Trimming and replanting stem plants?

Snip away. All the tops can be replanted and will continue to grow. All the bottoms will sprout new stems, usually 2 at the node below the cut. This is how you get the bushy growth you see in a lot of scapes.

The Ludwigia species may not like over crowding. In that case, pull the plants and replant the tops only.

Yes, the plant will still grow after you cut off the top

Wisteria would probably still grow if you set the whole thing on fire and replanted the charred husks. I got some of this in a petsmart tube, and it was obviously from trimmings (no roots to be seen). Now it's crowded out 1/3 of my 20gal in about a month. Time to trim the trimmings!

Crispino Ramos said:

The Ludwigia species may not like over crowding. In that case, pull the plants and replant the tops only.

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This is typically what I do with my ludwigia repens, just cut off the new growth and shove it in the substrate. The left over stem resprouts quickly and often times, two at a time. I typically just pull the old stem out and only use the new just because I prefer that look.