tobacco
莨 centers on the tobacco plant, specifically its leaves used for smoking or other products. The meaning is narrow and concrete, referring to the plant itself.
莨 combines the grass radical 艹 with 良, which likely contributes a phonetic element. The character was adopted to represent the tobacco plant, a relatively modern usage in Japanese.
The grass top 艹 sits above 良, like good (良) leaves growing from the ground. Picture a field of tobacco plants with broad, good-quality leaves.
For ロウ, imagine a row of tobacco plants drying in a long, low shed: row -> ロウ, and the low shed protects the good leaves.
Desmarestia tabacoides (species of algae)