First step of any of these DIY projects is to check if I can just buy the solution.  At first glance, the price jump between most capacitive sensors and low end TDR sensors is pretty big but some cheaper examples of TDR sensors exist at the $50 price point:

https://www.vegetronix.com/Products/VH400/

More than I'd like to spend on each plant I want to water, so there's actually a chance it makes sense to create my own solution that will be cheaper if I need to make several. 

Alright, so how does that actually work? Well, turns out there a pretty good background article on that too:

https://www.edn.com/use-time-domain-reflectometry-tdr-for-low-cost-liquid-level-measurement-part-i/

https://www.edn.com/use-time-domain-reflectometry-tdr-for-low-cost-liquid-level-measurement-part-ii/

https://www.edn.com/use-time-domain-reflectometry-tdr-for-low-cost-liquid-level-measurement-part-iii/

The TLDR version is that the speed that a voltage step moves down a transmission line is related to the medium the electric fields pass thru.  In PCBs this is the typically FR4 fiberglass that makes up the board itself, not the copper of the traces itself.  This is true for cables and anything else too.  Also, an open or short circuit will cause that voltage step to be reflected back to the sender, it is possible to measure the time it reach the end of the transmission line and come back from the sending side by looking at the final voltage step.  

So the basic concept is that by measuring that time, one can learn something about the medium the step is traveling thru.  For example, by dunking the line in water will cause the time to increase when compared to the time when the line is in open air.  

More subtle differences, like that between dry soil and damp soil are also possible and that seems to be the working principle of these TDR soil moisture sensors.  

But how subtle is that difference? I figured it would make sense to test this out first, so I will build a pulse generator I can use with my oscilloscope as a proof of concept.