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The Spider Cactus: Cute, not Creepy

The Spider cactus is native to South America. Gymnocalycium denudatum is an unbranching cactus that stays quite small, even at maturity. The cacti’s large blooms dwarf the plant and create a beautiful visual during spring and summer.

Gymnocalycium denudatum
UHLIG Kakteen

Spider Cactus Appearance

The Spider cactus features a single, spherical stalk. The body is dark green and glossy with rounded ribs. These ribs produce spines growing out of a fuzzy center which lay flat against the plant and resemble small spiders. The Spider cactus plant grows 4 inch ( 10 cm) tall and 6 inches ( 15 cm) wide. In spring and summer the plant’s creamy-white blooms appear. The large flowers reach a diameter of 3 inches (7.5 cm).

Spider Cactus Light Requirements

The Spider cactus plant prefers full sun from a south-facing window. All-day, indirect sun in a bright room also works well. If the plant is placed outside, choose a spot in partial shade to prevent scorching. Avoid full shade placement as this leads to less blooming.

Water Needs

In spring and summer, provide water for your Spider cactus once or twice per week, depending on the plant’s size. Practice the soak and dry method, allowing the soil to dry between waterings, then soak the soil thoroughly. Let the water seep through the pot’s drain hole, then dispose of any collected water in the drip tray to avoid root rot. 

Temperature and Humidity

Temperature and Humidity

The Spider cactus plant thrives in temperatures between 70 to 95 degrees Fahrenheit ( 20 to 35 degrees Celsuis) making it ideal for both indoor and outdoor growing. While the plant loves heat, it does not tolerate high humidity. Average household humidity levels of around 40 to 50 percent are acceptable but avoid placing the plant in kitchens or bathrooms where moisture is higher.

Soil and Fertilizer

Grow the Spider cactus in a fast-draining soil that allows water to run through freely and not pool on the surface. A cactus soil or a mixture of 60 percent vermiculite, 20 percent peat, and 20 percent sandy soil works well. Feed the cactus once per month from spring to fall with a liquid fertilizer. Use a fertilizer low in nitrogen and high in phosphorus and potassium for best results.

Spider Cactus Propagation

The Spider cactus will propagate from both seeds or offsets. To propagate by seed, collect the seeds from ripened fruit. Clean and dry the seeds, then surface sow into a container filled with cactus soil. Cover the seeds lightly with the soil. Keep the soil moist and cover the top of the container with plastic wrap to keep the soil damp. Once seedlings are established, move each to its own pot. Please note that plants grown from seed could be a hybrid of your particular cactus depending on how it was pollinated and with what other species.

If your Spider cactus produces offsets, take a cutting from the offset with a sharp and sterile knife. Allow the cutting to rest for a few days on a paper towel to form a callus. When ready, insert the calloused end into a pot filled with cactus soil and water lightly. Keep the plant warm but out of direct sunlight until roots form.

Wintering a Spider Cactus

Wintering a Spider Cactus

Let the Spider cactus plant go dormant in the winter to increase blooming next season. Choose a cool place with temperatures around 50 degrees Fahrenheit ( 10 degrees Celsius). Withhold water during this time and reduce light exposure until spring. Do not fertilize the cactus during dormancy. In the spring, begin feedings of a liquid cactus fertilizer once per month.

The Spider cactus is a compact variety with a very unique appearance. Its tolerance for a wide array of temperatures makes it a great choice for many growing zones and households. The cactus is slow growing but makes up for this with its extra-large blooms when mature.

Spider Cactus FAQ

Why is My Spider Cactus Turning Yellow?

Two common causes of a yellowing Spider cactus plant are incorrect watering and too much fertilizer. Adjust waterings as needed and dilute your current fertilizer to half strength to correct the issue.

Will My Spider Cactus Grow Outdoors?

Yes, the Spider cactus grows well in USDA zones 9b to 11b. 

What are Common Pests to Watch For with My Spider Cactus?

Spider mites and aphids are common pests that infest a Spider cactus plant. Spider mites are small insects that appear on the plant and often turn it red or yellow. Spray your plant with a houseplant insecticide to rid it of these pests. Correct aphid infestations by spraying the plant with water to remove a few insects or use an aphid plant treatment product for larger infestations.

How Do I Fix Root Rot on My Spider Cactus?

Root rot is typically a result of overwatering. Correct root rot by removing the plant from its pot and trimming away affected roots. Repot the plant in fresh soil and follow up with a treatment of one part 3 percent hydrogen peroxide and two parts water. Mix and pour this blend into the dry soil to combat any remaining bacteria.

When is the Best Time to Propagate My Spider Cactus Seeds?

Harvest and propagate Spider cactus seeds once the fruit has become very ripe, typically in July.