To prove:
1. Place sand down, place cacti on the sand, and place a solid block one block up and to the side of the cactus.
2. Speed up the game and/or wait a few minutes.
3. You should observe cacti growing into spaces with blocks cardinally adjacent.

To fix:
1. Add block detection system for blocks cardinally adjacent to the target growth spot, triggered every time a cacti grows.