The idea of an “underwater cactus” is a common misunderstanding. True cacti are desert inhabitants, specifically adapted to arid and semi-arid environments, and are not suited for prolonged submersion in water. This article explores why cacti cannot thrive underwater, what happens when they are overwatered, and suggests aquatic plants that might visually resemble them.
Cacti and Their Relationship with Water
Cacti originate from dry regions and possess specialized features for conserving moisture. Their fleshy, expanded stems serve as reservoirs to store water collected during infrequent rainfall. A thick, waxy cuticle covers their surface, significantly reducing water loss through evaporation.
Instead of broad leaves, most cacti have developed spines. These modified leaves minimize surface area exposed to the sun and reduce airflow, limiting water evaporation. Spines also collect dew or fog, directing condensed moisture to the plant’s shallow, widespread root system for quick absorption. This rapid water uptake and water storage allow cacti to endure long dry periods. Their unique Crassulacean acid metabolism (CAM) photosynthesis further conserves water by opening stomata at night to absorb carbon dioxide when temperatures are cooler.
Aquatic Plants That Resemble Cacti
While true cacti are terrestrial, some aquatic plants have spiky or unusual forms that lead to visual comparison. One example is Crassula aquatica, or water pygmyweed. This succulent thrives in fresh to brackish water bodies, including salt marshes, vernal pools, and muddy banks, often living partially or fully submerged. It forms small mats with tiny, fleshy, pointed leaves that can appear greenish-yellow to reddish-purple.
Another group of plants with a spiky appearance in aquatic settings are certain Echinodorus species, known as Amazon sword plants. Popular in aquariums, these can grow fully submerged, with some varieties featuring lance-shaped or broader leaves that give a spiky impression. Terrestrial Euphorbia species can be spiky and succulent-like, but they are not true cacti nor aquatic. These examples highlight that water-adapted plants have distinct biological differences from desert cacti, despite superficial resemblances.
What Happens When Cacti Are Overwatered
Providing too much water to a desert cactus leads to root rot. Cacti are adapted to dry conditions, and their roots require periods of dryness and good aeration. When soil remains waterlogged, oxygen becomes scarce, suffocating roots and creating an anaerobic environment. This fosters the growth of harmful fungi and bacteria, such as Fusarium species, which attack root tissues, causing decay.
Visible symptoms of overwatering include the cactus stem turning soft and mushy, often accompanied by yellowing or browning. Black or brown spots, particularly at the base, can indicate tissue damage. The plant may also appear wilted despite moist soil, as damaged roots lose their ability to absorb water and nutrients. In advanced stages, a foul odor may emanate from the rotting tissues, indicating significant decay.
Best Practices for Watering Cacti
Proper watering is important for desert cacti, balancing their need for moisture with their intolerance for constant wetness. The “soak and dry” method is recommended: water thoroughly until it drains from the pot, then allow the soil to dry out completely before watering again. This mimics natural desert rainfall patterns. Frequency varies; during spring and summer growing seasons, watering every 2-4 weeks when the soil is dry is common, while in fall and winter dormancy, this reduces to once a month or less.
Use well-draining soil formulated for cacti, typically a gritty mix (e.g., sand, perlite, or pumice) to prevent water retention. Pots with drainage holes are necessary to ensure excess water can escape, preventing the plant from sitting in standing water. It is better to underwater a cactus than to overwater it.