How can he successfully kill without detection, if his critical problem-solving skills are
this abyssmal?!?
He could have, but given that he doesn't actually understand how plants work or even why sunlight is bad for him, it's easy to see why he wouldn't think of that.
Nah, he knew full well that the flower needed sun, and that it's bad for him. And he clearly knew that objects existed even when he didn't perceive them (
Object permancence), alongside knowing the sunlight was outdoors at day.
All that should have been enough for him to puzzle together that he could leave it outdoors, close the door (and it would still exist! :O), way til day (sunlight prowls outdoor) and then night (sunlight dies), open door and profit.
The only issue is that he didn't know how often or much it needed water (reason it were dying that quickly is probably because he flooded the pot, not because a lack of sunlight). But if he thought it needed a lot, all he had to do was set up a system to continuously drip water on it while it was outdoors.