Ludwigia repens: the red stem plant that does not need CO2
A fast-growing stem plant from North America that offers genuine red colouration in a low-tech tank — provided the light is strong enough. Here is how the colour switch works and what to do when the stems turn green.
Where Ludwigia repens comes from
Ludwigia repens J.R.Forst. belongs to the Onagraceae family (the willowherb family), and is native to North and Central America — predominantly the southeastern United States, Mexico, and the Caribbean, where it grows in marshes, shallow ponds, and along the slow-moving margins of streams. Its species name, repens, means creeping; in its native habitat it often spreads horizontally across wet substrate as an emergent plant before sending stems upright.
Unlike the other plants in this series, Ludwigia repens is not from Southeast Asia or South America. The temperate-to-tropical range of its native habitat explains its unusually wide temperature tolerance and its ability to thrive in both warm tropical aquariums and cooler temperate setups without much adjustment.
Several varieties are common in the trade: standard L. repens (green above, red beneath), 'Rubin' (deep wine-red throughout, more demanding), 'Super Red' (intensely red, requires high light), and narrower-leafed forms. This guide covers standard L. repens, which offers the best balance between ease and colour.
Why it turns red
The transition from green to red in Ludwigia repens is driven primarily by light intensity. Under low light, the plant is predominantly olive-green; as light intensity increases, the undersides of the leaves develop a strong red-wine colour first, followed by the leaf surface itself turning reddish-olive to brick-red. The undersides remain red even in moderate light — this is a consistent identification characteristic of the species.
The second factor is iron. Iron is required for chlorophyll synthesis; low iron causes the red pigments (anthocyanins) to dominate as chlorophyll production falls behind. Paradoxically, this means some iron deficiency can intensify apparent redness, but true deficiency leads to yellowing and poor growth. The practical target is to dose iron regularly (most balanced micro-fertilisers include it) and provide strong light, rather than trying to engineer iron deprivation.
CO₂ is not required for red colouration and is not the limiting factor. This is one of the most commonly repeated misconceptions about red aquarium plants. Ludwigia repens specifically demonstrates this: its red colour is as vivid under natural ambient CO₂ levels with strong light as in a CO₂-injected tank.
How to keep it
| Parameter | Value |
|---|---|
| Lighting | Moderate to high — 50–100+ µmol/m²/s PAR; colour intensity directly proportional to light |
| CO2 | Not required; accelerates growth but does not determine colour |
| Temperature | 20–28°C (tolerates 15–30°C — wide range for a stem plant) |
| pH | 6.0–7.5 (tolerates 5.0–8.0) |
| Hardness | Soft to moderately hard — 4–12 dGH |
| Fertiliser | Regular iron supplementation important; balanced macro and micro dosing |
| Substrate | Flexible; any substrate that supports root anchoring works |
| Growth rate | Fast — one of the faster stem plants; 3–5 cm per week in good conditions |
The colour test
Turn over a leaf and look at the underside. In Ludwigia repens, the underside is always red, even in tanks with dim lighting. If the underside is green or pale, the plant is either a different species or genuinely unhealthy. The upper leaf surface ranges from olive-green (low light) to deep brick-red (high light), so the underside is the most reliable indicator of the plant's identity.
Where it works best
Ludwigia repens is a midground to background stem plant. A single stem is unimpressive; a group of 5–10 stems planted in a tight cluster creates the visual impact the species is known for. The contrast between the reddish stems and the surrounding green foreground or background plants is one of the most effective simple colour contrasts in aquascaping.
Because it is a stem plant, it is planted directly in the substrate in bunches — unlike Java Fern, Java Moss, or Anubias, which attach to hardscape. Space stems 2–3 cm apart within a clump. A clump of 7–9 stems positioned at the midground left or right of the tank (rather than dead-centre) gives a natural, asymmetric result typical of aquascape layouts.
The plant's fast growth rate means it will reach the water surface within a few weeks and begin growing horizontally along the top. Left untrimmed it shades lower plants and loses lower leaves. Regular trimming is part of keeping it well.
Pruning and taking cuttings
Propagation is by stem cuttings — the simplest method for any aquatic stem plant. When stems reach the desired height (typically 20–30 cm), cut them with scissors 5–8 cm above the substrate. The remaining lower stems will produce new side shoots and continue growing. The severed tops can be replanted directly into the substrate, spacing them 2–3 cm apart; they will develop new roots within a week or two.
This "top and replant" method is how most hobbyists grow out Ludwigia repens from a small initial bunch to a full background planting. Three initial stems, trimmed and replanted monthly, can fill a 30-cm planting zone within a few months.
What goes wrong
Turning green. The most common complaint. Usually caused by insufficient light — either the fixture is too weak, or the plant has grown into a shaded area of the tank. Move the stems to a brighter position, increase light intensity, or reduce the photoperiod if there are algae issues and increase intensity instead. Check that no floating plants or large overhanging leaves are shading the Ludwigia from above.
Losing lower leaves. Normal in fast-growing stem plants — lower leaves are shaded by upper growth and die back as the plant grows. Trim the stems back regularly (keeping them at 15–25 cm) to prevent the lower section from becoming a leafless stalk. Regular trimming maintains a bushy, full plant rather than a tall, bare one.
Pale or yellow leaves with red tinge. Usually iron deficiency. Increase micro-fertiliser dosing, especially iron. If using a substrate without nutrients, consider root tabs placed near the base of the clump — Ludwigia feeds effectively through its roots in nutrient-rich substrates.
Melting after introduction. Like many aquatic plants, L. repens may shed leaves when transitioning from emersed (shop) to submersed conditions. New growth will be adapted to the aquatic environment. Allow 2–3 weeks for the transition before worrying; remove dead leaves to maintain water quality.
More plants in this series
- Anubias barteri var. nana — the indestructible epiphyte
- Java Fern (Leptochilus pteropus) — the fern fish won't eat
- Java Moss (Taxiphyllum barbieri) — the moss that goes anywhere
- Amazon Sword (Echinodorus grisebachii) — the plant that started planted tanks
- Monte Carlo (Micranthemum tweediei) — the honest carpet plant guide
- Cryptocoryne wendtii — rosette, runner, survivor
- Vallisneria spiralis — the original background plant
- Rotala rotundifolia — the colour-shifting stem plant
- Eleocharis parvula (Dwarf Hairgrass) — the carpet that tests patience