Some species have roots coming from all over the place, especially if the light isn't penetrating down to the bottom. Other species propagate themselves by forming little plantlets on their leaf tips, complete with roots.
However, if you have roots in funny places on plants that shouldn't have them on funny places, it would suggest a growth regulator/hormone problem. But it's an unlikely scenario, unless you are using rooting hormone, or something similar.
As far as I know, all stem plants develop "aerial roots". These are plants that will sprawl in nature, growing taller and taller until they rest partly on their side on the bottom of the lake or stream. If anything causes the bottoms to die, like the loss of light from shading, the plant can float away and reestablish itself easily somewhere else. If the water level drops real low, the plant can feed itself from lots of roots along the stem, all now resting on the lake bottom. So, it is probably a survival mechanism for that type of plant.
HC is a stem plant too, and it uses those aerial roots to grow across the bottom of the lake or stream, or our aquarium substrate. Some low growing Hygrophila species do the same.
So, this would be considered unavoidable on a stem plant such as Luwigia Repens?
I'm just getting started with my first "high tech" planted tank and this is the first time I've encountered these aerial roots. Just planted 6 days ago. Specimens were from AZgardens.com and were lovely; dense bi-color foliage with close (~1/2") leaf grouping. After 6 days they have grown approx. 3"-4" in height, but every node now has aerial roots up to 3" long and the foliage density is way down...that is, there is more stem between nodes...about an inch.
Is this normal?
BTW, this is an amazing board. Thanks to all who contribute!
Ludwigia repens gets big aerial roots all the time, and they get long. The ones on mine get trimmed off; I don't consider them attractive.
One inch between nodes is normal. They may have grown theirs emerged, or used some other method to increase density. Plants often go through an adjustment period, and visibly change due to the difference in growing conditions. Some times it's for better, some times for worse. You'll get to know the various growth forms as you gain experience.