Care Diary: Thelocactus hexaedrophorus

I picked up a few in early 2020 thinking I’d grow them for a few months and then sell them; instead, I still have all 4 in the hopes I can set seed and produce my own! They stay small and are highly rewarding little cacti to grow.

Size when I bought my first 4 in January, 2020
(4″ pot)

Natural Distribution 

Thelocactus hexaedrophus is native to the north eastern region of Mexico, specifically Nuevo Leon, San Luis Potosi, and Tamaulipas. There’s two forms – nominate and ssp. lloydii, which seem to be differentiated by a subtle difference in tubercle appearance as well as elevation. The nominate form of T. hexaedrophus grows at about 1000 meters to 2000 meters above sea level, while T. hexaedrophus ssp. lloydii grows at about 2200 meters to 2300 meters above sea level. 

In the wild, it can be found growing in large numbers mixed alongside numerous other cacti species. It’s commonly found in the savannahs and grasslands of the region, neighboring other species such as Myrtillocactus geometrizans, Ferocactus latispinus, Opuntia imbricata, and Astrophytum myriostigma.

The cactus grows as a single stemmed plant, generally globe-shaped, rarely taller than about 3″ and about 6″ in diameter. Has fat, prominent tubercles that can develop some farina on the flesh, and usually slightly flattened on the tips rather than being pointed. Spines can vary in color, length, and quantity, typically dependent on the exact locale or cultivar. 

Cultivation Requirements 

These are nearly as easy to grow as an Astrophytum, which is logical given that they’re often found in close proximity to the common Bishop’s Hat. The hardest part when it comes to these unassuming little cacti is simply having patience for them to grow. 

Soil 

A nice, well-draining cactus soil is ideal. When I repotted mine, I used about 50% pumice with a quality succulent soil to ensure good drainage. They do have a tap root, so ensure your pot is deep enough to accomodate it without there being too much excess soil around the cactus. 

The key is that the soil dries within a few days of a thorough watering. You should only need to repot your Thelocactus every 2 to 3 years, and when you do, leave it dry for at least a week after repotting to ensure the delicate roots have time to heal. 

Two of my Thelocactus in early 2021, about a year after purchase. 

Watering

Thanks to their thick taproot and then fibrous smaller roots, they like regular water in summer but not too much. As long as your soil drains well and dries within a couple days of a thorough watering, you can water every few days in summer when the cactus is in peak growing season.

While they’ll accept plentiful water in summer, they don’t necessarily need it. They tend to take the water and put it right into (relatively) rapid growth. This can be a boon if you’re trying to grow a decent sized show plant in a shorter period of time, but it also has the opposite effect. The excessive water tends to make the cacti grow tall and lanky, looking overly plump and untidy if paired with too much shade in addition to too much water. 

Light

As with most cacti, these need nearly full sun – but as these are a smaller species that most often grows in the shelter of larger plants, they should be protected from the brunt of midday heat and full sunlight. Full sun during summer months will burn your Thelocactus, particularly if it is a hot day. I learned this the hard way when first moving mine into my greenhouse last year! 

You’ll notice the burning is closer to the sides and soil line on my cacti at left; that’s because the burn came from the sun coming up, rather than directly overhead, and was combined with the rocks on the substrate being quite warm. 

The first few days of my greenhouse being fully constructed I hadn’t realized it wasn’t quite ready for plants (it needed the fan installed), and these sorts of scorches and burns were pretty common. 

Fortunately, if the burn is mild or small, the cactus will outgrow it given enough time. 

The important takeaway is to slowly acclimate your new or young Thelocactus to hotter or brighter sunlight exposures, and to protect your plant at all stages from intense sunlight and heat. Ideally, your cactus will experience full sun in the morning up until around 12- 1pm, and then should be shaded until the worst of the heat is over, with full sun being acceptable after 3 or 4pm. 

Fertilizer for Thelocactus

These easy going little cacti appreciate a dilute, balanced fertilizer during their growing season, or one rich in Potassium and Phosphorus but low in Nitrogen. You can use a dedicated cactus fertilizer (which should be formulated to be low in Nitrogen), which I did for the first two years I had these cacti, or a balanced fertilizer at no more than half strength. I currently use a 20-20-20 fertilizer at 1/4 strength every other time I water, which is once or twice a week at least during peak growing season. 

I also have my cacti in a fairly high inorganic mix (mostly pumice), so they need more fertilizer than a cactus growing in more organic medium (more soil). If you have 50% or less pumice in your soil mix, you shouldn’t need to fertilize more often than once a month or so. Too much fertilizer, particularly fertilizer with more Nitrogen in it, will result in plants that are soft and retain too much water, making for poor growth and increased risk of rot.