Growing Sulcorebutia arenacea

An uncommon genus is the Rebutia or Sulcorebutia genus, which can form small clusters of plants (sometimes just a single main ‘head’), often with small, dense spines. They tend to stay small, developing large clusters over time, and while cute aren’t always very striking on their own.

Until they bloom, that is. This species, Sulcorebutia arenacea (might be just Rebutia arenacea now) produces sprays of showy, sunshine-orange-yellow flowers in bursts in late winter all through spring. Unlike some other rebutia species, this one is pretty even when it’s not blooming, making it a great candidate to grow for a show plant or just to highlight in your plant collection.

Habitat in the Wild

Found only in a small area of Bolivia in South America, and is known only from this single type locality. While not actively threatened by human encroachment on its habitat due to the remote and small nature of where it is found, it is still considered near-threatened just because there’s so few of them.

Their natural habitat is a seasonally dry valley, meaning they get little to no rainfall through the cold winter months.

Their population is not particularly dense, and they are easiest to spot during the blooming season. They typically bloom in late winter and continue to produce flowers through spring, and in mild years they may even continue to bloom all the way through to summer. 

Soil and Potting for your Sulcorebutia arenacea

Being highly prone to rotting if over-watered, a well-draining soil mix is essential for your little sulcorebutia. Particularly if growing on a windowsill indoors, you’ll need to ensure your soil mix is heavily on the inorganic side to ensure proper drainage, and keep the roots from being soggy for too long. 

I pot mine in a mix very similar to what I use for my Ariocarpus and Astrophytum, despite both genera being from very different climates to this plant. The requirements are the same: keep the roots from staying soggy! 

The mix is pretty simple: 

• 25% cactus and succulent soil 
• 25% orchid bark (the small chip kind)
• 50% pumice 

You can buy a pre-made blend if you’d prefer, there’s a ton of options on Amazon, but I find they tend to go overboard on supplements and additions that make it hard to be consistent. When I’ve used other pre-made mixes, they often go heavier than I like on the coconut fiber, larger chunks of bark, or have inconsistent mixing that leads to dry spots in my pots. I personally buy my materials in bulk at the local nursery supply store, but if you can’t find them locally, the links above should help. 

If you’re mixing it up yourself, you can even use regular houseplant soil instead of a dedicated cactus and succulent blend. You’d just add the houseplant soil in small quantities, mix, and see how it looks. Your blend should look like someone dusted the pumice and bark with a bit of “regular” dirt, but seem like it’s mostly the pumice and bark. Most soil mixes you can buy online to be shipped to you will be either extremely gritty, to the point of being too inorganic (like Bonsai Jack’s blends) or they’ll be too organic (as the soil I linked above). 

Really don’t want to mix it yourself? Here’s a couple of bags that (based on the photos of the soil) may work instead:

• This stuff from Spike & Bloom looks pretty good 
• O-FarFarm is an interesting name, but the mix also looks about right 
• This bonsai mix also looks pretty good 

They all have some organic material (typically peat moss or ground coconut), but lean heavily towards a mix of inorganic materials including various rocks, pumice, and charcoal along with orchid bark. You’re paying extra for the fluff in terms of charcoal and fancy rocks, as the plants will grow just fine in plain pumice, but you do you, boo.  

What is more fun is adding your top dressing, as these cacti appreciate having some rocks and grit around their “neck” (between the body of the cactus and right as the roots begin) to help keep water from sitting right against the body of the cactus. I used ground cover stones that I had on hand, but you can use whatever you’d like. Colored glass, aquarium gravel, or just akadama clay, any of it works. 

Blooming in March, 2022

What’s the deal with the temperatures?

For cacti, temperature and light are huge factors in how much water they need. 

If your cactus isn’t experiencing temperature extremes as it would in its natural habitat, you need to adjust how you care for it to ensure good growth. 

I’d strongly suggest putting your cactus outdoors for as much of the year as possible to experience summer heat and the beginning of winter cold. This will help keep growth compact and healthy, and encourage blooms! 

Want flowers? Let your Rebutia get cold in winter!

Without a winter cool down, where the cactus gets below 40F at night for a few weeks, you won’t find your cactus producing flowers the following season. In San Diego, it’s easy enough for me to provide this by simply keeping the cactus outdoors. If you’re further north, or in the city, you may need to get creative. Keeping them by a cold window can help, or try keeping them outdoors (but dry) up until the first snow or hard frost warning comes in. They can tolerate temperatures to the low 20s F if kept dry, so early winter might be your best bet to trigger a period of winter dormancy. 

Light for your Sulcorebutia

For the best appearance, these need strong light to keep them compact. Mine is in my greenhouse under 40% shade cloth, but would be in full sun the entire day without the cloth.

The more northern you are, the less likely you’ll need the shade cloth even during the middle of the day. Keep the plant outdoors if you can, and under a patio or sheltered roof to keep it from getting rained on when it doesn’t need water.

My fellow Southern US growers, we need to plan for blistering hot summers and intense sunshine. Again, a sheltered patio spot works well, especially if it has shade during the hottest part of the afternoon. Some nice direct morning sunshine is ideal.

April 2024, late start to blooms this year.

Showing the darker green trending towards red coloration of ideal light exposure.

In winter, particularly if growing these indoors, you should plan on supplemental lighting to keep the growth looking nice and compact. Do not use those goofy purple/pink/blue lights – they aren’t bright enough for cacti to do well (at least, not if you’re trying to get the cheap clip-on ones). 

Use a proper grow bulb or even a light bar over your indoor cactus bench to help them get through the winter. Something like these light bars may work, but I’ll admit I haven’t tried it myself. Even other San Diego growers I know who grow plants indoors have admitted they tend to just buy used lights used for growing marijuana, as those have enough light intensity to work. 

A light meter is extremely useful to help determine if your indoor grow lights are meeting your cactus growing needs. This thread from Reddit of a user figuring out the lighting needs for overwintering their cacti is a good example of someone investing in the right sort of lighting and testing to ensure it accomplishes what they need. If you’ve only got the one little sulcorebutia, then a light grow-bulb hung down above the cactus, or pointed at it, along with being next to a window may be enough to get you through the worst of winter. 

February, 2022

Fertilizing your Rebutia

For maximum bloom-splosions, you’ll want to give your cactus some food – but you also don’t want to overdo it before the weather starts to warm up.

I don’t put mine on a special feeding schedule separate from my other summer-growing cactus species, and it seems fine with that. If I had to guess, they store up energy for the following season’s blooms during the summer growing season.

My routine for the last few years has been to use a balanced 1:1:1 fertilizer at half the strength recommended, and I fertilize about once a month through spring and early summer, and then about once a week when the weather is nice in early to late summer. Once we start getting heat waves, I cut back, and by fall I’ve stopped fertilizing completely. 

Why did I link a 20:20:20 fertilizer and not a 1:1:1? Because the two ratios are the same; the higher number on the fertilizer jug just means that the fertilizer is more concentrated, but the macronutrients are still in equal proportions. This makes that fertilizer extremely cost effective (more bang for your buck) and a small jug lasts quite a while. 

You can get a dedicated cactus and succulent fertilizer, which is what I did for several years when I first started, but I realized I was just paying more for less fertilizer. 

I also used Fish Emulsion or Fish Fertilizer for a while, but I didn’t see any real difference apart from my greenhouse stinking to high heaven for days afterwards.