What are the consequences of watering my plant?
Max returns to her dorm room once in each of the first three episodes, but is only given a chance to water the plant in the first two episodes. You will always be given a chance to water the plant in episode 2 whether you watered it or neglected to in episode 1. If you don't see this option, it's likely Max has another task at hand which should be finished, after which you'll regain access to all the other objects in the room, including the plant.
In addition, Max is supposed to receive a text message from her mother reminding her about the plant in the beginning of episode 2. It says:
Hey honey! I hope you didn't forget to water the plant we gave you but don't drown it either! mommy
However, those who played through the first two episodes before the third one came out on May 19 did not receive this text message due to a bug which was only fixed prior to episode 3's release. Unfortunately, by the time they do receive it at the start of episode 3, it is too late, as there is no option to water the plant then. Since this text message was supposed to appear earlier in the game, some players may be compelled to replay the second episode if it would have influenced their decision to water the plant (or not). Others may choose to pretend the text was always there and they simply missed it by their own negligence, and move on for the game's sake.
Here are all the possible outcomes:
You water the plant in both episodes
The plant is over-watered, and killed. Anyone experienced with gardening knows why over-watering is dangerous; in this case, once a day turns out to be too much for poor Lisa.
You water the plant in episode 1, but not episode 2
The plant remains healthy in both episodes 2 and 3.
You don't water the plant in episode 1, but water it in episode 2
In episode 2, the plant's leaves discolor. In episode 3, after watering, the plant will be healthy again.
You don't water the plant at all
The plant dries out and dies.
Interestingly, when you examine the plant in episode 3, Max will comment on it differently depending not only on the choices you make above, but on other actions as well...
Regardless of whether the plant is still healthy or dead by the third episode, you will no longer have the option to water it. The game files seem to suggest that the option was there, but it could have been dummied out as I haven't been able to find a way to trigger this option. Instead, whether the plant is alive or dead is reflected in the episode 3 postgame statistics.
Max does not visit her room again in episodes 4 and 51, and the plant's fate appears to have no influence on the plot whatsoever. How anticlimactic.
You do get to "visit" her room once in episode 5, but you only do so by jumping to when she first takes her wall selfie, before the events of the game.
If you water it twice, it will be drowned and will become brown/unhealthy by Episode 3.
It's a 5 part game so be patient with it. My guess is a lot of the choices you make in the first episode will have some effect in the future. There are a lot of choices you make that will trigger the butterfly icon that didn't have any effect. For example :
signing/not the petition from your teacher, making fun/comfort Victoria, rearrange/not the pics in Victoria's room, helping the bird from hitting the window, reading/not the files in the garage.
Looks like everything in the post game statistics will have a future impact of the game. Hopefully at the fifth episode you don't need to choose 3 colors :P