I have this dumb cane plant, but it is getting tall and I'd like to cut it down to size. Found info on the internet but I need clarification.
The plant was in a low light situation for a few years and grew quite tall and thin. We recently moved to a brighter location, and it is looking healthy on the top again.
However, now the plant is too tall and won't support itself. Question, how hard can I cut it back ?
You can see that the stem grows off the side of an old larger stem. If I cut the growing stem back to 6 inches, then there would be no leaves remaining at all. Will this kill the plant, or will it simply regenerate a new stem?
I do plant on propagating the top portion of the new growing part as a back up. Any tips on that?
It'll produce a new stem if the top is cut off. Potentially more than one, even. (You do have to watch the watering while it's working on that, though -- it'll stay wetter a lot longer than you're used to, without the leaves to transpire water. Too much water and the stump will rot before it can resprout.) This will happen faster if the plant is in a warmer, brighter spot, but should still happen if the plant's in the original location.
I've propagated cut-off tops by 1) direct-sticking them in potting mix, and 2) by rooting them in water first and then transferring to soil. There's some risk of rot with the former. Rooting in a sterile medium (like damp vermiculite) is a nice compromise between the two, though I haven't done that with this particular plant before.
They'll also produce new growing tips from sections of the cane, provided the sections are long enough (about 3-4 inches / 8-10 cm). I've had good luck with Aglaonema (related, if not quite the same plant) in vermiculite, in a covered container to keep the humidity high while rooting is happening. I've managed to start a few new Dieffenbachias by laying canes horizontally on potting soil too, but that hasn't worked as well. Plants started from cane sections also take longer to develop full-sized leaves.
Further to what mrsubjunctive has to say, cane sections for propagation need to be at least 3-4 nodes long (nodes are the scars on each section of the cane from the leaves). I just gave one of my Dieffs a healthy haircut, and will see about 10 new plants from what I pruned. I tend to bunch groups of cane or cane sections together, so that as they grow they support each other. My current pot of crowns, for example, has 4 plants in a box grouping - it will help to keep the plant upright even when it gets more top-heavy and starts flowering.
I go with the "cut it and stick the cut sections directly into damp potting mix" method, which has always worked for me; I'd add that I won't water, but rather just mist the surface of the soil, until I see new growth developing. This helps to prevent rot. Water-rooting has its advantages, but the roots are weaker than what's developed with soil-direct planting, and I've found that the plants are more prone to shock when transferred from water to soil.
EDIT - if you're cutting sections of cane blind (without any leaves) you can check for branching nodes along their lengths - these look like little nubbins of green. I'll go take a picture of mine, because it's hard to explain them well. If you can get at least one if not two or three of these branching nodes on each section of cane you're going to propagate from, you'll see leaves much more quickly because the plant was already thinking of starting to produce them at those points.
OK, here's what I'm talking about in terms of nodes. In the first photo, the nodes are well developed and starting to branch. In the second and third, they're still in the eruption phase. You want at least one if not more of these in each section you cut for propagation - it speeds the leafing out time considerably and ensures the survival of your new plants.
(Years ago, my mum used to refer to Canada's then-Prime Minister Dieffenbachia, as she was more familiar with plants than politics.)
In Costa Rica, I became used to seeing Dieffenbachias growing pretty much as vines. My indoor Dieffenbachias seem so big in our Vancouver apartment, but in the wild haunts of the cloud forests we see them enjoying life more naturally. Everything is big there, and so a Dieffenbachia as a vine makes sense.
I mention this because it changed my perspective in cutting the houseplants back when they reach the (mere) 8-foot ceiling height. It also explains why Dieffenbachias, when ignored, sometimes grow at angles closer to the horizontal, when there is no alternative. These are quite snaky plants.
I have successfully cut ours back, from time to time, by taking my largest kitchen knife and using it as a machete, checking for safety first and then making one single very fast slash to make a very neat cut, thereby chopping the stem in two in that one single swing. If everything is done just right, I am thusly left with plus-one Dieffenbachia and my original number of fingers.
It is that simple. Other writers on this thread have given you all you need to know to be an expert. You are very likely to go through the actual process and find yourself saying, "Well, that was easy." Best of luck.