Poppycock.
This is exactly the kind of information that places like The Spruce, eHow, etc. are terrible at. They are just regurgitating info from other "sources" they find on the web, just like you can do. Witness their constantly vague and incomplete "advice". Either they don't really know, or the writers have no technical writing skills (or are just unable to write concisely). Probably both. Notice there are never any references, or even casual references in the text itself? It's like they consider themselves infallible, and there is no need to back up their claims, because they are the expert in everything. Even the "all knowing" wikipedia isn't so great, but at least you can look up references to see if what it is regurgitated has basis.* Google is your friend, but one always needs to be vigilant with the sources. Google Scholar is your better friend.
At one time, I was also interested in cutting propagation of Liriodenderon tulipifera, as I have one of the few trees that survived minus 32°F unscathed. Tried once with eight cuttings. Two callused, and one grew one single root, but eventually perished.
Here are some old, but still valid sources I had saved that you can consider:
https://academic.oup.com/jof/a...
https://www.scionresearch.com/...
I can't necessarily say that no 18" cutting will root, but I can say that no commercial propagator
ever takes 18" cuttings - of anything. Cutting propagation of Tulip poplar in just water is utterly ridiculous. Except for easy to root woody plants, trying to root with fall cuttings is pretty unheard of. It seems early to late summer Tulip polar cuttings are best to try, and in the juvenile state (regardless of softwood or hardwood cuttings). It's always been my with woody cuttings that actually root from mid September on, those roots die over winter here in colder Minnesota (but sometimes new roots initiate the following late spring,early summer). At the same time, for cuttings from the same species that root earlier, those roots survive the ensuing winter.
More sun is always better
IF cuttings can be kept turgid and temperatures don't get too high. This is usually impossible in amateur set ups, so bright indirect light is usually best. A Nearing frame, or something that approximates it works well.
It sounds like you might want to invest in a good book like this one to get started:
The Reference Manual of Woody Plant Propagation by Dirr and Heuser
*Here is a challenge: look somethings up in wikipedia that you already know a lot about. Read it with a skeptical eye, and when you find something that doesn't seem quite right, look up the reference given (if there is one!). I will lay odds that the original reference did NOT say what the wikipedia author claimed.
