That’s how I ended up with my Obregonia denigrii. It was peak COVID season, the plant bubble was looming large, and like everyone else, I was stuck at home with little else to do except ogle photos of other people’s plants online. Some specimens get insanely fluffy, like this Obregonia denigrii from the Intercity Show in 2005. Several other specimens pictured online at the time seemed to be equally fluffy, although in hindsight, these were probably grafted specimens from Thailand or South Korea. Whoops.
I saw the notable @cactusupdate post one of these for sale, and decided I’d give it a go with absolutely zero knowledge of the species beyond the fact that it existed and would get bigger. Normally, this would be a terrible idea, but as I came to learn – these are closely related to Ariocarpus, and are something of an inbetween of Ariocarpus and Lophophora. I happen to be in the perfect climate and area for growing Ariocarpus, and as such, growing the Obregonia turned out to be much more straightforward than I was expecting.
Obregonia denigrii in habitat
These little cacti are native to a very small area in Tamaulipas, Mexico, where they share their growing area with Ariocarpus trigonus and Astrophytum myriostigma. The semi-desert area gets regular rain, but these cacti grow in limestone soil thick with gravel and stones. The soil is poor, lacking in organic matter, and as such, dries out rapidly and has little in the way of nutrients.
The plants develop thick tap roots, which hold moisture to hold them over during dry spells. In habitat, they’ll sink down to soil level during dry periods. They’re often found in the worst soils, where erosion is a regular concern and the limestone can harden into a substance almost like a cast over over the soil.
For an idea of what the plants look like in habitat, I’ve linked a photo from Llifle here of one seen in habitat in San Jose de Salamancas, Tamaulipas.
Obregonia denigrii at the SDCSS Winter Show and Sale, February 2022
Growing Obregonia denigrii in cultivation
In habitat is one thing – but in cultivation, these are a different beast. Llifle describes them as incredibly challenging as seedlings, taking 4 or 5 years to reach 1 or 2 inches in diameter. That’s about as long as one thumb or finger joint, for me, and most of my other cacti species that I’ve grown from seed reach that size in 2 years or less!
Slow growing often means hard to grow, as there’s very little room for error. When a plant is only producing small increments of growth, its very susceptible to overwatering or soil that’s too rich. So to that end…let’s talk soil!
Obregonia denigrii June 2024
Soil and Potting for your Obregonia denigrii
Slow growing and with a thick taproot, you don’t want to overpot these. If you’re just getting started (as I was), you’ll want to stick to a pot a little bigger than the plant itself. Mine at left was about 3″ across when I potted it into a 4″ square pot.
As yours increases in size and you become more confident in its growth and general needs, you can upgrade it to a larger pot with an eye to how large it may get. These mature to about 6″ across, at most, making them excellent as container specimens and rarely needing particularly large pots.
When it comes to soil, it’s important to remember how they grow in habitat: in very poor, very gritty, and very fast-draining rocky soil with little organic matter. Their appearance in habitat is almost flat, with each leaf-like tubercule curved and nearly flat against the one below it.
Almost all specimens you see in cultivation will be better watered and plump compared to a wild counterpart. While they’ll enjoy more water and richer soil, providing too much of a good thing can lead to a plant that’s so overfed it’s prone to rot, disease, and pests. When allowed to stay too moist or to stay too plump for too long, the tap root is prone to rotting, and it gets a plump, stretched appearance that rapidly becomes unattractive.
A soil mix similar to Ariocarpus works well, something well-draining and high in inorganic material. The ingredients will likely look familiar, but the ratio may be surprising:
For me, with greenhouse conditions (120F+ summers, and 80F+ days 10 months out of the year), I lean more into the organic ingredients in that list: a touch more orchid bark and a little more soil. My greenhouse gets very hot but also very dry, with a fan moving air and lots of sunlight.
You can look up weather for a city very near to the small population of these cacti in habitat, Ciudad Victoria, and you’ll see the average summer temperatures are between 90 and 95 for at least 8 months of the year – but they get more rain than we do here in San Diego (nearly 35 inches a year on average compared to our 6 to 10 inches). So: they get a lot of high temperatures, but they also get rainfall on a regular basis year-round, with the least amount of rain falling in winter months.
Being in even hotter weather and experiencing zero natural rainfall in summer, therefore, means I need to offer options to keep the cactus engaged and actively growing during the hot summer months it prefers to grow.
I describe all of this because if you aren’t in a super hot climate for summer months, and your Obregonia isn’t outdoors in roasting high temperatures, you need to use less of the organic material in your soil. If your summers are hot, but humid, with lots of sunshine and baking heat, this cactus should be outside and in 75% pumice with 25% cactus soil, and no orchid bark at all. If you’re in an area with lots of clouds and summer rain, with summer temperatures rarely getting above 90F, you may even want to go purely inorganic and just sprinkle some worm castings on top of the soil in spring and call it good.
These guys need some crappy, nutrient-poor soil to grow in, and only thrive with more organic material if they’re hot and bright enough to grow. Which brings us to the next part of the trifecta of successfully growing cacti…
Sunlight for your Obregonia denigrii
This is not a species that can grow indoors or in cloudy climates without a LOT of help.
They’re well adapted to extremely bright, intense sunlight, and without direct sunlight they will slowly die.
As with all slow-growing cacti, the death may be slow enough that you think you’re okay. It’s fine. It’s not stretching, it’s not overtly rotting – you’re fine, right?
Nope.
In summer months with adequate light, you should see new tubercules emerging from the middle of the cactus and slowly increasing the size bit by bit. A true sign of proper lighting is that it blooms for you in mid summer, with cute white flowers showing off a reddish center.
Below, I’ll show my own plant with a couple months of summer growth. These don’t get huge, but when you’re watching them closely, the growth is noticeable.
Obregonia denigrii, winner of the Intermediate division of the summer show & sale in 2023
August 2022, when I first acquired the cactus
November 2022 – see the outermost tubercules already poking over the side of the pot?
If you look closely, you can also see one edge of the pot already bulging from the tap root, thanks to me potting it off-center in the square pot. In the summer months, when these are happy, hot, and brightly lit – you’ll see noticeable growth!
I have mine in my greenhouse, where daytime highs in summer are 120F+ for at least 5 months of the year, and over 80F for nearly 10 months. The greenhouse is situated in full sun, so during summer months, I have 40% shade cloth placed on the ceiling. I keep these near the front, where they get some morning sunlight without the benefit of the shade cloth, and that has seemed to help with nice, compact growth.
If you have to consider growing yours indoors or against a windowsill, I strongly recommend trying to grow an Astrophytum myriostigma for at least a few months before springing for one of these. If your A. myriostigma continues to grow in a nice, compact manner, with no stretching, continued speckling (if it’s not a nudum variety), and general nice, tidy appearance, you likely have your lighting dialed in.
The Astrophytum has to actually grow, not just not die. Having an Astrophytum for a few months that just looks the same, with no change, means it’s likely slowly dying. Use a ruler to measure the height, width, and take photos to keep an eye on the apical growth point (the top/middle of the cactus). No change = your conditions aren’t dialed in.
These guys need it at least as bright as an Astrophytum myriostigma does to actively grow, and thrive with high temperatures. Without that, it’s significantly harder to keep them happy and healthy – so be warned!
My reciept for my claims: first place in the 2024 Summer Show & Sale!
Watering Obregonia denigrii
So, the internet says to water these about once a month during the growing season, and not at all over the winter months.
I ignore all of this and water it on the same schedule as my arios: every week during the hottest part of summer, every two or three weeks in spring and fall, and not at all in winter.
Again, though, I’ll point to my climate: it’s HOT here! My soil dries out fast, it’s really bright, and I see my Obregonia occasionally start to wrinkle and shrink to a less plump and spoiled growth habit. If yours is never showing signs of mild suffering (each tubercule getting thin, wrinkling of the flesh), you should be watering less.
During summer months, it should be peak growing season, so if you’re not seeing any signs of growth – something’s up.
A loose baseline to follow is: if you’re keeping your Obregonia outside, where it’s getting above 90F more than 75% of the time, you can probably be as laissez-faire as I am about watering.
If you’re in an area with more clouds, it’s not as hot as often, or it’s very humid a lot of the time (Florida, I’m looking at you) – you’ll need to play it more by ear. Water less and wait to see signs of water stress before you water them: wrinkled leaves will be your clear sign it’s time to water the obregonia.
Fertilizing Obregonia denigrii
If you’ve potted yours in a highly inorganic mix, especially if you chose fully inorganic due to your climate, fertilization needs to be part of your routine.
For me, I use a simple 20:20:20 fertilizer at half strenght (yep, the same thing I use for everything these days!). I’ve also added some decomposed granite as a top dressing, which likely leeches some trace minerals over time.
If you’re using a completely inorganic medium, something more tailored to cacti may be more appropriate. Without organic material to make up some of the gaps, you’ll need to pay more attention to the nutrients available for your cactus, and a purpose-mixed blend can make more sense.
Given how often I water my plants, repotting them every couple years, or otherwise offsetting the fertilization, I’ve come to realize the balanced fertilizer meets my needs 99% of the time. I also have a lot of plants! If you’re babying a handful of them, then by all means – I highly encourage getting tailored fertilizer for them.
I do suggest including fish emulsion and/or worm castings as part of your fertilization routine if you’re using a highly inorganic soil mix (or even fully inorganic). Adding one or both to the cactus early in the growing season can give enough of a boost to carry through the summer growing months. This species doesn’t need more than one boost with the organic/all natural fertilizers, though. Once should be plenty, and can even be overkill if you repotted your Obregonia denigrii within the last year.
Sculptural and unique, Obregonia denigrii is an uncommon species in cultivation – but worth adding to your collection if you’re seeking rare, compact, and unusual in appearance. Happy specimens produce flowers throughout the summer months, with cheerful yellow stamens, a pop of red center, and white petals.
If you’ve successfully been growing Astrophytum, these should be next on your list of fun species to collect.
While they are most often grown from seed, they’re not self-fertile, so to produce seed you need at least two specimens that are not the same clone (i.e. tissue cultured).
Sadly, as of this post, I only have one specimen – so no seeds for me. But maybe, maybe, someday soon… 😉
