Growing lotus in aquaponics

The most common mistake among beginners is confusing lotus and water lily. Both plants are aquatic and ornamental, but they belong to distinct botanical families with opposite behaviours.

The lotus, of the genus Nelumbo, raises its leaves and flowers well above the water on rigid stems that can reach 1.50 metres. The water lily, of the genus Nymphaea, floats on the surface, its leaves and flowers level with the water.

The lotus is a perennial rhizome plant that overwinters at the bottom of the water, sheltered from frost. Its growth is vigorous in summer and completely dormant in winter. The water lily follows a similar cycle but always remains at the surface.

Choosing a lotus when buying therefore means checking the genus name. Nelumbo nucifera is the sacred Asian species, Nelumbo lutea the North American species with yellow flowers. Any other name most often refers to a water lily.

The lotus's strength lies in its extraordinary root system. The rhizome and its network of rootlets colonise the entire substrate and water column, creating a vast exchange surface with the aquatic environment.

This root density massively absorbs the nitrates and phosphates produced by fish waste and the breakdown of food, two unavoidable stages of the nitrogen cycle in aquaponics. The lotus also captures certain heavy metals documented by phytoremediation studies.

Compared with lettuce or basil, the lotus offers ten to twenty times more root biomass, which translates into a much stronger purification capacity per plant. A valuable asset in heavily stocked ponds.

Be careful, however: this absorption power is concentrated almost exclusively during the active growth period, from April to August. For the rest of the year, its filtering role decreases sharply.

Moreover, in an aquaponic system, too high a density of aquatic plants can create competition for your fruit and vegetables. It is therefore essential to find the right balance between ornamental and edible.

The lotus requires precise conditions to express its potential in aquaponics. Three parameters dominate: temperature, pH and light. None of them is negotiable.

In terms of temperature, the plant starts its growth above 20 °C during the day and flowers around 25 °C. The ideal range is between 18 and 30 °C in season. Below 15 °C, the lotus stagnates. Beyond a prolonged 32 °C, it suffers.

The ideal pH is between 6.5 and 7.5, which matches the usual preferences of most aquaponic fish. The water depth above the rhizome ranges from 15 to 60 cm for dwarf varieties, up to 1 metre for the giants.

Light exposure is the most frequently underestimated factor. The lotus requires at least 6 hours of direct sunlight per day to flower. In the shade, it develops leaves but produces no blooms.

Integrating a lotus into an aquaponic system requires a few technical choices. Three approaches dominate, depending on the size of the installation and the goal sought.

The first solution is the aquatic planting basket, placed at the bottom of the main pond. The rhizome is protected by a heavy substrate, clay soil mixed with pozzolana, covered with a layer of gravel. A method suited to small and medium ponds.

The second solution integrates the lotus into a dedicated settling pond, upstream of the return to the main pond. The plant then treats the most heavily loaded water, which maximises its filtering role while protecting it from fish.

Whatever solution is chosen, selecting a suitable substrate is central. An adult plant can cover up to 1 m² of water surface, which means space must be planned from the design stage.

The lotus has a more complex relationship with the nitrogen cycle than it appears. Its nutrient consumption follows a marked seasonality that absolutely must be anticipated.

From April to August, in full vegetative and floral growth, the plant massively absorbs nitrates and phosphates. This is the period when its filtering role reaches its maximum, relieving the system and stabilising water quality.

From September, the metabolism slows down. The leaves yellow and then die, the plant stores its reserves in the rhizome and its absorption drops sharply. A system that relied heavily on this living filter may see its nitrates rise again quickly.

The right reflex is to anticipate this seasonal imbalance by slightly reducing the fish's feeding in early autumn, increasing monitoring and providing a filtering backup to get through the winter without accumulation.

Overwintering the lotus is the step that separates lasting ponds from installations that must be redone every spring. The rhizome cannot tolerate frost, and its winter survival determines the entire spring restart.

Two strategies exist. The first is to leave the rhizome in place in the pond, provided there is a minimum depth of 60 cm above the rhizome. At this depth, the water does not freeze in mainland France, even during prolonged spells.

The second takes the rhizome out of the pond before the first frosts, cleans it carefully and stores it submerged in a bucket of fresh water in a frost-free cellar, between 4 and 10 °C. The safest method for shallow ponds, balconies or terraces.

During dormancy, plant filtration stops. Adjust the fish's feeding to the drop in temperature, monitor the parameters twice a month and gradually resume from mid-March when the water exceeds 12 °C at night.

The choice of variety determines the success of the installation. Lotuses fall into three main categories according to their adult size, and each suits a different type of pond.

Dwarf varieties such as the Yu Tang Jin Ma lotus, the Xiao Gongzhu or the famous Momo Botan are grown in 30 to 60 litres of water, perfect for balconies and small ponds. Height 40 to 60 cm.

Standard varieties such as Alba Striata, Flavescens or Ying Ying reach 80–120 cm, flower generously and suit medium ponds of 200 to 500 litres. Ideal depth 30 to 60 cm.

Giant varieties such as the High Cotton produce spectacular flowers on stems of 1.50 metres and beyond. Reserved for large landscaped ponds or professional installations.

Beyond ready-to-plant rhizomes, the lotus starts very well from Nelumbo nucifera lotus seeds. A fascinating method that lets you discover the plant's complete cycle.

The lotus seed has an extremely hard shell that keeps it viable for a long time, sometimes more than a thousand years according to archaeological studies. To germinate, it needs to be scarified. You gently file the shell on the side opposite the attachment point until you see the white kernel.

Then soaked in lukewarm water at 25 °C with daily water changes, the seed germinates in three to ten days. The first floating leaves appear in two to three weeks, then the first aerial leaves in five to six weeks.

In the first year, the young plant does not reach flowering. Patience, then: the second year rewards you with its first flowers and the start of a fully productive cycle.

Beyond its filtering function, the lotus brings the pond a visual and symbolic dimension that is hard to match. Flowering occurs in July–August, lasts several weeks and marks the peak of the water garden.

Each flower lasts three to four days, opening in the morning and closing in the evening. A mature plant can produce a dozen flowers per season, each measuring 20 to 30 cm in diameter depending on the variety. The display attracts pollinators and admiring eyes alike.

Beyond pure beauty, the lotus carries a rich cultural dimension. A symbol of purity and rebirth in Asian traditions, a sacred plant of ancient Egypt, it speaks to the imagination of anyone who lingers to contemplate it.

In an aquaponic approach that seeks to combine production, ecological balance and aesthetic pleasure, the lotus ticks all three boxes at once. Few other aquatic plants reach this level of versatility.