From Creatures Wikia
The Stopper Plant is a strange plant indeed; it grows beside the Norngarden 1's pool, and can be set to one of three states; repel creatures from the left, repel creatures from the right, or inactive. Much like a simpler version of Emmental's Selective Force Field, the Stopper Plant repels creatures which attempt to move past it when it is active. The Stopper Plant will not hurt your creatures, but pushes them harmlessly away. The side to which the pink, inedible berries face will be the direction creatures cannot approach from. When no pink berries are showing, the Stopper Plant is dormant and will not impede traffic in any direction.
A potted version that is moveable and repels in both directions is also available for download from The Norngarden. A potted Stopper Plant is capable of repelling creatures from both directions simultaneously.
The original Stopper Plant is an integral part of Norngarden 1.