the rape-turnip
葑 refers specifically to the rape-turnip, a type of turnip. Its meaning is narrow and botanical, without broader abstract extensions.
葑 combines the plant radical 艹 with 封, which likely provides the sound. The character is used for a specific plant name.
The grass top 艹 sits over 封, like a turnip plant growing in a sealed plot of earth. Think of the rape-turnip as a plant that is cultivated in a contained field.
For ホウ, imagine a farmer hoeing the soil around a rape-turnip: hoe -> ホウ, and the hoe helps the turnip grow.