swamp, shore, fifth month
皐 centers on a low, wet landscape: a swamp or marshy shore. The "fifth month" meaning is a specialized application, likely from the old lunar calendar when such areas were prominent or from a related homophone.
皐 is thought to combine 白 (white) with a lower element representing a marsh or shore. The exact historical development is uncertain, but the character has long been associated with wetlands and the fifth month of the lunar calendar.
The top 白 (white) suggests pale, misty water, and the lower part looks like a tangled shoreline. Picture a white mist rising over a swampy shore.
For コウ, imagine a crane (こう) standing in the swamp: the crane's call echoes across the shore, and こう cues コウ.