Fairy Castle Cactus Care Information – Acanthocereus tetragonus

If your sunny windowsill needs a little more magic, consider adopting a fairy castle cactus. This columnar cactus got its name because the multiple clumping stems grow up in a formation reminiscent of the turrets of a medieval castle. 

Each stem of the fairy castle cactus is a five-sided fleshy branch with wooly spines on each ridge. It is generally bright green, but variegated types are also available. 

The fairy castle cactus loves sun and heat, originating in a range including the southernmost United States, Mexico, Central America, the Caribbean, and northern South America. However, it will grow happily indoors, in a sunny windowsill. It is easy to care for, resilient, and widely available, making it an extremely popular houseplant.

fairy castle cactus behind a pink background

Quick Guide to the Fairy Castle Cactus

Sun Requirements Happy in full sunlight
HardinessNot cold hardy. Perfect for USDA zones 10-11
ToxicityMidly toxic to humans and pets
Primary growth seasonSpring through fall. Dormant in winter
Typical sizesMini-cacti / Up to 6 feet
FlowersLarge white flowers

Sun Requirements

When outdoors, the fairy castle cactus is happy in full sunlight or mostly sun with a bit of shade. Indoors, it prefers a sunny south-facing window with direct sunlight. If your sunlight is limited, consider adding a grow light.

Fairy castle cacti will bend towards the light. Rotate the pot of your indoor cactus every week or two to ensure it continues to grow straight up instead of leaning to one side.

Hardiness

Fairy castle cacti are not cold hardy. They can be grown outdoors in USDA zones 10-11, but they cannot tolerate temperatures below 30 degrees Fahrenheit – even temporarily. 

The good news is you can grow them indoors – provided that you keep the cacti in a warm and sunny south-facing window. In zones colder than 10-11, they can be grown indoors in the cooler months and taken outside after the last frost date, and brought back into the house in the fall. Or, you can get cold-hardy succulents

Toxicity

Fairy castle cacti are only mildly toxic to humans and pets, less harmful than many cacti and other succulents.

Primary growth season

Fairy castle cacti grow spring through fall. They are dormant in winter.

Typical sizes

Although you can purchase fairy castle cacti as mini-cacti, they can grow up to six feet tall given a suitable environment.

Flowers

Fairy castle cactus can flower, but it happens very rarely, on cacti at least ten years old, and only at night. These flowers are large and white. If you purchase a fairy castle cactus with small colorful flowers, they are probably straw flowers glued on for decoration by the merchant. You can pull them off or leave them to fall off naturally over time. Sometimes, if you do not remove them, the dye from the straw flowers will stain the cactus.

Growing the Fairy Castle Cactus

mini fairy castle cactus

Transitioning to a New Growing Environment

Fairy castle cactus respond well to repotting, using the instructions below. 

If fairy castle cacti are accustomed to living indoors (in your home or the garden store), be cautious about moving them outdoors. Transitioning abruptly to full, direct sunlight can cause them to sunburn. 

Any time you plan to take a fairy castle cactus outdoors — whether you are planting it outside or just letting it spend the summer in a pot outside — make sure the transition is gradual. Either gradually increase the number of hours per day it spends outdoors or start it in partial shade before moving it to a sunnier location.

Thinking about getting another succulent? Read our guide here.

Container and Soil

Fairy Castle Cactus in a white pot

Like all cacti, the fairy castle cactus needs excellent drainage to prevent root rot. 

When planting outdoors, you should amend the soil to ensure it is rich in nutrients but loose enough for good drainage. They can grow well in soil with lots of gravel or even in soil-filled niches in rock walls. Consider a raised bed or other planting options that promote good drainage.

Indoors, the fairy castle cactus needs a pot with drainage holes in the bottom so you can use the “soak and dry” watering method described below. The ideal pot is unglazed clay or terracotta, as these pots absorb excess water from the soil to protect the plant’s roots.

Repot your fairy castle cactus in a loose, well-draining soil mix meant for cacti. You can also make your cactus mix. A simple recipe you can use is two parts ordinary potting soil, two parts sharp sand, and one part of a gritty amendment that promotes drainage. This last part could be perlite, pumice, pebbles, bark chips, pieces of broken pots, etc. 

Repotting

woman putting soil in a pot

The fairy castle cactus goes through growth spurts, but typically it grows slowly, especially as a mature plant. As the roots grow (and crowd), you will need to report the succulent. Remember, it can grow up to 6 feet tall, so you will need to increase the pot size. 

Watering

Overwatering is the biggest cause of problems with the fairy castle cactus. It can cause root rot, as well as bacterial and fungal problems. The fairy castle cactus needs more water in its growing seasons (spring through fall) and less frequent watering during its winter dormancy (roughly half as often as during the growing season). This could mean roughly weekly in the summer and twice monthly in winter.

Like most cacti, the fairy castle cactus appreciates the “soak and dry” method. Place the pot in a sink, gently water the plant until water streams out the bottom. Wait a few minutes for the water to absorb throughout the soil as much as possible, then drench the plant again to make sure the potting mix is thoroughly wet. Let the cactus drain fully, then move it back to the window. Let the potting mix dry out completely before watering again.

Corking

close-up picture of fairy castle cactus

Some people overwater their fairy tale cacti as a result of misunderstanding the phenomenon called “corking.” Corking occurs when segments at the cactus base turn brown and hard, with a wood-like texture. 

Watch this video about corking:

Corking is often perceived as the cactus drying out too much, leading owners to overwater. However, corking is a natural part of the growth of a cactus. It occurs to provide more stability and prevent breakage when the cactus becomes top-heavy. 

Corking is non-reversible; You can’t correct it with extra water. The more water you provide, the plumper and heavier the cactus will be, thus increasing the succulent’s need to stabilize at the base. If you don’t like the look of corking, you will need to propagate your cactus and start over with a smaller plant.

Fertilizing

woman adding fertilizer to cacti

Fertilize your fairy castle cactus during the active growing season only, not during the winter months.

Choose an organic fertilizer meant specifically for cacti or succulents (such as this option). You will want to fertilize once a month during the growing season.

Pests and Disease

close-up picture of a fairy castle cactus

Maintaining proper growing conditions, especially avoiding overwatering, will help you avoid most diseases and pests that could affect your fairy castle cactus, such as bacterial or fungal infections. This, in turn, will reduce the prevalence of pests like fungus gnats.

Keep an eye out for mealybugs. These critters look like crumbs and tend to live in a white web-like substance inside the nooks and crannies of your cactus. You can kill them with a bit of isopropyl alcohol on a q-tip or in a spray bottle. 

Spider mites are occasionally a problem. They can be addressed in the same way, using isopropyl alcohol, or using a neem oil spray.

Propagation

fairy castle cactus with other cacti

Theoretically, fairy castle cactus can be propagated by saving seeds. However, because they flower so infrequently and produce so many branches, propagating from cuttings is much easier.

Use a sharp, sterile knife to cut off a branch for propagation. Then leave the cuttings lying on a plate or table in the open air for several days. By doing so, it will allow the raw cut area to heal over, forming a callus. It also avoids bacterial or fungal growth at the cut end.

After the callus has formed, you have the option of dipping the end into a rooting hormone. It is typically unnecessary for propagating the fairy castle cactus, but it can speed up the process.

Plant the callous end in moist soil. Moist soil is ideal, but don’t overwater. Keep in mind that until you’re cutting grows roots, it will not absorb water. 

Patience is key. Although this method is much faster than growing your cactus from seed, you shouldn’t expect it to grow more than 6-12 inches tall the first year after propagation, although it will likely produce several arms.

The Wrap Up

You don’t have to have a magical green thumb to grow a fairy castle cactus. As long as you give them plenty of sun and not too much water, they will thrive as a houseplant, and in the proper zones, they make a great outdoor plant as well. And who knows? If you make a wish and sprinkle a little fairy dust (fertilizer) as needed, you may eventually be lucky enough to see the elusive fairy castle cactus flower.

About The Author

Emily Cordo is a Master Gardener and DIY remodeling enthusiast. She is co-owner of a small garlic farm in central Indiana, built on the values of permaculture, organics, and biodiversity.