Can Mermaid Weed and Undulata Grow Together?

Mermaid Weed and Undulata share a workable water window around 20 to 28 °C, pH 6 to 7.5, and 2 to 12 dGH.

Both plants are comfortable in freshwater, so salinity is not a meaningful obstacle.

Flow is workable if the layout gives Mermaid Weed moderate flow and Undulata gentle, low-flow water.

The care split shows up in light or CO2. Mermaid Weed wants high light and recommended added CO2, while Undulata wants low light and no added CO2.

Both plants naturally lean toward the midground and background, which is why spacing, pruning, and final mature size matter more than they do in a more staggered planting mix.

Mermaid Weed reaches about 40 cm tall by 10 cm wide, while Undulata reaches about 25 cm tall by 20 cm wide. Use those mature sizes for the layout, not the small nursery portions you bring home.

Shade is not the main concern here, which makes the layout easier to keep balanced over time.

Mermaid Weed is typically rooted in substrate with nutrient-rich substrate preferred and feeds mainly as a mixed feeder. Undulata is typically rooted in substrate with nutrient-rich substrate preferred and feeds mainly as a root feeder. That difference can make the pairing easier to arrange than two plants fighting for the exact same root or attachment zone.

Mature size is not the main thing working against this pairing, so normal maintenance is usually enough to keep the scape readable.

Mermaid Weed brings moderate growth, moderate maintenance, and intermediate difficulty. Undulata brings slow growth, low maintenance, and beginner difficulty. If one grows much faster, trim that plant before it starts making the other look like the problem.

The practical watch-outs are that one plant is much more light-hungry, so the scape will need placement and trimming discipline; and that both plants tend to work in the midground and background, so spacing matters more than usual.

The strongest reasons to try the mix are that they share a workable temperature window around 20 to 28 °C; and that their flow preferences sit close enough to tune one layout around both plants.

Use this pairing when you are willing to manage the scape, not when you want a plant-and-forget combination. Start with more spacing than you think you need, then adjust once both plants show their real growth pace.

The simple success test is whether both plants still look healthy after the faster grower has been trimmed several times. If one keeps declining after routine care, the layout is probably asking too much of it.

This pairing is best treated as a layout decision, not just a water-parameter match. Mermaid Weed and Undulata can work together, but only when you intentionally manage spacing, shade, and maintenance so the stronger grower does not quietly turn the other into dead weight.