Is this correct...?

When stem plants get too tall for the tank you cut the stem then replant the top section and discard the bottom?

That’s the conventional wisdom, but it means ditching your established root system every time you want to shorten a plant and then hoping that it re-roots successfully before succumbing to the stress of having its vascular system eviscerated. It seems to work for most people (me included) and I think having a quality substrate and proper water chemistry and nutrients helps with the re-rooting. Also, by moving the upper part of the plant down you get rid of lower sections that can get leggy or funky.

Depends on the plant and the condition of the bottoms. With most Ludwigias, the bottoms tend to drop all their leaves as soon as they get shaded so pulling the bases and replanting the tops is the way to go with those. The same goes with E. stellata.

Other plants, like Rotala rotundifolia, retain their lower leaves so you can quickly build a dense stand by replanting the tops among the existing stem bases.

I’ve been staying away from a lot of stem plants because I don’t know I’m not sure how to replant them. How do you replant all those plants then get them to stay in the ground without floating back up? I trim then replant and I usually see half of them swirling around in the water a short time afterward. Especially after water changes which is a pain!

I have ahad alot of problems getting my stems to stick int he SS. Do you all use lead weights, or just persistance in planting them in the SS?

Replanting success for me seems to vary with the type of plant. If I can, I'll replant the stem in the middle of a water change, when the water level is lower. Then, I do my best to "squeeze" the substrate in around the base of the plant to hold it down. If the trimming continues to float, I'll shorten it a bit and/or prune off any leaves that look excessive/bad. In a few cases, I have had to resort to plant weights or just giving up !

Seeing as how roots seem to form around the internodes/leaves of the plants (in the water column) I've been trimming them at a node and leaving a bit of matter on the stem as an "anchor" long with the roots. I've worried about potential rot but it hasn't been an issue so far.

Does uprooting all the stem plants everytime you cut distrub the substrate thus encourages algae?

I don't think it would. Most stem plants don't have massive root systems anyway, so this shouldn't really stress them too bad, if at all. Furthermore, the plants will likely be sucking in more nutrients while reestablishing their root systems and growing out after a trimming...It doesn't seem like it will give algae much of a chance, but someone can correct me if I'm wrong.
I haven't tried trimming in this way yet...Usually I just chop off the tops and sometimes replant if possible. I'll try doing it this way and see what happens. Seems that it would look a lot nicer. A lot of my stem plants have "funky"-ness and messy debris down near the substrate(shaded areas), so this would seem to keep things looking more attractive. Some also have roots that started growing out from the stalk early on, before the tank got settled. I'll try doing this and report any findings.

As with most things, take your time and be careful.

If you go yanking the stem out as quick as possible, you may pull up some unwanted debris/additive from the substrate, but even if you do, it would be such a small amount in comparison to the overal tank that I doubt very much that it would ever cause problems.
I gently ease the stems out and never have any problems.

I've not really had any problems with planting in my 2-3mm course gravel as long as you plant the stem deep enough (i.e, not just under the surface gravel).
I find 2-3mm smooth gravel (in another tank) to be slightly harder due to it's round grains.
You can almost forget about planting stems in the larger 10mm gravel. I've tried it many times. You have to use weights to keep them rooted there.

Stems might still uproot if...
a) you have quite strong water current.
b) you have fish that "play" with your gravel.

I cut mine and leave the bottom planted and only replant the tops if I want a dense planting and to get them planted I clean off about 3 to 4 inches of the bottom leaves then I place the bottom of the stem in my Eheim tongs and push it all of the way to the bottom of the substate, they never float.

Whats happens if you just cut the top portion of the plant and leave the rooted part in the substrate?
Won't it just send out new leaves to start over?

My husband foolishly left some kind of computer-geek tweezer thingy laying on his desk. It's about 6 inches long and has a blunt but very narrow tip. A godsend! I gently grab the very end of the stem with the doo-dad and drive it as deep into the substrate as I can. Then I gently "ooch" the thing out, leaving the stem behind. This way I can get stems planted deeply without beating them up. Nice little gadget that!

Kathy

Pruning the top (leader) will cause the stem to branch lower down. This can look ok in time as more branches hide the original stem, but IMO the harshly cut tops just look too unnatural. This is most noticable in stems which produce a few large leaves.

Wow, 1 year in the hobby and I'm still learning! I was doing it wrong all this while!