Hygrophila polysperma

Indian Waterweed is a famously resilient and fast-growing aquarium plant, making it an excellent choice for those just starting their aquatic gardening journey. This tall stem plant, originally from Asia, is one of the easiest species to care for in the hobby. It's incredibly adaptable and doesn't demand specific or stable water conditions, thriving in a wide range of temperatures, pH levels, and water hardness. You don't need intense lighting or supplemental CO₂ for it to grow, although it will flourish even more with them. Its primary role in an aquarium is as a background plant, where its rapid vertical growth can quickly create a lush, green backdrop.

One of its greatest benefits is its ability to improve water quality. It absorbs a large amount of waste products from the water, which helps to starve out algae and keep the tank clean. In the process, it releases plenty of oxygen, benefiting fish and other inhabitants. Planting is simple: just push the stems into any substrate like sand or gravel, and they will quickly take root. Its straightforward care and significant benefits make it a rewarding addition to any freshwater tank.

Known by hobbyists as a prolific and nearly indestructible species, Indian Waterweed is a classic stem plant valued for its versatility and rapid growth. Its ability to adapt to a vast spectrum of aquatic environments makes it a reliable choice for almost any freshwater setup, from low-tech beginner tanks to high-tech planted aquascapes. It tolerates a broad temperature range and is indifferent to water pH, growing well in both slightly acidic and alkaline conditions. Furthermore, its resilience extends to water hardness, as it thrives in everything from very soft to very hard water without issue.

Lighting is a key factor that influences its growth pattern. In low-light conditions, it will still grow but may become 'leggy,' with more space between its leaf sets as it stretches towards the light source. Conversely, moderate to high lighting encourages more compact, dense, and bushy growth. While it does not require CO₂ injection, providing it will significantly boost its already fast growth rate, making it an excellent candidate for aquarists looking to quickly fill out a new aquarium. Its rapid consumption of nitrogenous compounds and phosphates from the water column makes it a valuable tool for nutrient control. This high nutrient absorption directly competes with algae, helping to maintain a clean and balanced ecosystem, especially in tanks with a high bioload or during the initial cycling phase. This process is coupled with high oxygen production, enhancing the overall health and stability of the aquarium environment.

Due to its potential height, it is best placed in the background of an aquascape. Regular trimming is essential to prevent it from outgrowing the tank and blocking light from reaching other plants. The trimmed tops can be simply replanted into the substrate to create a denser, more hedge-like appearance. It will root in any common substrate, including inert sand and gravel, though growth becomes even more vigorous in a nutrient-rich soil or with the use of root-tab fertilizers.

Hygrophila polysperma is a herbaceous perennial belonging to the family Acanthaceae. Its taxonomic status is validated, and it is widely recognized within the scientific and aquarist communities. Native to the Indian subcontinent and other parts of Asia, this species typically inhabits lentic or slow-lotic freshwater environments such as ditches, ponds, and the marshy edges of streams. Its IUCN status as 'Least Concern' reflects its robust, widespread population and exceptional adaptability.

The morphology of H. polysperma is characteristic of a fast-growing stem plant. It features a simple or branching stem with leaves arranged in an opposite decussate pattern. Submersed leaves are typically lanceolate to oblanceolate, though their size and even shape can exhibit significant phenotypic plasticity based on environmental conditions. For instance, under intense lighting, the internodal distance shortens, resulting in denser foliage. The plant's ability to thrive aquatically is supported by its capacity for adventitious root formation at stem nodes, allowing for secure anchorage and nutrient uptake directly from the substrate or water column.

Physiologically, H. polysperma is remarkable for its broad tolerance to diverse water chemistry parameters, including a wide range of pH, general hardness (GH), and temperature. This resilience is a key factor in its success both in aquaria and as an invasive species in regions outside its native range, such as North America. Its metabolic efficiency is high, characterized by rapid nutrient assimilation, particularly of nitrogen and phosphorus compounds. This makes it an effective agent for bioremediation in closed aquatic systems, where it mitigates eutrophication by outcompeting algae. It can grow both submersed and emersed; in its emersed form, it may produce small, pale blue to violet flowers in the leaf axils, facilitating sexual reproduction, though vegetative fragmentation is its primary mode of propagation.

Propagating this plant is exceptionally straightforward, making it an ideal species for aquarists of any experience level to practice vegetative propagation techniques.

In an aquarium setting, the species reproduces asexually through stem cuttings. While it is capable of sexual reproduction via flowers and seeds when grown emersed, this method is rarely utilized by hobbyists due to the ease and reliability of asexual propagation.

To propagate, simply use aquascaping scissors to trim the top portion of a healthy, mature stem. The lower, rooted section of the plant will respond by sending out new side shoots, often resulting in a bushier and fuller appearance over time. The cutting should be a few inches long; it's beneficial to remove the lowest one or two sets of leaves to prevent them from decaying under the substrate after planting. This trimmed cutting can then be inserted directly into the substrate, where it will readily develop a new root system.

Environmental factors can influence the speed of establishment, though cuttings will root under almost any condition. Success is nearly guaranteed, but for the most vigorous results, provide moderate to high lighting and a nutrient-rich environment. While not essential, CO₂ supplementation will accelerate root development and new leaf growth significantly. The plant is not selective about substrate and will anchor and thrive in sand, gravel, or specialized aquatic soil.

Newly planted cuttings require no special care. Ensure they receive adequate light and are not completely shaded by neighboring plants. To create a dense background effect, plant numerous cuttings in a close group. Due to its inherent hardiness, issues such as melting or fungal infections during propagation are extremely uncommon, making it one of the most reliable plants to multiply in a freshwater aquarium.