A successful Monday - got the entire sensor suite running properly.  

One notable - and very easily preventable - issue that I ran into: I had wired the RTC into a ground rail that didn't tie back to the Arduino's ground.  Always double-check your grounds!  It's easy to overlook, but it can cause a lot of confusing data problems.  A teachable moment, at least.

Moving forward:

Testing in the greenhouse begins this week.  I'll replicate the previous study's conditions as much as possible to see what kind of correlation we find.