Plant Species of the Borrego Desert:
Ericameria paniculata, Blackbanded Rabbitbrush

Table of Contents

Introduction
Species Description
Geographic and Elevational Distribution, and Habitat
Abundance
The Black Band
Comparison with Similar Species

Fig. 1. Ericameria paniculata, blackbanded rabbitbrush, in bloom, in its habitat of usually-wide washes with deep sand that are flooded with water seasonally.

Top: View of a single plant in San Felipe Wash just north of Kenyon Cove on 25 November 2020, by Tom Chester. Desert willows are growing behind the rabbitbrush.

Bottom: View of a wonderful display in Canebrake Wash on 25 November 2020, © Juergen Schrenk. Ironwood trees are growing behind the rabbitbrush.

Click on the photos for larger versions.

Introduction

Ericameria paniculata, blackbanded rabbitbrush, is one of our most glorious species when in bloom, made especially delightful since it usually blooms in November and December in the Borrego Desert, when few other species are in bloom.

Ericameria species are often called goldenbushes, since in full bloom they can appear as a solid mass of gold from a distance; see Fig. 1. Until 1993, this species was known as Chrysothamnus paniculatus. Chrysothamnus species are called rabbitbrushes, even though the scientific name Chrysothamnus means "golden bush"! (Ericameria means simply "heath-like leaves".)

No one knows for sure where the name "rabbitbrush" originated. One speculation is that rabbits or jackrabbits shelter in it and burst out when approached. We have seen evidence of jackrabbit predation on young plants of E. paniculatus.

E. paniculata occurs only in restricted areas in the Anza-Borrego Desert State Park (ABDSP) region, since it lives almost exclusively in sandy washes with deep sand that gets thoroughly recharged by water flow seasonally. The species drops out when the sand does not get deeply recharged by water flow, or when the sand becomes too shallow to hold much water. See Geographic Distribution for details.

E. paniculata is one of the less-abundant shrubs in the ABDSP region. There are probably only something between 5,000 and 15,000 total plants here, inside a total area of about 10% of the Borrego Desert below 3000 feet elevation. See Abundance for details.

The famous "black band" is a very-curious characteristic of this species. Well over 90% of all plants have one or more short black sections on their twigs, which are caused by a fungal infection, said to be a smut fungus. The black bands are most apparent on dead stems, but are found on live stems, too. We don't know whether the fungus kills those stems, or whether the stems die naturally and the fungus takes over as they die.

The black band is an easy way to identify the vast majority of specimens of E. paniculata. E. brachylepis and E. teretifolia are the most likely species to be confused with E. paniculata. Both of those species are significantly shorter plants, and only rarely grow in washes. When they do, they are usually at the very edge of the wash. Hence if you see a robust plant growing in a wash, it is very likely to be E. paniculata. It is easy to confirm that determination by looking for the black bands.

We have only seen a handful of baby plants of this species so far, but we haven't spent much time looking for them, so we don't know how common seedlings are. We found three patches of young plants in a survey of Nolina Wash, so significant recruitment has occurred in recent decades.

For many good photographs of this species in the ABDSP region, see:

Species Description

The following measurements came from plants near SR78 / Grapevine Canyon Road and from plants near SR78 / Pinyon Wash.

E. paniculata is a shrub, typically five to six feet (1.5 to 2 m) tall and six to ten feet (2 to 3 m) wide, with five to ten sturdy usually-erect stems from the base of the plant that combine to form a rounded bush. Some of those stems on the edges of the plant sometimes become horizontal. Each plant can have 50 to 300 ultimate twigs that are the result of the current year's growth. The shortest twigs are about 3 inches (8 cm) in length, with the tallest on each plant varying from 12 to 20 inches (30 to 50 cm); see a photograph of the current year's blooming stems that have grown from the flowering stem of the previous year.

Twigs are leafy, with green thread-like leaves ~1 mm wide and 10 to 35 mm long. The leaves have numerous resinous glandular pits that make the leaf sticky. The resin often dries to white dots on older leaves. (Such leaves are typical of many Ericameria species.)

The twigs are topped by short flowering twiglets, with each twig containing about 100 flowering heads. This arrangement is called a panicle, which is the origin of the paniculata in its species name.

The individual flower heads are small, just ~4 mm (0.15 inch) wide, composed of 5 to 8 yellow disk flowers and no ray flowers.

What makes this species so showy in bloom is that an individual plant can have a total of 5,000 to 30,000 flower heads per plant, producing a total flower area of 0.5 to 3 square feet (0.05 to 0.4 m^2), which can be spotted from tens of feet away.

Geographic and Elevational Distribution, and Habitat

E. paniculata is endemic to the desert in California, southern Nevada, extreme southwest Utah, and northwestern Arizona, living within a square 350 miles (550 km) on a side; see Fig. 2. Even though it is found within 15 miles of the Baja California border, it is not found in Baja. Our plants in the ABDSP area are at the southwest limit of its range.

Fig. 2. The SEINet geographic distribution map of E. paniculata as of 11 December 2020. See also the latest SEINet map (the northern outlier point in their map has a typo in its latitude as of 11 December 2020; it has been cropped out of the map above).

The geographic description presented here for the ABDSP area has been compiled from 375 records with good locations from survey locations by Tom Chester and companions, Calflora records, iNat observations, and vouchers. The iNat observations and voucher locations were reviewed to eliminate potential misdeterminations and poor locations. The accepted locations are from:

Fig. 3 shows the known locations of E. paniculata in our area from those 375 records, as well as one small area in upper Fish Creek where we have seen specimens while we were on a car trip, but that have no documented locations except for one plant.

Fig. 3. The geographic distribution of E. paniculata in our area, with known accurate locations marked with red dots. One area marked by a red line contains plants that have not been GPS'd. The blue lines show county borders, with the counties labeled. See also larger maps of the northernmost group, the central group, and the southernmost group.

E. paniculata mostly lives in three disjunct areas; a map for each area is linked to its name:

In all of these areas, E. paniculata begins to occur when there is deep sand in the washes, and the flow in the washes becomes great enough to deeply recharge the sand with water, at least seasonally.

Any of these areas could be examined in detail to support this; here is an examination of the Collins Valley area in more detail. Fig. 4 shows the geographic distribution of E. paniculata in the Collins Valley area.

Fig. 4. The geographic distribution of E. paniculata in the Collins Valley area. Coyote Creek extends from the top left of the map to the bottom right; only a small portion of it is shown by a blue line. Indian Creek has a number of washes associated with it, roughly within the parallel blue lines.

The bulk of the population of E. paniculata here is along the Indian Creek washes at the south end of Collins Valley. A dense population of E. paniculata begins when the sand gets deep enough, and is deeply recharged with water seasonally, and fairly abruptly ends when either the waterflow is too low to deeply recharge the sand, or the depth of the sand becomes too little. A few plants are found a bit farther east just before Indian Creek joins Coyote Creek .

At Santa Catarina Spring, ground water is permanently close to the land surface over a large area, which allows other plants to take over, primarily mesquite in the spring and saltbushes at the edges of the spring. In the narrow gap in the exposed bedrock below the spring, the high ground water allows willows and other fresh-water-loving plants to grow. E. paniculata picks up again in the broader sandy area below the narrow gap when the water table drops too low to support willows. E. paniculata then stops when the seasonal water flow becomes too low to deeply recharge the sand in the Borrego Valley basin.

In order to take advantage of that well-watered deep sand, E. paniculata has very deep roots. Fred Melgert and Carla Hoegen came across a specimen growing at the bank of a wash, where erosion exposed its upper roots; see Fig. 5.

Fig. 5. The taproot for E. paniculata is massive, needed to support such a large plant, and extends deep into sandy washes to provide moisture also needed to support the large plant. Photograph by Fred Melgert and Carla Hoegen. Click on the photograph to go to their original iNat observation.

There is one place that we know of where this species is not growing in well-watered deep sand. Five plants grow along the lower side of SR78, perhaps 20 to 30 feet above San Felipe Creek as it emerges from Sentenac Canyon. Apparently the road fill and possible alluvium deposits store enough water to allow not only this species, but at least one mesquite plant, to grow there. This area gets not only rain runoff from the road, but runoff from the cliffs above the road. See two plants, including a discussion and pano shots of the area, here and here.

Detailed results from the Nolina / Pinyon Wash survey are given in Fig. 6, which also show the importance of deep sand and significant water for this species. The number of plants per mile is much lower in Nolina Wash than below its junction with Pinyon Wash, since below the junction the wash receives water from both Nolina and Pinyon Wash. The number of plants is very low above the mouth of Nolina Canyon, where the bedrock walls on both sides create a much narrower wash than below the mouth. See also Fig. 7, which shows the elevation distribution of the GPS points from this survey, and a map of the GPS points showing E. paniculata locations near SR78 and in Nolina Wash.

Fig. 6. Plot of the number of plants of E. paniculata in Nolina / Pinyon Wash per mile, vs. distance from SR78. The points are from individual short sections where we wrote down the number of species seen in a short distance. More points were taken where there were gaps in the distribution. The two highest points are from small areas with many young plants. The red line is the average for the entire length of each line segment.

The number of plants is markedly lower above the junction of Pinyon and Nolina Wash, where the wash only has water coming from Nolina Wash. The number of plants is extremely low, but not zero, above the mouth of Nolina Canyon. Above that point, the plants were only found in widely-spaced small clusters. As a result, no points for the number of plants per mile are plotted there; only the average for that length is given.

The elevation distribution of E. paniculata is shown in Fig. 7, where it is plotted versus longitude. In our area, E. paniculata lives mostly at elevations of 500 to ~1800 feet, with a small number of plants found up to 2800 feet, and what is probably a waif at near sea level. A similar now-dead waif is found at 500 feet in San Felipe Creek at Borrego Mountain.

Fig. 7. Plot of elevation vs. longitude, with the source of the observation labeled. The quasi-linear features are typically from a number of GPS points taken along a single wash or drainage, as seen in Fig. 3. The almost vertical blue points in the middle of the plot are from Pinyon / Nolina Wash.

Abundance

Adding up the area of polygons that enclose the populations, E. paniculata is found inside a total area of about 100 square miles. E. paniculata plants occupy only a small amount of that bounding area of 100 square miles. 100 square miles is about 10% of the total area of the Borrego Desert below 3000 feet elevation, which is roughly 1000 square miles.

E. paniculata is one of the less-abundant shrubs in the ABDSP region. We estimate that there are between 5000 to 15,000 plants of this species. For comparison, our most abundant species, creosote and burroweed, have something like 10 to 100 million plants each here, and are therefore something like 2,000 to 5,000 times more abundant.

Our abundance estimate comes from adding up estimated numbers in each area in which it is found. The following goes from areas with more confident numbers to areas with less confident numbers:

The total of the above is 4300 to 10,100 plants, which we'll round to 5,000 to 15,000 plants to account for uncertainties in our maximum estimate.

The Black Band

Athough sometimes the black bands are so numerous and prominent that they can be seen from a distance, often they are not a striking feature and you have to look for them. An individual band is typically only 5 to 10 mm (0.25 to 0.5 inch) long, and is often found only on the dead twigs. Those dead twigs are often hidden by newer growth, so it is easy to feel that the black bands are purposefully hiding from you.

Fig. 8 shows close-up views of the black band on four different bushes, and Fig. 9 shows more typical views in the field of plants with fairly-obvious black bands.

Tip: If you can't find a black band on a shrub you think is E. paniculata, look at a few of its close neighbors. You are almost guaranteed to find black bands if you look at several plants.

The black band is caused by a fungus; see the microscopic images by Wayne Armstrong.

Fig. 8. Photographs of the black band on E. paniculata, all by Terry Hunefeld. Click on the photos to go to the original iNat observation.

Fig. 9. Photographs of the black bands on E. paniculata, all by Tom Chester.

Comparison with Similar Species

E. brachylepis and E. teretifolia are the most likely species to be confused with E. paniculata. Both of those species are significantly shorter, and only rarely grow in washes. Fig. 10 shows typical plants of these two species in their habitat; compare with the photographs in Fig. 1 at the top of this page.

Fig. 10. Photographs of two species that are most similar to E. paniculata, showing their normal hillside habitat and shorter height. Top two photographs: E. brachylepis. Bottom photograph: E. teretifolia.

All photos by Tom Chester. Click on the pictures for larger versions.

Hence if you see a robust plant growing in a wash, it is very likely to be E. paniculata. It is easy to confirm that determination by looking for the black bands.

Another way to discriminate these species requires examination of the phyllaries, the bracts surrounding the flower head. The phyllaries of E. brachylepis have a raised midvein that is often brownish, especially in age, and the heads are shorter and wider. The phyllaries of E. paniculata are weakly keeled, producing a ridge that looks a bit like a raised midvein, but the keel is not as prominent as the raised midvein of E. brachylepis. The heads of E. paniculata are elongate, longer and more slender than the heads of E. brachylepis.

The phyllaries of E. teretifolia have prominent gland tips, which neither of the other two species have.

Fig. 11 shows the difference in appearance of those phyllaries between the three species.

Fig. 11. Top: Flowering heads of E. brachylepis (left) and E. teretifolia (right). Bottom: Flowering heads of E. paniculata.

All photos by Tom Chester.

Voucher data provided by the participants of the Consortium of California Herbaria (ucjeps.berkeley.edu/consortium/) on 10 December 2020.

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Updated 12 February 2021