be sure to look at the photographs below and at Gutierrezia sarothrae and the various species of Chrysothamnus and Ericameria.

Petradoria pumila
Petradoria pumila variety pumila (Rock Goldenrod)
Asteraceae (Sunflower Family)

Foothills. Roadsides, woodlands, openings. Summer.
Prater Ridge Trail, Mesa Verde National Park, August 13, 2005.

Petradoria pumila puts on quite a show in July and August.

Petradoria pumila
Petradoria pumila variety pumila (Rock Goldenrod)
Asteraceae (Sunflower Family)
 

Foothills. Roadsides, woodlands, openings. Summer.
Dolores River Overlook, July 14, 2004.

Petradoria pumila is a very common plant found in extensive colonies lining trails and roads and filling meadows.  It first attracts attention to itself in early summer with bright green, dense, six inch diameter clusters of vertical leaves.  Then, in mid-summer (when this photograph was taken), its four-to-ten inch stems are topped with a mass of tiny, attractive, golden/yellow flowers.

The base of the plant often has a whorl of gray dead leaves from the previous year’s growth. This whorl could be confused with the very similar whorl of Stenotus armerioidesPetradoria pumila is also confused with Gutierrezia sarothrae but the latter usually blooms later in August and September and is taller, woody, bright yellow, in more rounded clusters with very narrow, short leaves.

"Petra" is from the Greek for "rock: and "doria," is, according to William Weber, "an old name for a Goldenrod".  "Pumila" is Latin for "dwarf".

A specimen of this plant was first collected for science by Thomas Nuttall "in open situations on shelving rocks" in the Rockies (Intermountain Flora).  Nuttall named the plant Chrysoma pumila in 1841 and Edwin Greene renamed it Petradoria pumila in 1895.  This is the only species in the Petradoria genus, but see the discussion below about whether this species has two varieties.

Petradoria pumila
Petradoria pumila variety pumila (Rock Goldenrod)
Asteraceae (Sunflower Family)
 

Foothills. Roadsides, woodlands, openings. Summer.
Dolores River Overlook, July 26, 2009.

Petradoria pumila
Petradoria pumila variety pumila (Rock Goldenrod)
Asteraceae (Sunflower Family)

Foothills. Roadsides, woodlands, openings. Summer.
Dolores River Overlook, July 14, 2004.

 

Petradoria pumila var. graminea
Petradoria pumila var. graminea (Grass-like Rock Goldenrod)
Asteraceae (Sunflower Family)
 

Foothills. Roadsides, woodlands, openings. Summer.
Lone Mesa State Park, July 14, 2011.

These clumps of Petradoria are very unusual.  In fact, I have never seen anything like them, for Petradoria pumila commonly grows in small upright clusters to about six inches in diameter.  The pictured robust clusters are 15-24 inches in diameter.  Leaves are very narrow and flowers are few in each head.  These characteristics lead me to conclude that these plants are Petradoria pumila variety graminea.

For a good discussion of the differences between Petradoria pumila variety pumila and variety graminea, read Tom Chester.  He concludes that the Petradoria genus has minor gradations within the species and that varieties or subspecies are probably not warranted. Most floras agree with this assessment; Stanley Welsh's A Utah Flora retains the variety graminea.

Petradoria pumila variety graminea
 

Petradoria pumila var. graminea (Grass-like Rock Goldenrod)
Asteraceae (Sunflower Family)

Foothills. Roadsides, woodlands, openings. Summer.
Lone Mesa State Park, July 14, 2011.

The photograph at left and those below show individual clusters of plants from the massive display in the above photograph.

Petradoria pumila variety graminea
 

Petradoria pumila var. graminea (Grass-like Rock Goldenrod)
Asteraceae (Sunflower Family)
 

Foothills. Roadsides, woodlands, openings. Summer.
Lone Mesa State Park, July 14, 2011.

Petradoria pumila var. graminea leaves are more slender and linear than those of P. pumila var. pumila.

Petradoria pumila variety graminea
 

Petradoria pumila var. graminea (Grass-like Rock Goldenrod)
Asteraceae (Sunflower Family)

Foothills. Roadsides, woodlands, openings. Summer.
Lone Mesa State Park, July 14, 2011.

Range map © John Kartesz,
Floristic Synthesis of North America

State Color Key

Species present in state and native
Species present in state and exotic
Species not present in state

County Color Key

Species present and not rare
Species present and rare
Species extirpated (historic)
Species extinct
Species noxious
Species exotic and present
Native species, but adventive in state
Eradicated
Questionable presence

Range map for Petradoria pumila