Roots above substrate

[TomO]

What would cause my some of my stem plants to throw out so many roots from above the substrate?

[Daniel]

Those are called adventitious roots. It allow plants to propagate vegetatively from leaves and stems and is completely normal. Many plants propagate almost exclusively from leaves and stems and almost never from seeds.

[WeehawkenFish]

That 2nd pic looks like pennywort.  I don't even plant mine I just float it on the top and it grows like crazy.  Totally normal for it to want to feed from the water column.  

[TomO]

So to propagate them, trim and replant horizontally?

[JettsPapa]

8 minutes ago, TomO said:

So to propagate them, trim and replant horizontally?

 

To propagate which plants?  The stem plants, like water wisteria?  If yes, you can cut them and plant, but you don't need to plant them horizontally.

[TomO]

The ones in the pictures with the adventitious roots. Or do I trim them on the stem between the roots and plant the vertically?

[JettsPapa]

42 minutes ago, TomO said:

The ones in the pictures with the adventitious roots. Or do I trim them on the stem between the roots and plant the vertically?

You can trim stem plants pretty much between any two nodes and stick the portion you trimmed off into the substrate.  The parent plant will sprout new growth and the cutting will grow into a new plant.