rush, of which various things are made
蒯 centers on a type of rush plant used to make various items. The meaning is narrow and concrete: a specific plant material.
蒯 combines the plant radical ⺾ with a phonetic component that originally represented a type of rush. The exact historical development of the right side is uncertain, but the character is consistently associated with the rush plant.
The top ⺾ (plant) and the right 刂 (knife) suggest cutting a rush plant to make things. Picture harvesting rushes with a knife to weave mats or baskets.
For カイ, imagine a person named Kai weaving rush mats by the river: Kai -> カイ, and the rushes are cut with a knife (刂) from the plant (⺾).