Whether you’re an experienced succulent keeper or new to the hobby, it’s easy to see why you’d want to cultivate Astrophytum asterias, a.k.a. the Star cactus. These little succulents are unique, hardy, and produce stunning blossoms!
Astrophytum asterias ‘Star cactus’ care and propagation are both easy and rewarding if you have a little patience! They grow slowly but do well in pots indoors as long as they get full sun, warm temperatures, thorough but infrequent watering, and fast-draining soil.
The rest of this article will explore Star cactus care in-depth. This includes how to care for Star cacti, how to propagate them, and how to deal with pests and disease.
What is a Star Cactus?
Astrophytum asterias, commonly known as the ‘Star Cactus,’ is a small, slow-growing cactus species that belongs to the family Cactaceae. It is native to central and northeastern Mexico, where it grows in rocky and gravelly desert areas.
The plant has a distinctive star-shaped appearance. The stem of the cactus is globular and can grow up to 10 cm in diameter. The stem is divided into eight to ten deep ribs, and the ribs are lined with white or yellowish-white areoles that produce small, hair-like spines. The spines are usually only a few millimeters long and can range in color from white to reddish-brown.
The Star Cactus blooms in the summer, producing yellow flowers with orange-red centers that are about 5 cm in diameter. The flowers are followed by spherical fruits that are green, pink, or dull red in color.
How To Care for a Star Cactus
Astrophytum asterias, commonly known as Star cactus, Sand Dollar cactus, or Sea Urchin cactus, is native to Texas and Mexico’s dry, sandy deserts. As its nicknames suggest, it looks like a sand dollar or a sea urchin.
Though it grows very slowly, the Star cactus produces some of the most beautiful blooms of any succulent. They come in yellow, red, and orange, and flowers occur yearly in summer or fall.
The Star cactus comes in six varieties, all of which are relatively small, dome-shaped, spineless, and have white-dotted ridges in a star pattern. In addition, all six types require the same sort of care.
Temperature
Star cactus flourishes at around 70° F (21° C) or above, making it perfect for indoor plant cultivation! However, it lives in zones 8 to 11 and can withstand temperatures as low as 20° F (-7° C) for short periods. It will also survive in temperatures of more than 100° F (38° C) if watered more frequently.
Temperatures under 50° F (10° C) or over 95° F (35° C) will slow the plant’s growth and send it into dormancy. However, this cactus likes a period of cool weather each year in the winter. A Star cactus may fail to flourish if not allowed its regular dormancy period.
Humidity
The Star cactus is native to desert climates and prefers low humidity. A humid environment can cause problems with root rot and other fungal infections. However, you can still grow Star cactus indoors in humid places, especially if you have air conditioning!
It’s also possible to combat high humidity by watering less frequently, using an extra fast-draining soil mix, and providing good airflow and direct sunlight.
Pots
Using pots with plenty of drainage holes in the bottom is essential. Succulents do not do well in pots with no drainage holes! Ceramic and terra cotta pots are by far the best choice for your Star Cactus because they help disperse moisture and assist with draining.
Terra cotta can also be a helpful indicator of when to water because terra cotta darkens when wet. This shows where the soil is retaining moisture deep inside the pot.
The size of the pot is also an important factor. Your pot should be no more than 2 inches (5 cm) in diameter larger than the plant itself. A large or deep pot is not recommended because the extra soil holds water and raises the risk of overwatering!
Repotting should be done yearly at the end of spring or the beginning of the summer to help your Star cactus grow. Upsize your pot by no more than 2 inches (5 cm) in diameter.
Soil
Star cactus does best in soil that is highly porous and water permeable. That means airy and fast-draining! Star cactus roots are extremely sensitive to moisture and need plenty of oxygen.
As with most succulents, regular potting soil is much too rich and moisture-retaining for this plant. However, regular potting soil can be used if you mix it with plenty of sand, gravel, vermiculite, coco coir, perlite, pumice, or peat moss.
A good rule is to use one part of potting soil to at least two parts sand, gravel, vermiculite, etc. Your cacti should not sit in moist soil for more than a few days. If you can feel moisture in the top 2 inches (5 cm) of soil three days after watering, your soil is not draining fast enough, and your plant is at risk of getting root rot!
As I mentioned above, your cactus may need extra protection from overwatering if you live in a humid climate. In this case, you can use one part potting soil to four parts sand, gravel, vermiculite, etc. This will increase drainage and help combat the extra moisture in the air.
Water
Like most succulents, your Star cactus likes to be watered deeply but infrequently and to be allowed to dry out completely in between. One way to water your Star cactus is to place it in a sink to let the water flow freely through the soil and come out of the drainage holes in the bottom of the pot. Alternatively, you can set the pot in another container full of water and let it soak for up to half an hour.
Watering times can differ depending on the season, how much sun your plant gets, what kind of soil you use, the size of the pot, and how much airflow there is. For this reason, it is best to test the soil for moisture before you water rather than stick to a specific schedule.
You can test for moisture by sticking your finger into the soil 2-3 inches (5-7.6 cm). If the soil feels cold and sticks to your finger in little clumps, your plant does not need to be watered. The soil may still feel cool when dry, but your finger will come out dusty with no dirt clumps. Do this test about once a week.
Generally, your plant will only need watering once or twice per month unless the weather is very hot or the pot is extremely small. Likewise, your plant may not need much water during the winter. When the weather is cold and moist, the soil will take much longer to dry out completely.
Check the soil every few days to prevent over-drying if you use an extra fast-draining soil mix to combat a highly humid climate. This testing method will soon show you how much and how often to water. Eventually, you will not have to think about it as much.
Light
The Star cactus needs direct sunlight for at least six hours daily to flourish and blossom. Less than 6 hours will stunt its growth and prevent it from producing blossoms. It would be best if you only shaded it when the weather is extremely hot for long hours. Prolonged exposure to high temperatures can cause scorching even to these hardy little cacti.
You can place the Star cactus outdoors in your garden or patio if you live in zones 8-11. However, the Star cactus will also do very well indoors, pretty much anywhere, if positioned in the correct window. A south or west-facing window is best.
South-facing windows get the longest, hottest sunlight and are, therefore, best for cacti and succulents. A west-facing window will also do, although it gets less light and heat than a south-facing window.
Fertilizer
Like most succulents, the Star cactus appreciates only light fertilization during its growing period in the summer and early fall. It may be tempting to offer this slow-growing cactus extra fertilizer to speed its growth, but doing so can have adverse effects.
In fact, over-fertilization can burn the roots of your plant and prevent it from drawing in water and nutrients properly. It’s best to use no more potent fertilizer than a 10-10-10, or diluted 20-20-20, added to water. This Triple 10 All Purpose Liquid Fertilizer from Amazon.com is a good choice that can be added to water and easily diluted.
Give the fertilizer mixture only when you water in the growing season. Stop fertilizing when the weather starts to cool in fall, as your plant will be readying itself for dormancy. Do not fertilize in winter at all.
If your plant is small and needs more frequent watering, dilute the mix further to avoid over-fertilizing. Younger plants may be more susceptible to root burn from fertilizers. I recommend fertilizing at most 3-4 times per growing season, regardless of the size of your plant.
How To Propagate a Star Cactus
Unfortunately, propagating Star cactus isn’t the easiest. However, it can be done with a bit of care and finesse! First, you will need more than one Star cactus, as these plants must be pollinated to produce seeds. Second, you’ll need a shallow container to sprout the seeds.
The best choice is something that closes but isn’t totally sealed and lets in plenty of light. A clear Tupperware with holes poked in the lid will work great. It acts like a tiny greenhouse, holding heat and moisture inside and allowing for a small amount of airflow. Your soil should be moist but well-draining, similar to the mixture described above.
To pollinate, collect the pollen from one flower with a paintbrush or other small implement. Then transfer the pollen to another open flower. When the pollinated flower dies, the seeds can be collected from inside and sprinkled onto the soil mixture in your greenhouse container.
The seeds should germinate within a week, but it will take years for the tiny cacti to mature! For detailed instructions on propagation, check out this awesome youtube video:
Caring for Baby Star Cacti
After 5 – 6 months of growing in their little greenhouse container, the baby Star cacti will be ready for transplant into a larger container. They should also get a change of soil. Their new soil should be a drier mix with more sand and gravel.
In addition, the baby cacti will no longer need to be covered. They should be provided full sun exposure but protected from high temperatures.
After this, the baby cacti can grow together in one shallow container until they are large enough to pot singly in 1-2 in (2.5-5 cm) nursery pots at about 2 years old.
Pests and Disease
Although the Star cactus is a hardy plant, it is susceptible to pests and disease, especially if not cared for properly. Overwatering, exposure to extreme temperatures, and lack of sunlight can weaken your plant to these problems. Common issues in Star cactus care include root rot, mealy bugs, and aphids.
Root Rot
Root rot is a fungus that attacks roots when they’ve sat in moist soil for too long. The best way to prevent it is to use well-draining soil and avoid overwatering.
If your plant does end up with root rot, you’ll have to remove and dispose of the soil, wash the roots, cut off any infected roots, and then repot in fresh soil. Finally, water the plant with 1 part water to 1 part 3% hydrogen peroxide to prevent reinfection.
Insects
Mealy bugs and aphids are insect pests that feed on succulents like the Star cactus. The first step to avoiding a pest infestation is regularly inspecting your plants.
If you notice specks (aphids) or fuzzy white lumps (mealy bugs), immediately quarantine the infected plant. Next, spray the infected plant liberally with 70% isopropyl alcohol.
If alcohol doesn’t kill the infestation, try a homemade mixture of dish soap and water as described in this youtube video:
Other natural pest control options include neem oil spray and ladybugs.
Lastly, there are plenty of store-bought chemical insecticides, but many are harsh to plants, people, and animals.
In a Nutshell
- Astrophytum asterias, the Star cactus, is hardy, slow-growing, and easy to care for.
- Likes temperatures between 50° F (10° C) and 95° F (35° C).
- Grows well indoors in almost any climate or outdoors in warm, dry climates.
- Prefers low humidity.
- Needs fast-draining soil.
- Water deeply but infrequently.
- Requires a minimum of 6 hours of bright, direct sunlight each day.
- Fertilize during the summer with 10-10-10 water-soluble fertilizers.
- Propagates by seed.
- Susceptible to root rot, mealy bugs, and aphids.
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