Gymnocalycium horstii
Family: Cactaceae​
​Common Name: spider cactus
Origin: Gymnocalycium horstii originates from Uruguay, eastern Paraguay, southern Brazil, and northeastern Argentina, on rocky outcrops in the Pampas grasslands.
Description: G. horstii is a distinctive, globose cactus with minimal spines that curve toward the plant body, and slightly defined ribs. It begins as a single stem but develops into a small aggregation as the species ages. From mid spring to late summer, beautiful pink to white flowers can form, showy and large in comparison to the cactus.
  • Interesting Features:
    • The spider cactus is very similar in appearance to Gymnocalycium horstii subsp. ​buenekeri, a subspecies that develops shiny-appearing stems. Often, the two species are confused.
    • G. horstii is critically endangered in its natural habitat, with wild populations severely fragmented from human activity.
  • Uses: In cultivation, Gymnocalycium horstii is used as a houseplant, and within xeriscape or desert gardens.
    • It is uncertain, but species within the Gymnocalycium genus may contain mescaline, the compound that gives Lophophora williamsii (peyote) its psychoactive abilities. However, some species may also contain anhalonine – a strychnine-like compound – and should not be ingested.

Cultivation and Care:
  • Watering: Moderate 
    • Water frequently during summer, as G. horstii requires more water than the average cactus – up to weekly for potted plants. However, always allow soil to fully dry in between waterings, and suspend irrigation in winter.
    • During spring and autumn, water as needed.
  • Soil: Plant in a light cactus or succulent soil that is well-draining, with a pH between 6.1 and 7.8.
  • Lighting: Provide the spider cactus with partial shade or indirect sunlight, but avoid direct light, as this can cause sunburn. Additionally, if cultivating outside, choose a site that receives filtered sunlight and mid-day shade.
  • Propagation: Gymnocalycium horstii can be propagated from woody stem cuttings, offsets, or seed collected from overripe fruits. However, seed is the preferred method of propagation, as some plants rarely produce offsets.
  • Cold Tolerance: 30°F, USDA hardiness zones 10a-11.
  • Special Accommodations: To encourage rich growth, feed during the summer with a high potassium fertilizer.
    • As one of the greatest contributors to the endangerment of G. horstii is illegal theft by collectors, always ensure that cacti are purchased from a valid source. 

Sources:​​
  1. Llifle Encyclopedia of Cacti
  2. World of Succulents
  3. Garden
  4. Cactus Limon
  5. Dave's Garden
  6. Garden Tags
  7. Window Sill Cactus
  8. High Times
  9. Cactus in Habitat