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The zone that would be most difficult for a plant to live in is the  Benthic zone. It is the zone at the lowest level of a body of  water.
The habitable zone defines the region where a planet might be able to retain liquid water on its surface. Any closer to the star and water would vaporize away; any farther, and it would freeze to ice. But water in its liquid state is what scientists are after, since that is thought to be a prerequisite for life.