Get the seed left by the plant girl in the Glen. You will need to plant it in the Refuge.
Equip the seed and talk to the giant robot near the entrance of the Refuge. He will explain that you need to plant the seed in the garden. So leave it in the pot, but you need soil.
Go to the apartment block (top of the elevators, top-left path) go all the way to the end, the gardener's apartment has some plants outside. So talk to him and he will give you some soil. Give it to the seed. Next, you need water.
Go to the vendor street. One of the vendors will have "water pills". He gives you one as a sample so go give it to the plant.
Next, play as normal until the ending. For the final choice, you need to
Then, you will get the achievement.