bracken, fernbrake
蕨 refers specifically to bracken, a type of fern, and by extension fernbrake, the thicket where it grows. The meaning is narrow and botanical.
蕨 combines the grass radical 艹 with 厥, which likely provides the sound. The character was created to name the bracken plant.
The grass top 艹 sits above 厥, which looks like a dense thicket. Picture a patch of bracken ferns growing in a wild, tangled brake.
For ケツ, imagine a kettle (ケツ) sitting in a fernbrake, steaming among the bracken. The kettle sound hooks the reading ケツ.
bracken
kamraj (Helminthostachys zeylanica); tunjuk-langit; di wu gong
freshly budded bracken
water sprite (Ceratopteris thalictroides); Indian fern; water fern; Oriental waterfern; water hornfern
Dryopteris lacera (species of wood fern)
Japanese painted fern (Athyrium niponicum); pictum
furled bracken fronds
type of iron sword with curved pommel (Nara period)