Good Field Guides for Opuntia

I’ve been trying to learn the Opuntia species and I can’t seem to get it down. I’m located in Texas, US, and we have quite a high diversity of species scattered across the state. I’ve been trying to study the species throughout my time on Inat but I can’t seem to get it down. I am a very visual learner, so I generally go through field guides/other observations and cross-reference plants in habitat (or vice versa) in order to learn a species. This hasn’t really worked with Opuntia and I can’t seem to find a good field guide. If anyone has any good resource recommendations they would be greatly appreciated!

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Cactus of Colorado is really good, but I’m not sure how many species will overlap

Opuntia is a difficult genus, only made more challenging by poor taxonomic treatments in the past.

The fact is that in the latter half of the 20th century, after a substantial amount of names had been published in the 1800s and early 1900s, the trend for a few authors became to over lump the genus down to a few “main” species and explain away everything else as hybrid swarms, with the genus seemingly tending toward one giant hybrid swarm with no reproductive barriers. When really, a few years of field experience with the plants would likely provide any competent botanist with the knowledge that this notion is false. While it is true that hybridization does happen in Opuntia, hybrid swarms do exist in areas, and they are more promiscuous and morphologically plastic than many genera, hybrids are the exception and not the rule. You can draw many parallels to Quercus in this way.

Not only did this baseless trend confuse those attempting to understand the genus even more, but it has had serious negative impacts on the conservation of entities that have been completely ignored and reduced to synonymy with 0 explanation whatsoever. One of the most egregious examples of this is the narrowly endemic Opuntia zebrina which was first described by John Kunkel Small in 1919. After being poorly known by many for decades, Benson (1982) reduced it (and many others) to a synonym under O. stricta/O. dillenii. This treatment, out of convenience, was copied over several times by many sources and not once backed up with field work at the very least. A species can’t be assessed or monitored and listed if it has been almost completely forgotten for decades, and now O. zebrina is likely deserving of critically imperiled status, being known from only a handful of sites which continue to be ravaged by rising sea level, storm surge, and the invasive Cactoblastis cactorum. It continues to survive at its few remaining sites where it grows next to Opuntia dillenii s.s. and produces no known intermediates.

Opuntia is not alone in being shrouded by this kind of thinking though, pioneer of North American Rubus taxonomy L. H. Bailey had this to say about the situation with Rubus, which I feel applies very well to that of Opuntia as well:

“It is common opinion that Rubus is botanically the most difficult genus in North America, particularly in respect to the native blackberries. Two reasons are mainly responsible for this attitude: prejudice against understanding the plants; lack of field experience and of adequate herbarium materials. When these defects are corrected, the genus lends itself readily to systematic study.

Scraps of specimens without annotations are mostly hopeless, particularly when the possessor of them does not know the plants to which they belong. As such collections have accumulated, the practice has developed of assuming that what is not clear is the fault of hybridity, and confusion has thereby been multiplied. No other North American genus has been treated so shiftily.

Much of the difficulty in understanding Rubus is due to the admixture of forms in clearings, abandoned quarries, burned-over fields, and similar places where invading vegetations have not become stabilized and the less adapted elements are not yet defeated. These areas appear to be full of nondescript Rubi, as if they were hybrids and incapable of definition. These cases are clarified when one recognizes the fact that about one hundred and fifty species of American birds feed on the fruits of Rubus: one would expect such medleys in any areas adapted to receive them, al though the extent of actual propagation of species by these means yet awaits investigation. The fact that one does not recognize the forms only means that the species of the neighborhood are not understood by the observer.

I find a strange reaction among collectors to the effect that when two or more species of Rubus grow together in a small area they must there fore be variants of one another or hybrids; yet the same area may support a dozen kinds of grasses or several species of ferns and no questions are raised. In a certain place two species of blackberries grew together, to the confusion of the collector; yet there was no confusion because Impatiens pallida and I. biflora were side by side in the same locality. In a wild place about fifty miles from my home Rubus canadensis and R. allegheniensis are intertangled over a considerable area, as if they came from common roots; yet several hours of digging out the bushes by two of us disclosed the root systems to be completely separate and distinct ; the plants had grown from seeds, undoubtedly dropped on the clearing by birds (Gent. Herb. 373). Before me sheet of Carex oxylepis collected by Svenson in Alabama with statement that the plant was growing with C. blanda, С. cherokeensis, С. flaccosperma С. willdenovii; we do not offer such in formation on labels of Rubus.

Collectors have mostly taken Rubus along with other plants in general field work or have satisfied themselves with uncritical materials in herbaria or within particular territory. Result is that Rubus has been the most neglected of any of our large and widespread genera. A genus not understood until collected.

Blanchard, who was an inveterate field man, asked what would be the result in Aster if we “pick out a half-dozen and then call all others hybrids of those” (Gent. Herb. 273). Contemporary authors in America do not accept it as a method in taxonomy. Inability to determine specimen does not indicate hybridity.”

Despite those who still continue to explain away things that are difficult to identify as hybrid swarms, there has been some decent progress in Opuntia taxonomy in the past few decades. The morphological plasticity of Opuntia, history of reticulate evolution, and the “regional looks” of Opuntia species resulting from these two things makes keys very difficult to write, but it can be done. Perhaps one of the best Opuntia keys ever written is that in Powell & Weedin’s Cacti of the Trans-Pecos and Adjacent Areas. Of course, that only covers the far western region of the state, so your best option is for the rest of the state is going to be Opuntia Web (https://www.opuntiads.com/), which has entries for most known species in Texas and other states, including original descriptions, general info, herbarium specimens, and photos. It is not so much a field guide as it is a database of organized and useful info on published names in the genus, but it is without a doubt one of the most valuable resources for those looking to understand Opuntia in the US. Such a convoluted taxonomic history involving long ignored and poorly known entities has meant that the only option for understanding the Opuntia diversity of some areas is using sites like Opuntia Web and going back to the original descriptions, matching them to the populations in the field, and observing how they “behave” in habitat, both alone and in regards to how they interact with other entities. Another resource you may find useful is the book Cacti of Texas in their Natural Habitat by Gertrud (@gkonings) and Ad Konings (@adkonings).

Feel free to tag me or @davidferguson in any iNaturalist observations you’re uncertain of also.

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I love the explication by @aidancampos , but as a reality check, it may be decades before there is a good treatment of Opuntia, and it may very well involve scientific concepts that haven’t even been developed yet!

In the meantime, it’s less important to try to make a species-level id than to make scientifically useful observations. Closeups of glochids, shapes of spines, old and new pads, flower details, fruit details, etc etc. Go back a month later and take some more pictures. (I had the singular pleasure of finding old Wikipedia photos of mine used to illustrate a new species, Heterotheca excelsior; I’m glad I took care to get a lot of detail into those images!)

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I knew that the genus had a lot of convoluted taxonomic history, but it appears that my understanding was only the tip of the iceberg. That’s all the more reason to study the species further. Thanks for the recommendations!