Messy aerial roots?

Territorial Instinct

Compare this with the same cuttings planted in a denser bush. In the 'bush' setting, the cuttings concentrated on growing upwards (to reach the light), had less branching and much less aerial root development.

Propagation Instinct

In general, when stem plants start to grow horizontally, often at the surface of the tank, the natural response is for serial roots to start to grow. In nature, this prepares the plant to colonise other areas when it is swept away by currents.

Branching Instinct

Aerial roots are usually associated with side shoots or branches. In many ways, the side shoot is a completely separate plant. When we prune (rather than replant) the tops of stem plants, this encourages side shoots (and therefore aerial roots) to grow. So over-pruning often results in an abundance of aerial roots.

Substrate Confusion

Coarse substrate (such as pea gravel or pebbles) or poor quality substrate can confuse the plants as to where the substrate really is. This can cause aerial roots to sprout in internodes lower down the stem before the stem has gained significant height. The plant is looking for a place to root but does not recognise the substrate as soil due to the large grain size or poor substrate quality.

Under-nutrition

Apart from the above examples, aerial roots can also occur in undernourished plants, where excessive roots are produced as the plant diverts more energy to nutrient-seeking functions. However, in such cases 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:

1. Plant in denser clumps. This works for stem plants that take crowding well (Rotala rotundifolia and its variants, Ludwig arcuata etc.)
   
   
2. 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.
   
   
3. Trim plants before they get too close to the surface. Near the surface, stem plants will usually bend or grow sideways. They would start to branch and aerial roots will almost always develop.
   
   
4. Use finer substrate. This is especially the case for carpets.