Cacti are members of the Cactaceae family, known for their remarkable adaptations that allow them to thrive in arid environments. The question of what grows on a cactus includes both the plant’s own reproductive structures and external life forms that use the cactus for support or sustenance. Their thick, water-storing stems and protective spines provide a platform for various biological interactions. These interactions define the complex ecosystem surrounding these resilient plants, ranging from beneficial partnerships to parasitic relationships.
Distinctive Flowers and Edible Fruits
The most distinctive growths on a cactus are its reproductive structures, which have evolved unique characteristics for survival. Cactus flowers are often large and vibrant, attracting pollinators like bees, hummingbirds, and bats, depending on the species. Many columnar cacti produce flowers that open only at night. These nocturnal flowers feature pale colors and strong scents designed to attract moths and bats.
Following successful pollination, the flower develops into a fleshy fruit that is a source of food for animals and humans. The fruit of the Opuntia genus, the prickly pear, is called a tuna or cactus fig and is widely consumed for its sweet, juicy pulp. Another commercially relevant example is the dragon fruit, or pitaya, which grows from epiphytic cacti like Hylocereus. Additionally, the flattened stem segments of the prickly pear, called nopales, are harvested and eaten as a vegetable.
Non-Parasitic Hitchhikers
External organisms that grow on a cactus without causing harm are known as non-parasitic hitchhikers. This relationship is called commensalism, where one organism benefits and the host is largely unaffected. These hitchhikers are epiphytes, or “air plants,” which use the cactus surface purely for physical support. This positioning allows them to reach higher light levels or avoid ground competition, especially in humid environments.
Epiphytes draw moisture and nutrients from the air, rain, and debris accumulating on the cactus surface, not from the host’s tissues. Lichens and mosses are common epiphytes, often appearing as a crust or fuzzy layer on the skin. In tropical regions, small ferns, bromeliads, or even other epiphytic cacti like Rhipsalis may grow on the larger host. The epiphyte conducts its own photosynthesis and absorbs water independently.
Pests and Disease-Causing Organisms
In contrast to harmless hitchhikers, a variety of organisms actively feed on the cactus, drawing nutrients and causing damage. Cochineal scale insects are a notable pest that feeds on the sap of prickly pear cacti, appearing as small, white, waxy masses. Their feeding weakens the plant and can lead to stunted growth. Mealybugs are another common sap-sucking pest, often found in the crevices and joints of the stem, leaving behind a fuzzy, cotton-like residue.
These insects can also infest the roots, severely inhibiting the plant’s ability to absorb water and nutrients. Fungal and bacterial infections manifest as growths, typically appearing as soft, discolored, or mushy patches on the stem, which signals rot. This rot is often exacerbated by overwatering, which allows these microorganisms to spread and compromise the cactus’s water-storing tissues.