I really enjoyed the graphics, especially the lighting! It took me a while before I noticed the scythe in the bottom right corner. An outline could help. The same goes for the plants. On my monitor it was hard to notice what state they were in.
It also took me a few attempts before I understood what's going on (for example I though that the purifier device didn't do anything, but in reality I just had to wait a bit longer near the water station to fill the can). But when I understood the mechanics it started feeling really cool. My highscore is 12 plants.
The sfx added a lot to the atmosphere and the feel of the game, but with the footsteps - I think that some noise supression could help since you can hear background noise. When playing I really felt like I was underground and isolated from the rest of the world. Maybe adding some ambience noise could work. It doesn't even have to be music.
It was a cool experience!
We also both used 6 hours and made a game about growing plants. Coincidence? I think not.