For this game, I was intrigued by the premise and that introduction which sets the perfect tone for how players should approach the game. Really, This could be consolidated to a one page RPG or even a brochure format. It's minimalist, which isn't an issue. I even felt it spoke to how sparse and devoid the world may be of life.
I think there are a few things that are missing which would make this a much more solid offering, though.
One: you should mention that this is a solo ttrpg for prospective players (even though I think there's an opportunity here for asynchronous play with other players where you write letters or notes to them instead of your siblings). You should also explicitly state what kind of dice players should use. There are SO many. I know from experience that you're likely thinking of 1d6 but since the instructions state you need "a green die" and "you'll roll a die" someone can come in with 1d4 (and have a very easy time) or a 1d20 and weep continuously lol. You can encourage players to try other dice but there really should be an optimal one for the experience you'd like to offer. The last bit was the dormant and wild portion. I understand leaving it up to the players imaginations but if the garden turns dormant...do you keep playing out the full 12 months/12 turns (I assume, too, that you weren't encouraging players to journal 365 letters in order to play) and can it come back? If the Garden is wild...do you stop then too? If it's 1d6 I'm assuming you can't even have a roll where it gets still yours to tend. As a player I'd ask that question for sure. Cause you may wish for the player to start again and just keep playing but that isn't stated in the instructions.
I still like the look of it all though. So simple and straightforward in concept. Very easy to grasp and the mechanics aren't labored. I can see this one as being very friendly to first time solo ttrpg players with some tweaks.