reed, bullrush
芦 centers on tall, water-loving grassy plants: reeds and bullrushes. The two meanings are closely related wetland plants.
芦 combines the grass radical 艹 with 戸, which likely contributes the sound ロ. The character originally referred to reeds or similar grassy plants.
The grass radical 艹 sits above 戸, like reeds growing tall beside a door. Picture a door opening onto a marsh full of reeds and bullrushes.
For ロ, imagine a row of reeds growing in a low, wet area: low -> ロ, and the reeds stand in a row by the water.