Can Hornwort and Large Ammannia Grow Together?

Hornwort and Large Ammannia share a workable water window around 22 to 28 °C, pH 6 to 7.5, and 4 to 12 dGH.

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

Flow is workable if the layout gives Hornwort gentle, low-flow water and Large Ammannia moderate flow.

The care split shows up in light or CO2. Hornwort wants low light and no added CO2, while Large Ammannia wants high light and recommended added CO2.

They naturally settle into different parts of the scape, which gives you more room to use each species for what it does best instead of forcing direct competition.

Hornwort reaches about 100 cm tall by 15 cm wide, while Large Ammannia reaches about 50 cm tall by 15 cm wide. Use those mature sizes for the layout, not the small nursery portions you bring home.

Shade is the biggest layout risk. If the taller or denser plant gets ahead, the other one can slowly decline even when water and nutrients still look fine.

Hornwort is typically free-floating with no substrate required and feeds mainly as a water column feeder. Large Ammannia is typically rooted in substrate with nutrient-rich substrate preferred and feeds mainly as a mixed 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.

Hornwort brings fast growth, moderate maintenance, and beginner difficulty. Large Ammannia brings moderate growth, high maintenance, and advanced 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 shade becomes a real risk here, especially once the taller or broader plant settles in; and that their substrate preferences are different enough that rooted nutrition should be planned deliberately; and that growth pace and maintenance rhythm are uneven, so the stronger grower can dominate if pruning slips.

The strongest reasons to try the mix are that they share a workable temperature window around 22 to 28 °C; and that their flow preferences sit close enough to tune one layout around both plants.

Skip this pairing for most display tanks unless you have a specific reason to experiment. A better long-term choice is a partner plant that shares the same water window and asks for less compromise in light, flow, or maintenance.

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.

Hornwort and Large Ammannia are usually better used in separate scapes built around different goals. The practical problem is not that one of them is a bad plant; it is that their long-term maintenance rhythm, spacing, or environmental preferences pull the layout in different directions.