roots
荄 centers on the root of a plant, the part that anchors it underground and draws nourishment.
荄 combines the plant radical 艹 with 亥, which likely provides the sound. The character is used for plant roots.
The top 艹 is a plant, and the bottom 亥 looks like a tangle of roots spreading underground. Together they show a plant's roots.
For カイ, picture a kite tangled in the roots of a plant: kite -> カイ, and the kite string is caught in the roots.