To unlock this achievement you must harvest fruits from the plants you have on your garden in Plumpet Island.
As you progress on the story, you will be traveling often to/from Plumpet Island to replenish your health, fast travel, buy stuff, sell stuff, craft weapons, craft potions and craft ammo.
Everytime you arrive in Plumpet Island remember to go to the garden (top-left part of the map) and:
- Check if you have new seeds in your inventory by selecting a plant spot;
- Plant the seeds and water them until the blue mark level;
- Harvest the glowing plants;
- Water the harvested plants until the blue mark level.
Keep doing this a few times when you are in Plumpet Island and you should get the achievement at some point during the second half of the story.
The fruits you get are used as ingredients for crafting potions and ammo, so they are all useful.
Also, the achievement tracker works really well here!