In late April and early May 2024 I was fortunate enough to be able to join a botanical tour of Baja California, Mexico between Tijuana in the north and Cabo San Lucas in the south. This floristically diverse region is filled with unique species, many of them endemic to the peninsula only, and is the sort of destination that is very fulfilling to travel to if you are inclined towards natural history and ecology. One of the highlights of the tour for me was finally visiting Ferocactus diguetii in habitat. This is among the largest and most massive of the barrel cacti and it’s a plant that I’ve known about for over 30 years, but had never visited before despite multiple prior trips to Baja. The fact that they grow on islands is the primary reason for the lack of easy access. Read onwards if you find the notion of giant cacti on remote desert islands as compelling as I do….

Ferocactus diguetii is probably the largest of all of the Ferocactus species, and the second largest of the barrel cactus group plants. The only one that seems to often become larger is Echinocactus platyacanthus, which occurs in mainland Mexico, primarily in the southeastern, eastern, and northeastern portions of the country. When I say “largest” I am speaking in terms of averages at adulthood for species that are typically single-stemmed, as opposed to occasional larger specimens in typically smaller species or multiple-stemmed ones. As with most things biological the term “largest” is squishy and malleable and contains plenty of exceptions or variances in how you define the parameters. Suffice it to say that F. diguetii often reaches gargantuan sizes for a non-columnar, non-woody, single-trunking plant.

One other barrel cactus that also achieves huge sizes is Ferocactus pilosus, a species from northeastern and east central Mexico’s Chihuahuan Desert. But that one is often multi-trunked, branching basally from ground level and sometimes having anywhere from 4 to 12 additional subtending stems emanating from the main central mother stem. While individual stems of F. pilosus are usually thinner and less massive than F. diguetii, in aggregate the clumps are just as heavy (or more so) than the usually singular trunks of F. diguetii. I’m sure that people will see why categorizing any one species as being “the largest barrel cactus” (as I am trying to do) needs to have various caveats and exceptions applied in the process. Regardless of the standards used, each one of these three species under discussion are utterly magnificent!

Many individuals of Ferocactus diguetii achieve heights of 8 feet/2.5 meters, and exceptional ones top out at over 13 feet/4 meters tall. This one was the second largest that we observed on our trip, topping out at roughly 10 feet/3 meters tall.

Found only upon several of the islands in the Gulf of California, miles offshore from the Baja Peninsula itself, Ferocactus diguetii has several subspecies that differ slightly from one-another due to long-term genetic isolation upon their insular homes. All share the trait of island gigantism, which is an ecological phenomenon whereby certain species become larger than average when compared to their more accessible relatives on larger landmasses. Gigantism can arise as the result of the safety afforded to island denizens, since predators are scare or nonexistent. By “predators” I mean the creatures that might eat the plant or animal in question, even though we usually think of predators as being animals that eat other animals. But to a barrel cactus, deer, cows, or bighorn sheep act in the same predatory niche as wolves or lions might to those same herbivores in the opposite direction. Safe from the hooves and mouths of ungulates, many island plants end up becoming larger than their mainland counterparts that are exposed to such pressures.

Here is an average-sized Ferocactus diguetii standing about 5 feet/1.6 meters tall. This would be a rather large barrel cactus in most other species, but here the safely isolated island ecosystem lacks many of the burrowing rodents and chewing ungulates that might otherwise chew into, undermine, or knock over barrels this size. Growth can easily continue interrupted for many decades or even centuries more, allowing them to become much larger than this over time.

The largest population of Ferocactus diguetii with the most numerous giant individuals occurs on Isla Cerralvo, the southernmost of the major islands in the Sea of Cortes. Located about 30 miles southeast of the city of La Paz, capital of Baja California Sur, Isla Cerralvo is part of the Cape Region at the southern end of the Baja peninsula. Other subspecies occur farther north on Isla Santa Catalina and Isla Carmen, and upon a few smaller islands. To my knowledge they do not occur upon the Baja mainland at all, despite being located only a relative few miles away and being populated with a number of other Ferocacti.

One could be excused for mistaking this Ferocactus diguetii trunk as belonging to a woody, arborescent cactus such as a saguaro or a cardon. While this plant is clearly columnar in shape, it probably wouldn’t be fully defined as a columnar cactus species the way the saguaro, cardon, and many others are. This is not an exact science, but in the cactus world the term “columnar” tends to refer to species with woody structures inside the core of the stem that enable tall heights and multiple branches. Whether the highly lignified parts are comprised of ribs fused together, or a solid rod in the core of the trunk, this wood is an important part of what makes some cacti able to become so tall and large, and to have multiple branches.

Barrel cacti (Ferocactus/Echinocactus primarily) lack this wood, and despite the large sizes they do achieve there are inherent anatomical limitations upon them once they become too heavy for internal turgor pressure and non-woody cellular structure to support alone. As such, despite their impressive sizes, barrels can never become nearly as tall or heavy as arborescent cacti such as Carnegiea gigantea (saguaros) or members of the genus Pachycereus (cardons) in Mexico or Trichocereus in South American countries, among many others. Barrels simply lack a woody core and that eventually restricts their ultimate height.

A majestic 13 foot-tall (4+ meter) individual of Ferocactus diguetii looks over the Sea of Cortes (~Gulf of California) from Isla Cerralvo, with the Cape Region mountains on the Baja peninsula in the distance. The large alluvial fan of sediments in the distance is the result of flash flooding out of a canyon, brought by hurricanes that rake across the region between July and October most years. Not every year sees a major hurricane while some years see two or three, but some sort of late summer or early autumnal tropical cyclonic system usually brings much-needed precipitation to this arid island every year. That said, sometimes droughts lasting two to three years can persist, challenging the survival of every terrestrial organism that lives upon the gulf islands.

Other large cacti also survive upon the gulf islands in the Sea of Cortes. Seen here is a nursery full of seedling cardons (Pachycereus pringlei) growing just a short distance from the high-tide line of the beach.

Cardons must evidently have some resistance to salt spray and high nutrient loading since they are sometimes the only major tall plant growing amidst dense colonies of breeding seabirds that perch upon the cactus stems and deposit layers of sodium, nitrogen, and phosphate rich guano around the cactus roots. This combined with the aridity makes survival challenging for most species upon these islands, especially the small ones that the seabirds favor. Larger islands with sizable surface areas such as Isla Cerralvo have better conditions for desert-adapted plants and therefore higher biodiversity in the plant life. In the background you can see organ pipe cactus (Stenocereus thurberi) and senita cactus (Lophocereus schottii) also populating the rocky slopes. The giant barrels are also found mixed in, although none are visible in this photo.

Ferocactus diguetii flowers have a slight range of colors from pinkish-yellow to reddish-orange. Like most barrels they can start flowering while still quite small and will continue doing so annually throughout their long lives, which can span centuries in some species including this one.

Here’s another Ferocactus diguetii exhibiting a slightly different flower color from the previous individual. Note the dense white apical wool. This is a common feature in many barrels (not all), especially in the species originating in very hot and sunny places such as the coastal deserts of Baja and Sonora, or comparatively cold-winter environments such as the Mojave Desert in Nevada, Arizona, and California USA. Apical wool is useful for protecting the meristematic cell-division point at the top of the cactus stem where new growth occurs. It reflects excessive heat and solar radiation and insulates against frost in both applicable seasons. Wool also shelters the developing flower buds and newly-pollinated fruits as well. It tends to wear off on areoles that are more than 2-3 years old, but by then the tougher tissues and lack of new growth or reproductive structures obviates the need for it, and normal photosynthesis can occur on the green skin beneath the spines.

As is the case with almost all other barrels, flowers are borne in a ring around the top of the plant just outside the meristem zone where cells divide and produce new areoles and spines. Usually areoles that are 1 to 2 years old are reproductive, with little to no flowering on older areoles located farther down the stem. Once a given areole has bloomed in a barrel cactus, it won’t ever do so again. Only one flower is created per areole. This is a common pattern in most cacti, although exceptions do exist where older areoles lower down the stem can flower on some species (Trichocereus), multiple flowers can come out of one areole (Lophocereus, Myrtillocactus), and all of the above plus repeat blooming annually can occur in some genera (Neoraimondia). In both Ferocactus and Echinocactus barrels, however, one flower one time only is the norm. Fruits are 1 to 2 inches long, spineless, scaly, and contain hundreds of small black seeds. The fruits are not particularly tasty although they are technically edible.

Ferocactus diguetii can grow equally well upon stony, exposed slopes and in the flatter, brushy areas dominated by various types of thorny or resinous, milky-sap plants. Seedlings have to compete with the bushes for many years but eventually they will top out over the crowns and enter life in the full Baja sun.

Ferocactus diguetii also does well on exposed cliff faces, especially upon deeply fractured ones where seeds lodge and seedlings can germinate. Plants might not reach their most enormous sizes here but they can still become quite large, and dense colonies might develop in some places.

A relatively small Ferocactus diguetii grabs a foothold in some seemingly soil-free clefts on a vertical cliff face. This plant was around 6 inches/15 cm diameter and is approaching the age at which it might begin blooming.

It is amazing how a barrel this large and heavy can remain clinging to life in a situation such as this one. Not only is it growing in nearly zero soil on a cliff face, but it somehow germinated beneath an overhanging rock. At least one large, muscular root is partially visible at the base, although there may be another one or two hidden by the plant’s body. How a couple of rather tenuous roots can both find enough moisture AND provide physical support to a plant that likely weighs at least 60 to 80 lbs (~30 kg) in a vertiginous location is utterly remarkable.

An osprey nest is visible to the center right of the photo, replete with least two chicks. My zoomed-in phone camera doesn’t provide all that much detail and the chicks aren’t visible, but I liked the nest situated amidst the barrels and a juvenile cardon nonetheless.

One of the parent ospreys rests atop a cardon cactus a short distance from the nest. A classic Baja Sea of Cortes scene if there ever was one.

Ospreys were pushed towards the edge of extinction in the 1960s and 1970s by the widespread use of the pesticide DDT. DDT has the ability to bioaccumulate in the food chain, becoming more concentrated in the cells and tissues of top predator organisms that sit highest on it. Once a critical threshold of DDT toxicity is reached, many birds are unable to successfully lay eggs or incubate them to completion since the chemical interferes with eggshell production. The thin-shelled eggs break easily and subsequently die, leaving the birds with great difficulty in producing enough offspring to keep the species afloat.

This problem of bioaccumulation and egg loss was proven to occur in numerous birds of prey like ospreys, peregrine falcons, and bald eagles, as well as in brown pelicans and other fish eaters. Once the use of DDT was banned in most places globally, bird populations were gradually able to recover over the next few decades. When I was a child seeing an osprey was a rare event and a cause for excitement. Now they are much more abundant and nests are common in both populated and unpopulated regions again. I still find them to be a cause for at least some level of excitement 50 years later, however….

The smallest seedling Ferocactus diguetii I have ever seen gets its start in yet another rock crevice. This little plant is the size of a fingertip and likely only 2 or 3 years old. Perhaps someday it will join its nearby cliff-dwelling brethren in becoming large enough to flower and help perpetuate the species. And this concludes my appreciative essay on the largest Ferocactus on earth. Thank you for reading.

To join a future Baja California tour or any one of a number of other wonderful botanical destinations across multiple countries, consider signing up with Guillermo Rivera and www.plantexpeditions.com . I highly recommend him myself and so do many of my other plant friends who have also been on one or more of his trips.