How to Grow Frailea castanea

frailea castanea

Written ByJen Greene

Posted: June 5, 2024
These tiny little cacti are adorable and remarkably easy to grow. With few spines and a small adult size, plus how well they thrive in partial shade, these are ideal for windowsill cultivation. Because they also self-fertilize (even without their flowers opening!), they’re excellent candidates for learning how to collect and grow cacti from seed!
frailea castanea

When I first received the plant, February 2022

Habitat and Distribution 

Found in south Brazil and northern Uruguay, these tiny cacti grow in stony fields and rocky outcrops. Because they are often found in grasslands, one of the biggest threats they face in habitat is from cattle either eating them or trampling them. 

Where they do grow, they are not very dense in population. Unlike seeing Astrophytum or Ferocactus in habitat, these tend to be more spread out. 


While sparse in habitat, these cacti are popular in cultivation and can usually be found quite readily if you search online marketplaces. I see them regularly at San Diego Cactus & Succulent Society meetings, and they pop up occasionally in Facebook cactus groups as well. 

Potting your Frailea castanea

Small size, even when these are adults, means small pots are needed! A 3″ pot will likely be just fine, up to 4″ if your plant is older/larger. They have thin fibrous roots, and appreciate very good drainage – especially during their growing season, when they’ll be very happy with plenty of water. 

frailea castanea

Potted in its show pot, settled in by July 2022

You don’t want your Frailea castanea to have too rich of soil, which can make it grow too much, too quickly. They are already relatively short lived for a cactus, sometimes only living for about 15 years, and giving them rich soil can hasten their demise.

I potted mine with my tried and true well-draining mix, but not quite as inorganic as the copiapoas or other species that really need fast drying roots.

I’ve also potted mine into a ‘show pot’, which is basically a nice pot intended to showcase the cactus. Here in San Diego, we have the big show and sale each summer & winter, and this cactus has made an appearance in recent years! 

On top, I’ve added top dressing just to showcase how pretty the cactus is. Japanese Akadama is popular as a top dressing, but I’ve been using small-pebble black lava rock lately as I like the drama it brings to the pot. Black lava rock can heat up and burn your plants if they’re not slowly aclimated to sun or outdoor heat, so just be cautious if you do use it. The larger chunks are much more likely to scorch your plants than the little ones. 

Watering your Frailea castanea

These are very easy to judge if they need or want water; they’ll look wrinkled and shriveled, and sink down into their soil when thirsty.

When watered well and actively growing, they’ll plump right back up!

They have an active growing season, where they’ll accept regular water – as days increase in length and nights are rarely below 50F, you can expect your Frailea to wake up. I mention “wake up” because they go somewhat dormant in winter, and if nights are below 50F, they need to be kept dry. 

If kept quite dry through winter, mine has tolerated nighttime drops to the high 20s and low 30s without much fuss. Crucially, though, I don’t water them at all in winter. Around October or November, I cut water way back, and by Thanksgiving they are dry and stay that way until there’s no risk of frost – typically March. 

frailea castanea thirsty

Thirsty Frailea castanea in February of 2024, showing how it’s sunken in, not plump, and there’s even a fold at the base 

The Frailea pictured above in February of this year shows how very sad they start to look near the end of winter. This is normal and even healthy! They evolved to have a rest in winter, and it helps encourage blooms to form as the weather warms up in spring. 

Did you know… Frailea can produce seed even if their flowers never open 

Frailea castanea (and other Frailea species) will only open their flowers on the hottest and sunniest days. Anything less, and the flowers never even get to the point where you can see the petals! The Frailea doesn’t worry about that, though. It’s cleistogamous, and it’ll produce seeds all by itself. 

Cleistogamous loosely means “closed marriage” – which refers to plants that self pollinate, which includes peas, peanuts, and pansies! 

frailea castanea seeds

Frailea castanea showing flower buds along with deep purple coloration from sun stress (May 2023) 

I’ve noticed that early in the year, when days aren’t that hot and mornings are often cloudy, my Frailea won’t even get blooms that seem fully formed. They’ll turn into little buds, and seem like they’re growing into flowers, and then suddenly it’ll just be a seed pod. 

Using tweezers to gently wiggle your buds/fruit will help you identify if it’s a fruit body (full of seeds) or if the flower bud is still considering how big it’ll get. 

I can’t overstate how much these really need a hot, bright day (preferably multiple of them) to fully open up their flowers. In the two years I’ve had mine, I’ve seen a nearly open bloom exactly once – and I live in super sunny San Diego! 

frailea castanea bloom

I didn’t realize at the time how uncommon it was for the bloom to open, and didn’t go back out to check on it later in the afternoon. 

All of this to say: if you never see your Frailea castanea bloom, that’s okay. If you’re growing it on a windowsill in a climate-controlled house, chances are, your cactus will never get warm enough to consider opening. 

A last note on water before moving on to light: 

Indoors, or in high humidity climates, you will need to water these far, far less than cacti grown outdoors. Careful watering to prevent too much growth, too quickly, is required, as is keeping the cactus dry in winter. 

It’s important to let your little Frailea suffer just a bit to keep the growth compact and slow: this will prolong its life! 

Lighting for your Frailea castanea 

In habitat, these grow in grasslands and rocky outcrops, where they’re shaded at least part of the day. You can tell when one of these has been grown in shadier conditions (or sunnier ones) by the color: shade-grown plants will be distinctly green-hued, either throughout the entire body or just at the base. 

For the really beautiful, deep purple tones, your Frailea castanea will need slowly increasing exposure to lots of sun. Restricting water can also help encourage that deep jewel hue. 

frailea castanea

Showing more green this spring, May 2024, along with some new growth emerging (the green between the ribs)

frailea castanea stressed

Stressed to the maximum in February, 2023, when it won first place at the SDCSS Winter Show & Sale. 

The color of your Frailea will be your biggest single indicator of light quality and intensity. A plant can be perfectly healthy and receiving adequate light to prevent etiolation even if it’s more green than anything else, but you can be far more confident that your light conditions are appropriate if the cactus is blushing with purple. 

These cacti should also grow with a distinct indent in the center, making them resemble a donut. It can be hard to see when the flower buds and fruit are nestled in the center, but even with those, you should be able to spot that the center is lower than the outer edges of the cactus. 

If you notice the center starting to come up and the top of the cactus looks flat, instead of like someone pressed the center in with a finger, that’s an enormous sign that your Frailea is not getting adequate light (no matter what color the cactus body is). 

It’s also fine for your Frailea castanea to fluctuate in color over the course of the year! You’ll see in the photos I’ve shared that mine has varied from more green to a very deep, dark purple. When I’m preparing the plant for show, I’ll stress it pretty heavily, moving it slowly to more sun exposure and watering it less. The rest of the year, I offer more water and keep it more sheltered from sunlight. 

My little Frailea in September 2022, with fruits that are ready to be collected and seeds harvested

Growing From Seed 

While these are supposedly short lived (at least, according to Llifle), they do produce seed very readily. You should be able to easily collect the seeds from the fruit when the fruits come off the top of the cactus with little to no effort. If you gently tug on the fruit and it still seems attached, let it be. When they’re ready, they’re so loosely attached you can use a fingertip to knock them off. 

I use tweezers to gently open the fruit onto a papertowel and scrape the seeds out. This catches any debris, and if the fruit still has some moist material (such as actual fruit flesh), it’ll dry out on the papertowel. Generally, by the time the fruit is ready to come off easily, the flesh has dried out. 

The seeds will look surprisingly large, and with your handy dandy tweezers, be easy to collect and either save in a paper envelope or sow them directly. I sow mine following the directions detailed here in my resources section on growing cacti from seed; 

Compared to Astrophytum, these are slower to germinate. Heat is crucial, and a seedling mat is a must-have even in spring and summer if you’re sowing them indoors. 

Interested in some Frailea castanea seeds to try for yourself? I have some from my own plant in my shop! Check it out by clicking the “Shop” link in the menu. 

You may also like…