Growing plants both within and out of an aquarium creates a semi-aquatic environment called a paludarium (swamp enclosure). This setup merges the aquatic world below the waterline with an emergent display above it. Plants capable of this transition offer a unique aesthetic, blurring the line between a traditional fish tank and a terrarium. Beyond visual appeal, introducing emergent growth provides biological benefits, such as enhanced natural filtration, as the plants efficiently draw nutrients from the water column.
The Biology of Emersed Growth
The capacity for plants to grow both submerged (underwater) and emersed (above water) is a remarkable biological adaptation found in many species that naturally inhabit riverbanks and floodplains. When a plant transitions from a submerged state to an emergent one, it undergoes significant physiological and morphological changes. This transformation is often driven by a need to access the higher concentration of carbon dioxide (CO2) available in the atmosphere compared to what is dissolved in water.
One of the most notable changes is the phenomenon of heterophylly, where the plant produces two distinct leaf forms depending on the environment. Submerged leaves are typically thin, often highly dissected, and lack a thick cuticle to maximize the absorption of dissolved CO2 and light in a low-intensity, aqueous environment. Conversely, emergent leaves develop a thicker structure, a waxy cuticle, and functional stomata, which are necessary to prevent excessive water loss and regulate gas exchange with the air.
Growing emersed provides the plant with virtually unlimited access to atmospheric CO2 and generally higher light intensity, which often results in faster growth and more robust health. The stronger stems and tougher leaves developed in the air are adaptations to withstand wind and direct sunlight. Furthermore, many aquatic plants will only flower and produce viable seeds when they are in their emersed form, allowing for sexual reproduction that is impossible underwater.
Top Species for Emergent Display
Selecting the right species depends on whether you seek a true aquatic plant that naturally adapts to air or a terrestrial plant that can thrive with only its roots submerged. Many popular aquarium plants fall into the first category, as they are naturally found along water margins that experience seasonal flooding. Species like Anubias and Microsorum pteropus (Java Fern) are excellent choices because their native habitats are riverbanks, allowing them to transition easily as long as their rhizomes remain moist.
Plants in the genus Cryptocoryne and many stem plants such as Ludwigia and Rotala also grow readily in an emersed state, often exhibiting a denser, more compact appearance than their submerged counterparts. Cryptocoryne species may temporarily “melt” their submerged leaves when transitioning, but they quickly produce new, thicker aerial leaves. These true aquatics generally prefer high humidity and moderate to bright lighting when grown emersed.
The second category includes common houseplants that function effectively in a hydroponic setup where only the roots are in the tank water. Epipremnum aureum (Pothos) is one of the most widely used options because it is resilient and highly efficient at absorbing nitrate and other waste compounds directly from the aquarium water. Other suitable terrestrial plants include:
- Spathiphyllum (Peace Lily)
- Various Philodendron species
- Dracaena sanderiana (Lucky Bamboo)
Only the root systems of these plants should be submerged to prevent the foliage from rotting. They thrive in moderate to low light and provide excellent natural filtration.
Setting Up the Emergent Environment
Creating a successful emergent environment, often called a riparium, requires careful control of the water-air interface. Plants that are transitioning from fully aquatic growth, such as Anubias or Rotala, need their roots anchored in a saturated substrate or directly in the water, ensuring the crown or rhizome remains above the waterline. For terrestrial plants like Pothos, the stem cuttings or the root ball must be positioned so that the leaves and nodes stay completely dry while the roots are immersed in the water.
The lighting needs for emergent growth differ from those for submerged plants. The light must be positioned above the emergent canopy and often needs to be higher intensity than the light used for the submerged portion of the tank. Full-spectrum LED grow lights are effective, providing the necessary red and blue wavelengths for optimal photosynthesis in the air. A consistent photoperiod of eight to ten hours is recommended to encourage plant health while mitigating algae growth.
High humidity is important, especially for aquatic plants making the initial transition to air-based leaves. Using a partial lid or frequent misting can help maintain the required humidity levels, ideally above 70%, to prevent the new aerial leaves from drying out. Emergent plants draw primary nutrients, particularly nitrates and phosphates, directly from the fish waste in the aquarium water. Supplemental fertilization is rarely necessary, as efficient access to atmospheric CO2 and tank nutrients supports vigorous growth.