×There's a pretty narrow margin for error, but there is a way to keep the briar from being closer than 4 expansions from the border thicket.

×Also note you can step on the top plate yourself.

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I can step on the first plate.
That leaves 4 pressure plates to be pressed by briar in consecutive order. I can "seed" the briar so that they open one by one, but by the time of the last (4th) growth, it's connected to the rest and kills me.

There is an additional space but it's covered by the yellow door, plus I wouldn't be able to open it without closing the entrance to the escape hallway

×Briar grows out by one tile on each growth, so you just want to make sure that you have four tiles of free space between the briar you place and the briar that's already there.

There's no need to have the briar connect on the very first growth - that won't reach on its own.

×My holds: Beethro's Brain, Entropy, Further Up and Further In, Washed Ashore, Hinterlands, Hypercube, jfflpnk, Lavender Levels.
Collaborations: Way Forward, Advanced Concepts 2