Cardinal flower: the round-leaved midground with coloured undersides
Lobelia cardinalis
The aquarium form of cardinal flower is an easy, slow, characterful midground — tidy rows of rounded leaves that are green on top and reddish-purple underneath, brightest under good light.
A bog plant grown underwater
Lobelia cardinalis is the cardinal flower, a North American marsh and streamside plant (family Campanulaceae) famous in gardens for tall spikes of vivid scarlet flowers. In the aquarium we grow a very different-looking thing: a compact, permanently submersed rosette-topped stem. Most aquarium plants are the ‘small form’ (sometimes sold as ‘Mini’), selected to stay low rather than bolt for the surface like the wild type.
Because it is a bog plant, nurseries usually grow it emersed — out of water — so a freshly bought plant is often in its air-grown form and has to convert to submersed growth once planted.
Green tops, coloured undersides
The look is distinctive: neat, rounded to spoon-shaped leaves arranged in tidy ranks up the stem, glossy green on top and washed reddish-purple underneath. That two-tone effect is its signature, and the underside colour deepens under stronger light — a light-driven response rather than something a particular fertiliser dose guarantees. It brings a soft, rounded structure that contrasts nicely with fine-leaved stems.
Easy, and slow in a good way
This is an undemanding, adaptable plant: it takes low light through to high and soft water through to hard. Light mostly controls two things — how compact it stays and how strongly the undersides colour up — so more light gives a tidier, more colourful plant. CO2 is not required but helps keep it dense. It is a root feeder as much as a column feeder, so it appreciates a nutritious substrate or root tabs; its slow, steady pace means it rarely needs the constant trimming faster stems demand.
| Parameter | Value |
|---|---|
| Lighting | Low to high; colour improves with more light |
| CO2 | Not essential; keeps growth compact and colourful |
| Temperature | 20–28°C |
| pH | 6.0–7.5 |
| Hardness | Soft to hard — adaptable |
| Fertiliser | Feeds from roots and water column |
| Substrate | Does best with a nutritious substrate or root tabs |
| Growth rate | Slow to moderate |
| Placement | Midground |
| Difficulty | Easy |
Structure in the midground
Its rounded leaves and slow growth make it a reliable midground plant that holds a defined shape between a carpet and taller background stems. Planted in a small group it forms a characterful clump; because it grows slowly it stays where you put it, which makes it easy to design around compared with vigorous stems.
Cuttings and side shoots
Propagate by topping and replanting cuttings, or by separating the side shoots that emerge from the base. New plantlets root slowly but reliably. Patience is the theme: this is not a plant you bulk up in a fortnight, but each cutting establishes into a tidy, long-lived clump.
What goes wrong
Melting soon after planting is the common one and usually reflects the emersed-to-submersed transition rather than a mistake — old air-grown leaves die back while new submersed leaves form. It is the same plant melt process seen in crypts; leave the plant in place and let it re-leaf. Stretched, pale growth with weak underside colour most often means too little light. Stalled growth in an otherwise healthy tank frequently points to a lean substrate, since this is a keen root feeder — add root tabs before assuming a water-column problem.
More plants in this series
- Bacopa caroliniana — another rounded-leaf, easy stem
- Alternanthera reineckii — classic red midground stem
- Staurogyne repens — low bushy green midground
- Rotala rotundifolia — fine-leaved colour contrast