死亡草种子 are obtained by harvesting 死亡草 while it is blooming. A Deathweed plant only blooms during a 满月 or a 血月. Its blooming state can be identified by the sickly green pods on its leaves. At this time it will drop 1-3 Deathweed Seeds.
Deathweed Seeds can be placed in empty 粘土盆s, Deathweed planters, corrupt or crimson 草, 黑檀石, or 猩红石 and grown into additional, harvestable Deathweed. Potted Deathweed will grow anywhere, even outside of 腐化之地. Use a Pickaxe and left-click on the flower or herb above the Clay Pot to harvest it.