So I gave up on the temperature monitoring for the time being, as watering it became more important.
With us wanting to take a few vacation days, I figured I should get this going so that we don't need someone to constantly water it.
I have been chipping away at it very slowly over the summer.
At the start of the summer I added a sprinkler water line going around the inside perimeter, with little tubes and dripper spikes going into every pot.
For a while I used just a bucket with a little fountain pump to send the water to the line to try it out and all was well.
I then bought a (clean and new) 120 liter garbage can that I use to hold the water. It's still using the little fountain pump because it seems to do the trick.
So I have the task of watering reduced to me just having to plug in the pump now 
I chose a garbage can because it was a fraction of the price of a rain barrel of roughly the same capacity. So the whole project is still affordable too.
I made the mistake of adding an organic liquid fish fertilizer into the water though, which then caused some kind of fungal or algae growth (something slimey) that clogged all of the little dripper tubes and took me a full day to clear up again. My neighbour is using a hydroponic setup in his greenhouse and his fertilizers don't seem to have that issue, so maybe the fish stuff is a bad idea. So the automatic fertilizing part needs some work - maybe I'll eventually get an in-line dispenser or something.
For the technology of it, I have it pretty much done except for assembly - I bought an electrical box and with outlet - the type that one would add to a workshop wall to add a plugin. I was able to mount the relay board into the bottom of it. Then I used an old computer power cable and wired it into the box and the relays. So basically you plug that into the wall and the two outlets are relay controlled. With the Arduino beside it I can easily control the power to the two outlets.
That left me with needing a way to power the Arduino board, so I bought a wall plugin to power cell phones off eBay (less than 2 bucks) - it has a spot to plug in a USB cable so I could plug the arduino into that. That works, but why use an extra outlet? So I took the adapter apart - the cover came off easily with one screw and nothing was glued. I then desoldered the power connections and added some "pigtails" (short electrical wires) that I could just wire into the same electrical outlet to get power. So far, so good.
Then to make it more compact still I desoldered the big USB connector and soldered on a short length of a USB cable with the end that plugs into the Arduino board.
That all works welll too.
That's where it's at right now. I have enough space in the outlet box to fit in the power adapter, but I still need to figure out how to mount it safely.
And then the trickiest part might be where I try to squeeze an Arduino Nano into the box too, although I'm thinking that because I want to use a 16x2 lcd display that I'll have to use an external box for that anyways.
Cheers,
-Nico
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