幌菊种子 are obtained by harvesting 幌菊 while it is blooming or using the 再生法杖 if it is on 沙 since 1.3 versions. In desktop versions of Terraria, a Waterleaf plant only blooms during 雨, when it can be cut with virtually any weapon or tool to produce 1-3 Waterleaf Seeds. In console and mobile versions, Waterleaf plants must be at least partially submerged in water before they will bloom and yield seeds.
Waterleaf Seeds can be placed in empty 粘土盆s or in 沙 and grown into additional, harvestable Waterleaf. Potted Waterleaf will grow anywhere, not just in Desert biomes, but will only yield seeds during 雨 (desktop) or when submerged (console/mobile). Use a Pickaxe and left-click on the flower or herb above the Clay Pot to harvest it.