“Aerial roots” refer to roots that often grow out of the internodes of stem plants midway up the stem, above the substrate.
Why do these roots appear? Are they cause for concern? What can we do if we find them unsightly?
Aerial roots are most common in sparse tanks, or when stem plants have a lot of space around them. In such environments, aerial roots develop naturally to support territorial expansion.
If they successfully root to substrate, new stems may develop from the internodes.
In the topmost picture above, a cutting of Rotala ‘blood red’ was placed as a single stem at a corner of the tank. It had a generous amount of space around it, and started to branch and develop aerial roots.
Compare this with the same cuttings that were planted in a tighter bush. In a ‘bush’ setting, the cuttings focused on growing upwards (to complete for light), had fewer branching, and much fewer aerial roots developed.
In general, when stem plants start to grow horizontal, often at the tank surface, serial roots start to grow as a natural response. In nature, this prepares the plant to colonise other areas when swept away by the currents.
Aerial roots generally accompany side-shoots or branches. In many ways, the side shoot is a complete separate plant by itself. When we trim (rather than replant) the tops of stem plants, this encourages side-shoots (and therefore aerial roots) to grow. So over-trimming often results in an abundance of aerial roots.
Coarse substrate (such as pea gravel or pebbles) or poor quality substrate can cause confusion in plants being able to sense where the substrate really is. This can lead to aerial roots spawning in internodes lower down the stem even before the stem has gained significant height. The plant is seeking a place to root but does not recognize the substrate as soil due the the large grain sizing or poor substrate quality.
Apart from the examples above, aerial roots can also occur in under-fed plants where excessive roots are produced as the plant channels more energy towards nutrient seeking functions. However, in such instances, the plant would often show other signs of poor nutrient access: smaller or poorly formed leaves.
If you find them unsightly, you can take the following actions:
Plant in denser clumps. This works for stem plants that take crowding well (Rotala rotundifolia and its variants, Ludwig arcuata etc.)
Instead of simply trimming tall stem plants, replant the tops and discard the old stems and roots. This is routinely done for most Dutch Style Aquascapes.
Trim plants before they grow too close to the surface. Nearer the surface, stem plants will typically bend or grow sideways. They would start to branch, and aerial roots almost always develop.
Use finer substrate. This is especially the case for carpets.