Carpeting plants that don't need CO2

Monte Carlo, Marsilea, dwarf hairgrass, and dwarf sag can carpet without CO2 injection. Expect 8-16 weeks and proper light. Realistic timelines inside.

A dense foreground carpet is one of the most visually satisfying things in a planted tank, and one of the most common reasons people think they need CO2 injection. The association is strong: competition aquascapes with flawless HC Cuba carpets all run high-tech setups with pressurized CO2, intense lighting, and daily fertilizer feeding. But if you're not entering IAPLC and just want a green mat across the front of your tank, several species can get there without injected CO2. It takes longer, and the result is a bit different, but it works.

What changes without CO2

Without CO2 injection, plants grow slower. That's the simple version. The longer version is that atmospheric CO2 dissolved in tank water typically sits around 3-5 parts per million, while injected setups push 25-35 parts per million. That's a 5-10x difference in available carbon, and growth rates scale accordingly.

For carpet plants specifically, this means three things. First, filling in takes weeks instead of days. Where a CO2-injected Monte Carlo might cover the foreground in 3-4 weeks, the same plant without CO2 could take 8-12 weeks. Second, growth may trend more vertical. Without abundant CO2, some species produce taller leaves and runners that reach upward toward light rather than spreading laterally along the substrate. Third, the carpet will be less dense. CO2 encourages branching in stem plants, and the same principle applies to carpet runners. More carbon means more branching points and tighter growth.

None of these are deal-breakers. They just mean different expectations.

The reliable low-tech carpet species

Monte Carlo (Micranthemum tweediei)

This is the default recommendation for a low-tech carpet, and for good reason. Monte Carlo has slightly larger leaves than HC Cuba (which makes it more forgiving), spreads through runners, and adapts to a range of lighting conditions. With CO2, it grows fast and hugs the substrate tightly. Without CO2, it still carpets, but expect 8-12 weeks to fill in a 60 cm tank foreground assuming you plant in small clumps about 3 cm apart.

The catch: Monte Carlo needs decent light even without CO2. If your light can't deliver at least 35-40 micromoles of PAR at the substrate, the plant will grow slowly and tend upward rather than outward. A nutrient-rich substrate (aquasoil or root tabs in inert substrate) makes a significant difference. It also helps to physically press down new growth with your hand during water changes for the first few weeks. This encourages horizontal spreading.

Marsilea species (M. hirsuta, M. crenata, M. angustifolia)

Marsilea are miniature aquatic ferns that resemble tiny clover leaves. They're among the most tolerant carpet plants and handle lower light better than Monte Carlo. M. crenata is the smallest of the three, M. hirsuta is the most widely available, and M. angustifolia falls in between.

The growth rate is slow. Very slow. Expect 12-16 weeks for a full carpet, and even then it might not be as tight as you'd like. The trade-off is reliability: Marsilea almost never fails outright. It just takes its time. In low light it produces larger leaves on longer stems, more like tiny lily pads than a tight carpet. With moderate light, the leaves stay small and close to the substrate.

Marsilea can go through a transition phase when first planted, especially if purchased in emersed form. The original leaves may melt and new submersed growth takes over. This is normal, not a sign of failure.

Dwarf hairgrass (Eleocharis species)

Two varieties matter here: Eleocharis acicularis (standard dwarf hairgrass) and Eleocharis "mini" or "belem" (smaller, shorter version). Both spread through underground runners and create a grass-lawn effect.

Without CO2, dwarf hairgrass grows and spreads, but the carpet is noticeably less dense than in high-tech setups. The blades tend to grow taller (5-8 cm instead of 2-3 cm) and may need periodic trimming to keep them short, which actually encourages lateral spreading. Strong light is important here. Moderate light produces sparse, leggy growth that doesn't look like much of a carpet.

The "mini" or "belem" variant stays shorter naturally and is often a better choice for low-tech tanks where you don't want to trim constantly.

Dwarf sagittaria (Sagittaria subulata)

Dwarf sag is the most forgiving option on this list. It tolerates low light, no CO2, hard water, soft water, and general neglect. It spreads aggressively through runners and will fill in a foreground reasonably fast (6-8 weeks in good conditions).

The trade-off is that it's the tallest "carpet" plant here. Individual leaves can reach 10-15 cm, and in low light they may stretch even taller. It's more of a short grass field than a tightly mowed lawn. If you're after the classic competition-style 1-2 cm carpet, dwarf sag won't get you there. If you want a green, living foreground that fills in reliably with minimal fuss, it's the best choice.

Honorable mentions

Pearlweed (Hemianthus micranthemoides): Not a traditional carpet plant, but it grows low and dense when trimmed aggressively. It's extremely easy without CO2 and fills in fast. The maintenance cost is frequent trimming because it grows quickly.

Moss (Taxiphyllum species): Spiky moss or Christmas moss attached to flat stones or stainless steel mesh can create a carpet-like effect. It works in very low light and needs no CO2. The look is different from a true carpet plant, more textured and less uniform, but it can be striking.

Substrate matters more without CO2

With CO2 injection and heavy water-column feeding, a carpet plant can get everything it needs from the water. Without CO2, root feeding becomes proportionally more important. A nutrient-rich substrate like aquasoil or laterite capped with sand gives carpet plants a consistent food source that doesn't depend on water column fert schedules.

If you're using inert gravel or sand, root tabs are a must. Push them into the substrate every 10-15 cm across the foreground, about 3-4 cm deep, and replace every 2-3 months. The substrate calculator can help you figure out how much substrate you need for proper depth.

Planting technique

For all carpet plants, plant in small portions spaced 2-3 cm apart rather than large clumps with gaps between them. More planting points means more origins for runner growth, which means faster fill. Use fine-tipped aquascaping tweezers and push each portion firmly into the substrate at a slight angle. The substrate should be at least 3-4 cm deep in the foreground, ideally sloping shallower toward the front glass.

A dry-start method (DSM) can dramatically speed up the initial establishment phase. Fill the tank with substrate, mist it thoroughly, plant the carpet species, seal the tank with plastic wrap, and let it grow emersed for 4-6 weeks before flooding. Emersed growth is much faster because the plants have unlimited CO2 from the air. This technique works especially well with Monte Carlo and dwarf hairgrass.

Realistic timelines

These assume moderate-to-good lighting, nutrient-rich substrate or root tabs, and planting density of small clumps every 2-3 cm in a 60 cm tank:

Monte Carlo: 8-12 weeks to cover, 12-16 for a dense carpet.

Marsilea: 12-16 weeks minimum. Patience is the entire game.

Dwarf hairgrass: 10-14 weeks. Trimming short at week 6-8 encourages lateral spreading.

Dwarf sag: 6-8 weeks. The fastest filler on the list.

These timelines assume no CO2. With a dry start phase, subtract about 4 weeks from each. Use the lighting calculator to verify your light levels before attributing slow growth to species choice when the fixture might be the bottleneck.