The word "adjacent" doesn't seem to be used consistently, which is annoying. The Spore Lily, which the player is introduced to first, doesn't want to be adjacent to any other flowers, and diagonals count as relevantly adjacent (fair enough, the bonus isn't that significant anyway). But for the Glacier Vine and Moonbell, diagonals do not count as adjacent, and so they don't provide their bonuses into the diagonals, which comes as a bit of a surprise when the player had learned that 'adjacent' ought to include those tiles.
To be clear, it's understandable why you as the dev don't want the bonus-giving plants to radiate their bonus to 8 plants instead of 4. Restricting the Spore Lily to have absolutely no neighbours even on the diagonals feels a little less fair, though it's not a completely unreasonable design decision. But those are different conditions, so you really ought to use different words to describe them.