Alternanthera cardinalis

The Cardinal Plant is a stunning freshwater plant that brings a splash of vibrant red and purple to any aquarium. Originally from South America, it’s a popular choice for adding contrast to the usual green foliage. Because it needs strong light and added CO2 to show its best colors, it's considered a medium-difficulty plant, making it a great challenge for aquarists ready to take the next step beyond beginner plants.

You should plant it directly into the substrate at the back of your tank. It grows quite tall, creating a beautiful, colorful wall that can hide filters and heaters. It works well in sand, gravel, or a special aquatic soil. To keep this plant happy and colorful, you'll need at least medium to high lighting and a source of CO2 gas. Without these, its leaves might look more green or brown than red.

One of the best things about the Cardinal Plant is how it helps keep your aquarium healthy. It produces a lot of oxygen for your fish to breathe and is excellent at absorbing waste nutrients from the water, which helps prevent algae growth. This makes it not just a pretty decoration, but also a hardworking part of your tank's ecosystem. Regular trimming will keep it looking neat and encourage it to grow thicker.

Alternanthera cardinalis, commonly known as the Cardinal Plant or Purple Hedge, is a highly sought-after stem plant prized for its intense reddish-purple coloration. Native to South America, it serves as a spectacular background element in a planted aquarium, creating dramatic contrast against green flora. Achieving its signature deep red hue, however, requires specific conditions, which is why it's rated as medium difficulty and is best suited for high-tech aquascapes.

Success with this plant is largely dependent on a triumvirate of care: lighting, CO2, and nutrients. High-intensity lighting across the full spectrum is non-negotiable. Without powerful light penetrating to the substrate, the lower leaves will drop, and the plant's coloration will fade to a less impressive olive green or bronze. The plant uses light intensity as a cue to produce protective anthocyanin pigments, which give it the red appearance.

Equally important is carbon dioxide injection. The high CO2 requirement is directly linked to its vibrant pigmentation and robust, dense growth. In a CO2-enriched environment, the Cardinal Plant can grow relatively quickly. This rapid growth necessitates regular pruning to prevent it from over-shadowing other plants. Propagation is straightforward for a stem plant: simply trim the top 4-6 inches of a healthy stem and replant the cutting directly into the substrate. This practice not only multiplies your stock but also encourages the parent plant to branch out with side shoots, resulting in a much fuller, bushier appearance over time. Strategic trimming can be used to shape the plant group into a desired form within the aquascape.

Given its fast growth in optimal conditions, this plant is a heavy feeder, a direct consequence of its high nutrient absorption rate. It will thrive when planted in a nutrient-rich substrate, such as aqua soil, which provides a steady supply of nutrients to its root system. Additionally, a consistent and comprehensive liquid fertilization schedule is essential. Pay special attention to iron and other micronutrients, as a deficiency will directly impact its ability to produce the red pigments. Its voracious appetite for nitrates and phosphates makes it an excellent tool for nutrient export, significantly contributing to the tank's biological load reduction and helping to outcompete nuisance algae.

While it tolerates a broad range of water hardness, it performs best in stable, slightly acidic to neutral water, conditions that are naturally fostered in a CO2-injected setup. Medium water flow is ideal, ensuring that CO2 and nutrients are distributed evenly across all leaf surfaces and preventing detritus from settling, which could invite algae. Like many tissue-cultured or commercially grown stem plants, it may undergo a period of 'melting' as it transitions from its emersed form to its fully submersed form. During this time, old leaves may die off, but as long as the stems are healthy, new, more colorful submerged-growth leaves will soon appear once the plant has acclimatized.

Alternanthera cardinalis is a cultivated variety of dicotyledonous angiosperm belonging to the Amaranthaceae family. Although often sold under the specific epithet cardinalis, in horticultural and scientific communities it is largely considered a cultivar of Alternanthera reineckii, and may be properly referenced as Alternanthera reineckii 'Cardinalis'. This nomenclature helps distinguish it from other A. reineckii varieties like 'Pink' or 'Mini'. The genus Alternanthera includes both terrestrial and aquatic species, with this particular plant being well-adapted for a fully submersed existence under specific limnological conditions.

Its morphology is characterized by a sturdy stem with an opposite decussate leaf arrangement. The leaves are typically lanceolate to ovate. The intense red-to-purple coloration of the adaxial and abaxial leaf surfaces is a result of high concentrations of anthocyanin pigments. The synthesis of these flavonoids is a physiological response to high-stress environmental factors, primarily high-intensity irradiation (high PAR, particularly in blue and red wavelengths). This serves as a photoprotective mechanism against photo-oxidation. The plant's high CO2 requirement suggests a C3 photosynthetic pathway, where enriched dissolved inorganic carbon (DIC) levels are necessary to maximize the efficiency of RuBisCO, prevent photorespiration, and fuel the high metabolic activity required for rapid growth and pigment production. Carbon limitation will result in stunted growth and a phenotypic shift to greener, less pigmented foliage due to the dominance of chlorophyll.

From a biogeochemical perspective, A. cardinalis exhibits a high metabolic rate, reflected in its high oxygen production and nutrient absorption capacity. During the photoperiod in a carbon-replete environment, its rate of photosynthesis can contribute significantly to the dissolved oxygen concentration of the aquarium. Its role as a nutrient sink is substantial; it actively sequesters nitrogen and phosphorus compounds (NO3-, PO43-) from the water column, incorporating them into new biomass. This rapid uptake corresponds to its high bioload reduction value, making it effective for phytoremediation in a closed aquatic ecosystem by mitigating eutrophic conditions and competing with algae for limiting nutrients.

Successful cultivation requires maintaining parameters that mimic its tropical South American origins but optimized for aesthetic display. A temperature range of 22-28°C and a pH of 6.0-7.5 provide a stable environment for its enzymatic processes. The required water flow is critical for breaking through the boundary layer around the leaves to facilitate efficient gas exchange (CO2 uptake, O2 release) and nutrient delivery. Its IUCN status as 'Not Evaluated' (NE) is typical for horticultural cultivars not assessed in the wild.