They don't need substrate.they are Fresh water plants that can TOLERATE!!!!! salt yet because of their comparatively slow growth are simply out competed for light , thats why you don't see them up and down the banks of every freshwater stream, river, and lake that doesn't drop below freezing.
The new growth that forms a "tip" that can should be clipped after at least two-three sets of leaves have emerged and opened (not including the two new leaves that are still "fused" together to form that "tip").
This will cause a split (2 out of 3 times, depending on your accuracy).
Mangrove propagules fall from the adult tree, and land in the water, and float around with just the tip sticking out of the water,(1/4"-1"(max))..and float until they touch something, sand, rock ,mud ,some are just capture by adult plants prop roots, and will start growing, sometimes for years before they can get a hold fast.
Someone early in the thread made a reference to "prop roots" incorrectly,
The roots common out of the bottom of the plant are simply roots, the ariel prop roots won't start emerging until it is at least a couple of years old and has in fact a nice thick "Bark" not green ,..but a gray/brown bark just like the trees around you in North America proper.
This is a wild prop floating past my dock (back yard)..this is the most submerged it gets,(note the Doctor fish in the background)
The second photo is of a prop floatting next to my dog's but,..and a wild one (to the right) witch I "split" three years ago with (Julian's clipping method I described above)-Dave
By the way Julian Sprung puplished the best + simple mangrove care book back in the 90's it's a small paper back affair, yet well worth tracking down!