Bishop’s Hat, Monk’s Hood Cactus, Goat’s Horn
The genus Astrophytum contains 6 species of cacti in total, all of which are native to North America. They are popular amongst keen cacti collectors and houseplant enthusiasts alike for their rock-like appearance.
They are sometimes referred to as living rocks although the name Astrophytum translates as ‘star’ ‘plant’ in Greek.
Position Full sun and good ventilation
Watering Infrequently and less in winter
Size Up to 1.5 meters tall
Climate Prefers warmth of 60 to 86 °F
Seasonality Winter dormant
Toxicity Non toxic
Flowers Annual flowers from mature plants
Each of the six species of Astrophytum cacti has different characteristics, most of which are round or column-shaped in appearance with a tendency to grow individually. In their natural habitat, they can reach heights of 1.5 metres.
All species are either green or green-blue and most have varying numbers of vertical ribs. Their outer surface can either be entirely smooth and bald or be covered in tiny white hairs that provide a silvery sheen. Likewise, some species are spineless and others have long twisted spines.
In summer and if well cared for both in terms of meeting watering needs and feeding, they will produce daisy-like flowers. These too can vary in colour from red, yellow, pink or white and often have a beautifully contrasting circle of colour near the centre of the flower.
These perfectly circular flowers are heavily petaled and appear right at the top of the cacti, opening only during daylight hours. Some say they give the appearance of pompoms atop a hat or cap and this is where many of their nicknames derive.
Types of Astrophytum
All of the six species have the same care needs which I’ve set out below in this article but here is a little taster of each of these perfectly individual cacti from this one-of-a-kind genus. Which one will be your favorite?
The Astrophytum Asteria cacti are possibly the best known and most photographed of the six species. This is a spineless variety and is distinctively stubby and star-like in appearance.
It has eight ridges each of which are covered with a row of descending dots and the entire greenish-gray surface is covered in tiny white hairs.
It is an easy plant to care for and is often referred to as Star Cactus, Sea-urchin cactus or Sand Dollar cactus.
Astrophytum capricorne (Goat’s horn cactus)
It’s the long curly spines of Astrophytum capricorne that give it its most commonly referred to nickname of Goat’s Horn cactus.
The 7-9 rounded and ribbed stems soon change to columns as the plant matures and can reach heights of 1m. One feature that remains throughout their growth is the white markings that form a decorative pattern on the stem.
Mature plants will produce beautiful yellow flowers during the growing season.
This is one of the more rare species of Astrophytum and is thought only to grow in one single location in the state of Nuevo Leon in Mexico and is considered critically endangered as a consequence.
It is very different in appearance from the other cacti in this genus with tentacle-like stems twisting and turning from its base. It is known to produce heavily petaled, whiteish-yellow flowers with an orange throat.
This one is something of a collector’s item and is extremely slow-growing.
Another rare and collectible variety is Astrophytum coahuilense. This one is often mistaken for Astrophytum myriostigma however, to the trained eye it is distinguishable by its more delicate grey patterning that covers the plant in its entirety.
This cacti has five ribs and flowers are yellow with a red circle near the centre.
Astrophytum myriostigma (Bishop’s hat or Bishop’s cap)
Bishop’s hat or Bishop’s cap as it is aptly nicknamed is another well-known and well-photographed variety of the Astrophytum genus. Similar to Astrophytum coahuilense it is covered with tiny white flecks and has five ridges each of which has markings or dots that descent from the central point at the top.
As a mature plant, this species will produce impressive yellow flowers with a red throat in summer.
Astrophytum ornatum (Monk’s hood cactus)
This is one of the largest and easiest to grow. It’s impressively fast growing too and can reach heights of up to 1.2m.
It has a dark green outer surface, approximately 7-9 ridges, and is star-shaped. And the flesh is dotted in white woolly scales. This one has ferocious-looking spines and grows in clusters from areoles that are spaced all over the cacti.
As is true of all plants, there are a few rules to follow in terms of understanding and mastering the basics of their care, and once you have nailed that, it should be plain sailing from then on.
Read on to find out everything you need to know about growing, nurturing, and caring for any of the species of Astrophytum. From what soil you should be using to when and how often you should be watering.
I’ll also be sharing tips on how best to propagate and the problems you need to look out for in order to keep your Astrophytum in tip-top condition.
In their native North America, mainly in areas of Mexico but also Texas, Astrophytum cacti are accustomed to intense desert heat and dry, arid terrain. Indeed, they will thrive and grow to their biggest in these conditions.
If you do live in such climates, then position your Astrophytum in full sun but provide a little shade during the most intense periods of heat.
When growing elsewhere, these cacti can still thrive provided they are brought indoors and are positioned on a windowsill that gets full sun for a large part of the day. Make sure they are well ventilated to keep them as dry as possible but avoid drafts and any potential exposure to frost.
Your Astrophytum will be able to withstand moderately lower temperatures in winter. In fact, moving it to a cooler position will help to encourage flowering in the following year.
Height & Spread
When grown in their natural habitat and depending on the particular species, an Astrophytum cactus can grow from 30 centimetres to 1.5 metres in height and between 10 to 20 centimetres in diameter.
These impressive dimensions are severely reduced when any of the species are grown in pots and especially when grown indoors. Understandable I guess.
These plants need a different watering schedule depending on the season. During the winter when they are dormant, they need very little water, if any at all.
Your plant will still grow during dormancy, however, it does so at a much slower rate and so has less requirement for water and nutrients.
I recommend checking the moisture level of the soil once per month from October to March. If the soil has any moisture at all, then hold of watering.
During the summer months, check the moisture of the soil every 2-4 weeks. You want the top 1 inch of soil to be completely dry before you make any attempt to water your Astrophytum.
My personal preference is always to be guided by the moisture in the soil rather than adhering to a regular watering schedule. It stands to reason any difference in temperature will affect how damp the soil is and it’s not all about diarising when your plants need water.
To test the dryness of the soil press your finger at least a quarter of an inch into the soil or prod the soil around the drainage holes in the bottom of the pot. If the soil feels completely dry, you can go ahead and water your cactus.
Keep an eye out for any signs of overwatering. These include soggy, mushy, or brown color around the base of the cactus. This is usually just below the surface of the soil line. You may notice that your cactus reduces or even stops growing. If you do notice this then stop watering immediately and consider repotting your cactus in fresh, well-draining soil.
I never advocate the use of a spray bottle or mister as a watering device for cactus. This is because their native habitat is dry and arid and they are not used to the high levels of humidity that a water mister can invoke. Also, allowing them to remain damp for extended periods will cause them to wilt or rot.
Astrophytum need a well-draining soil so their roots remain free from excess moisture and to allow them to get the oxygen they need to thrive.
I use coco coir or peat-free soil that I mix with minerals such as grit, sand, or perlite. Ready-mixed cactus and succulent soil is the easiest option and there are many to choose from with availability to either purchase online or at any good garden-supply retailer.
I’m a big fan of Espoma Cactus Soil Mix because it’s made from 100% natural ingredients.
It contains Espoma’s trademark Microbiome formula to help improve soil condition and promote root health. My cacti have always looked all the more healthy for me using it.
Alternatively, you can easily make your own good-quality well-draining cactus soil at home. Start with one-third organic matter such as a good quality peat-free compost or coconut coir. Mix this with two-thirds of minerals such as grit, sand, or perlite.
Planting in the right type of soil is going to encourage healthy growth and I find it’s a good idea to mix in a little extra grit to shop-bought cacti potting soil and to always sprinkle a layer of grit over the top of the soil. Not only does it look attractive, but it also helps to prevent root rot and disease.
My top tip for getting the best possible blooms each year on your mature Astrophytum cacti is to make sure they have a well-rested winter period. The easiest way to do this is to reduce the frequency and amount of water and also reposition your cactus into a cooler position. Believe me, when I say, doing this just for the winter months will increase the chances of your cactus flowering in the following year.
All six species of Astrophytum cacti flower during their growing season but only once they have reached maturity. Flowers vary in color from white, yellow, pink, or red but all are a sight to behold and look so unusual against the rock-like backdrop of the cacti.
Once flowers are spent, the blooms turn into hairy-looking berries that are either pink, red, or gray. Remember to collect the seeds if you’re planning on propagating.
How to fertilize Astrophytum
Feeding you Astrophytum cacti is essential if you want to encourage a good, strong root system, healthy growth and those all-important blooms.
Use a ready-mixed half strength cactus fertilizer to avoid any possible root burn or the prospect of over-fertilizing.
Liquid fertilizers should be diluted with water and applied when you need to water your cacti. Alternatively, Miracle-Gro do a great low N-P-K succulent and cacti formula that is already diluted and comes in a handy pump action bottle that can be administered directly onto the potting medium.
You really only need to fertilize Astrophytum during the summer months every 4-6 weeks. There’s no need to fertilize your Astrophytum in winter.
I only tend to repot my Astrophytum every 2-3 years. They are a relatively slow-growing plant so they won’t quickly outgrow the pot and can survive in soil with a low-nutrient value for some time.
When it is time to re-pot, use a new succulent or cacti pot that is slightly larger than the last, and always take the time to replace the potting mix with some fresh, well-draining cacti soil.
I tend to hold off watering for a few days to allow the cactus to acclimatize. Thereafter, water as described above.
How to Propagate Astrophytum
Astrophytum cacti can be propagated from seed. Use good quality shop-bought seeds or, if you’re lucky enough to own a mature plant, carefully harvest the seeds from the fruit that appears after flowering. For the best chance of success, use seeds that are as freshly harvested as possible.
Before you start propagating, allow the freshly harvested seeds to dry thoroughly. You can do this by laying them on a piece of kitchen paper in a well-ventilated spot but out of direct sunlight.
To prepare your propagation area, sprinkle a layer of cactus soil mixed with sand on a saucer or tray. The sand just helps to provide extra drainage and could be substituted for grit or perlite if you have it. I just find that sand works best because it is finer and the seeds don’t get lost in the soil mix.
Scatter the dried seeds over the mix and lightly cover them with a little more sand. Avoid placing the seeds in direct sunlight. Position in a warm, well-ventilated, and sheltered position that is bright but not in direct sunlight. If the weather is very hot, move your seeds to a slightly cooler area with better shade.
Water just enough to keep the soil moist and until the seeds germinate, which can take up to 8 weeks.
Once germinated, reduce the amount of water. They will take a year or so to develop into the familiar dome shape and 2 years plus for the ridges and spikes to form.
Common problems with Astrophytum
Astrophytum are subject to a few common problems. Knowing what they are can help you troubleshoot why yours might not be thriving.
This is a common problem of all species of Astrophytum. It occurs when the soil does not drain quickly enough, and too much moisture is trapped around the roots, or if your plant is not well ventilated. It can also occur, of course, if your cacti are simply being watered too often.
The tell-tale signs of root rot in a cactus include the top of the cactus becoming saggy or turning yellow, an unpleasant smell, or the base of the cactus that sits just below the soil level discoloring or becoming mushy.
To treat your Astrophytum for root rot you should first remove it from its pot. Then, very carefully brush off any soil that remains around the roots.
Roots that are brown or black and soft and mushy are rotten and need trimming off. Always use sharp, sterile scissors or a knife to remove root rot and allow the cut to dry before repotting with fresh soil in a clean pot.
Water thoroughly, but less frequently thereafter.
Mealybugs are those tiny but unsightly pests that are found in crevices of plants especially around leaf nodes and stem junctions. In Astrophytum cacti, mealybugs are often found in between the ridges and the base of the spines.
These creatures are wingless insects that love humidity and they – rather charmingly- will feed on the sap of your plant using their straw-like mouths to puncture into the foliage.
It is easy to spot mealybugs because of the distinctive cotton-like mass that they produce on the surface of plants. Whilst not pleasant to look at, if treated quickly they won’t do any lasting damage to your plant.
To rid your cacti of these tiny pests dab them off using a cotton bud that’s been soaked in diluted rubbing alcohol (such as surgical spirit). Call me neurotic, but I like to check over the plant daily and dab away any visible traces until they are gone.
In addition, every 7 days use a household insect spray, detergent, or soapy water to wash the mealybugs away until the infestation ceases. You can also use Neem oil to help discourage future mealybug infestations.
Scale are static shell-like parasites that can be found in the same hideouts as mealybugs. I.e. the ridges of Astrophytum cacti. These little critters feed on the sap from within the plant.
Your plant can look pretty sick and wilted during an Infestation but if treated quickly enough, can recover.
Treat scale as I have described above for treating mealybugs and as with all infestations, move the infected plant away from all other plants to avoid further issues. It’s also a good idea to give your other plants a thorough check and treat if needed.