Nausicaä's garden lab [1] is in the deepest cellars of the Castle. It is there she takes the spores she collected from the Sea of Corruption and grows them in clean water and clean soil. She discovers that the spores do not create Miasma when raised with clean water and clean soil and comes to the conclusion that the Sea of Corruption is slowly removing the poisons from the earth.
The garden lab has an automatic watering system and space for large numbers of plants.
She keeps the garden lab secret from the rest of the Valley because she doesn't want them to be afraid [2]. She even grows a Hisokusa, one of the most dangerous plants in the Sea of Corruption [3], as well as a Breadfungus plant.
Because she's going to war to fight in the Periphery Campaign, she shuts off the water to the plants and lets them die.
References[]
a See Page Conversions Between Editions to convert Deluxe Edition volume 1 (DE1) to other editions