grass, plants, grass radical (no. 140)
艸 centers on grass and plants as a general category. The radical sense is the same grass shape used as a component in other characters.
艸 is a pictograph of two grass sprouts growing side by side. The modern form keeps that doubled sprout image clearly.
See the two sprout shapes as blades of grass growing together: 艸 is grass.
For ソウ, imagine a field of grass swaying in the wind with a soft 'sow' sound: sow -> ソウ, like the rustle of grass.