This page explains how to recruit Turnip in Chrono Cross: The Radical Dreamers Edition.
Turnip is a walking, talking vegetable. That’s it, nothing will surprise us anymore.
How to Recruit
First, you need to complete the Water Dragon Isle sidequest prior to Fort Dragonia. This will net you the Ice Breath. Head for Hermit’s Hideaway (Another World) and search for a dry patch of soil near the center. Inspect this patch and it’ll say it’s too hot for vegetation to grow. That’s your cue to spray it with the Ice Breath.
Later on, after getting Serge back, head for Hermit’s Hideaway (Home World) with Poshul in your party. Inspect the same patch of soil and Poshul will end up digging up Turnip, who joins on the spot. However, Poshul is a missable character, so you can’t recruit Turnip if you missed her.