garden, field
圃 centers on a cultivated piece of land: a garden or a field where plants are grown, often enclosed or defined.
圃 combines 囗 (enclosure) with 甫, which likely contributes the sound and may relate to spreading or planting. The character originally referred to an enclosed garden or cultivated field.
The outer 囗 is a fenced enclosure, and inside 甫 looks like a sprout pushing through soil. Picture a fenced garden where plants are sprouting.
For ホ, imagine a garden hoe leaning against the fence: hoe -> ホ, and it's used in the garden.