This is probably a total newbie question but there seem to be roots growing out of my stem plants toward the substrate. What are these and if i cut the stem below these "roots" and replant it will it grow?
Yes it will grow!!!
If the stem is long, you can cut it half or more and replant it. Cut the root short too to make it easy to put under the substrate.
You will enjoy doing this when your plant grow more!!!
They are called aerial roots. I get them on my rotala, cabomba, and hygro all the time. Most of the time, if I don't trim anything, a new stem branches from that point, or if snip the top off below the roots a new stem grows from the substrate. It seems that my really healthy stems do this more than the ones that are sub par.
^^^Agreed! I've seen this a bunch with my healthier stem plants in the past. Sometimes it can be an eye sore, but you can always plant something near the base of your stems to hide the roots. I've used blyxa japonica and lobelia cardinalis for this purpose.
These roots are normal on stem plants. it is most commonly seen when theres not enough nutrients in the substrate and it is trying to draw some out of the water column. It still occurs when there is enough nutrients but just not a lot.