Pick a Prickly Pear

Note About Lichens and Star Wars

     Although this post will look at cacti in Colorado, I noted something in the new Star Wars movie that was relevant to this blog. In the final scene of Star Wars: The Force Awakens, brilliant orange lichens are clearly visible splashed across the rocks where the scene was shot. This is probably Xanthoria elegans or a related species, which were addressed in depth in a previous post. My hope is that those of you who have watched or will be watching the movie will look out for this detail and revel in the knowledge that your Star Wars viewing experience was enhanced by the glorious beauty of parietin-producing lichens. May the force be with you, readers! 

    

Colorado Cacti 

     While Colorado supports a wide range of climate zones, much of the state falls into the classification of semi-arid land dominated by brush and grasses(1). This is the ecosystem seen in the foothills, the eastern plains, and many desert-like parts of the state such as the San Luis Valley and parts of the Colorado plateau near Grand Junction. Cacti thrive in semi-arid climates, although those which grow in Colorado tend to be smaller and lower-growing than their impressive cousins such as the the saguaro of the Sonoran desert. 

      Cacti (Cactaceae) are an easily recognizable family of plants which in many cases have exchanged their leaves for prominent spines, with photosynthesis taking place in the succulent stems(2). The outer skin of a cactus is an impermeable, waxy cuticle which helps cacti to retain water in arid environments. While some plant families require patient observation to recognize in the field, most people know a cactus when they see one. It's always fun to encounter a well-fortified cactus growing alongside the trail, sometimes with showy flowers. Cacti tend to flower in the spring, and the cacti of Colorado produce impressively large colorful blooms relative to their size. 

     Flower guides for Colorado seem to snub cacti. This may be because despite their impressive flowers, many native cacti are relatively small, may not flower in large patches, and blend well with the soil when not in bloom. The two print references I typically use(3,4) only make a nod to the genus Opuntia without delineating species, while leaving other genera unexplored. This excellent site(5) notes five genera with ten species, but since it focuses on the four corners area it is possible that more cacti are represented than those which grow only within Colorado. At worst, the several native cactus species of Colorado are not difficult to learn to differentiate when considered in context some other plant families prominent in the region. Asteraceae, for instance, has hundreds of species in the Rocky Mountain region (7), most of which look infuriatingly similar. An intriguing density gradient map of plant species diversity which includes both Asteraceae and Cactaceae may be found here. 

     For the purposes of this post, I am going to focus on four species of cacti that I have encountered in traveling about the state. These are Opuntia macrorhiza, Opuntia polyacantha, Pediocactus simpsonii, and Escobaria vivipara. 

Opuntia macrorhiza, Pricklypear Cactus  

     O. macrorhiza is one of two pricklypear species found in CO. All Opuntia species are readily recognized for their patches of spines and flattened, green pads. I have encountered both Colorado species up to at least 7,000 feet of altitude in a variety of climates, although they seem to be most abundant in sunny dry areas. O. macrorhiza differs from O. polyacantha in its tendency to have longer spines growing from its areoles, larger pads, diminished spines towards the basal end of the pad, and occasional lateral creases of the pad flesh. The flowers of both species look highly similar and may be a range of colors including pink, yellow, peach, or even red. 

     All species of Opuntia that I am aware of are supposed to be good edibles, with the fruit providing a good base for jellies while the de-spined pads may be grilled or boiled as a green vegetable(8). 

O. macrorhiza growing in canyons near Colorado National Monument. 

Note the length of the spines.

New growth on O. macrorhiza. The pink structures appear to be still-living 

leaf primordia from which the hard, lifeless spines will eventually be produced.  

O. macrorhiza growing on North Table Mountain near Golden. The ID for this image was  based on the 

presence of longer spines more distally on the pad, with their presence diminishing towards the base.

Fruit on an Opuntia cactus, likely O. macrorhiza evidenced by the lateral creases 

and more pronounced spines more distal on the pad. This particular cactus was growing 

outside of an office park along the Cherry Creek in Denver. 

Opuntia polyacantha, (also) Pricklypear Cactus  

     

     The common name for this cactus does not differentiate it from its closely related cousin mentioned above. Both species of Opuntia in Colorado are simply referred to as a pricklypears. O. polyacantha looks highly similar to O. macrorhiza in most regards, and is equally edible. The main differences to look for are the slightly smaller pads, consistent presence of spines on the pad from top to base, and the shorter spines(9). The lateral creases mentioned as characteristics of O. macrorhiza are not indicated in sources that I can find as field characters of O. polyacantha, so it appears that horizontal wrinkles are a good indicator that a pricklypear is not O. polyacantha. 

Opuntia polyacantha in bloom on North Table Mountain near Golden. I made this ID solely on 

the basis of the consistent, closely spaced spines up and down the pad. 

Pediocactus simpsonii, Simpson's Pincushion Cactus 

     P. simpsonii is a squat, barrel-shaped cactus that inhabits sunny, rocky slopes in the foothills. The genus Pediocactus is populated by squat, barrel-shaped cacti(10). The lumpy swells below each areole are called tubercles, and Pediocacti have pronounced tubercles which may become more rib-like (vertically aligned and uniform) with age. Areoles are often woolly with trichomes and short, plentiful spines. Flowers tend to be funnel-shaped, and to bear scales (short sepals) on the outside of the petals. Cacti in this genus are frequently referred to as hedgehog or pincushion cacti for obvious reasons, although some other cacti may also be referred to by these common names. 

     All of these features apply nicely to P. simpsonii, and are visible in the photos below. This cactus is easily missed as it is not very large: only a few inches in diameter and growing even less tall. Look for it in sunny, South-facing exposures in the foothills. Its appearance can be quite variable with more tightly or closely spaced tubercles, tiny to largish stems depending upon age, and flowers which may be white, yellow, or even green(11). The spines grow radially from each tubercle, with a central spine(s) which may be black or brown. It may be quite small (>3 inches in diameter) as in the photos below, and ranges up to about 7 inches in diameter. 

     P. simpsonii is named in honor of James H. Simpson, an army engineer who led an expedition in Colorado(12). 

Pediocactus simpsonii crouching between some rocks on North Table Mountain. The yellow-green, 

fleshy flowers are easy to miss amidst the hot rocks and straw-colored brush that mark their habitat.

Same cactus, different angle. Note the relative size of the blades of grass (unidentified) in the  background of the photo. This particular individual is not large, with the stem spanning only a few inches in diameter. 

Escobaria vivipara, Common Beehive Cactus 

     E. vivipara is a widely distributed cactus, ranging from Canada to Texas(13). It enjoys the same arid, sunny habitats characteristic of other cacti in Colorado. E. vivipara demonstrates the marks of the genus Escobaria nicely. These include squat, rotund habit, and spines which grow from tubercles(14). You could be forgiven for confusing members of this genus with those of Pediocactus, as these and many other genera of stout cacti are best distinguished by very minute features of the flowers and seeds. 

     Adding to the confusion, both P. simpsonii and E. vivipara bear short, radial spines of which the primary (more central) spines are brown to black. However, E. vivipara has grooved tubercles, which give them a more polygonal appearance than those of P. simpsonii. I also have not been able to find any reference to P. simpsonii ever bearing pink/rose flowers, while E vivipara consistently does, which may honestly be the most useful field mark for a casual cactus enthusiast in this area.  

     One final feature of interest for E. vivipara is the density of its spines, which may be adapted to not only prevent predation by herbivores as in the previous species mentioned in this post, but also to provide shade to the cactus stem in sunny environments. I have not been able to find any specific reference to whether the spines of E. vivipara provide this function or not, but species with similarly dense, radial spines often benefit from the additional shade(15). 

Escobaria vivipara growing at about 6,500 ft in the foothills near Denver. 

Note the angular tubercles and the brilliantly pink flowers. 

     Thank you for reading about the small, spiky wonders of cacti! It was an educational post to research and I hope it was informative. I also hope that everyone who sees this has as pleasant a Christmas as I did while I worked on finishing this: quietly typing away while drinking wine and watching a spy movie with my family. Stay warm and Happy Holidays!

References 

1. Colorado. (n.d.). Retrieved from Wikipedia: https://en.wikipedia.org/wiki/Colorado#Eastern_Plains
2. Cactaceae. (n.d.) Retrieved from Wikipedia: https://en.wikipedia.org/wiki/Cactus 
3. Mammoser, D., & Tekiela, S. (2007). Wildflowers of Colorado. Cambridge, Minnesota: Adventure Publications, Inc.
4. Craighead, J. J., Craighead., F. C., & Davis, R. J. (1963). A Field Guide to Rocky Mountain Wildflowers. Boston, MA: Houghton Mifflin Company.
5. Schneider, A. (n.d.) Wildflowers, ferns, and trees of Colorado, New Mexico, Arizona, and Utah. Retrieved from: http://www.swcoloradowildflowers.com/index.htm 
6. Easter Colorado Wildflowers (n.d.). Opuntia macrorhiza. Retrieved from: http://www.easterncoloradowildflowers.com/Opuntia_macrorhiza.htm 
7. Kartesz, J.T., The Biota of North America Program (BONAP). 2015. North American Plant Atlas. (http://bonap.net/napa). Chapel Hill, N.C. [maps generated from Kartesz, J.T. 2015. Floristic Synthesis of North America, Version 1.0. Biota of North America Program (BONAP). (in press)].
8. Missouri Botanical Garden (n.d.) Opuntia macrorhiza. Retrieved from: http://www.missouribotanicalgarden.org/PlantFinder/PlantFinderDetails.aspx?kempercode=a248 
9. Eastern Colorado Wildflowers (n.d.). Opuntia polyacantha. Retrieved from http://www.easterncoloradowildflowers.com/_c_3frame.htm 
10. Tomlinson, N., & Robinson, W.(2008). ECHINOPSIS. Retrieved using Internet Wayback Machine from:http://web.archive.org/web/20080720010527/http://cssaustralia.org.au/content/view/72/104/
11. Schneider, A. (n.d.). Pediocactus simpsonii. Retrieved from: http://www.swcoloradowildflowers.com/White%20Enlarged%20Photo%20Pages/pediocactus%20simpsonii.htm   . 
12. Cactus Art Nursery (n.d.) Pediocactus Simpsonii. Retrieved from: http://www.cactus-art.biz/schede/PEDIOCACTUS/Pediocactus_simpsoni/Pediocactus_simpsonii/pediocactus_simpsonii.htm 
13. United States Department of Agriculture [USDA]. (n.d.). Escobaria vivipara (Nutt.). Retrieved from: http://plants.usda.gov/core/profile?symbol=ESVIV 
14. Cactiguide.com (n.d.). Genus: Escobaria. Retrieved from:http://cactiguide.com/cactus/?genus=Escobaria 
15. Mauseth, J. (n.d.). Cactus spines. Retrieved from: http://www.sbs.utexas.edu/mauseth/researchoncacti/spines.htm