mustard, rape, dust, trash, rubbish
芥 centers on small, scattered, or discarded things: mustard seeds, dust, trash, and rubbish. The plant-related meanings (mustard, rape) are specific applications of this small-particle idea.
芥 combines 艹 (plant) with 介, which likely contributes the sound. The character originally referred to mustard plants, and the 'dust/trash' sense may derive from the small, scattered nature of mustard seeds.
The plant top 艹 over 介 suggests tiny mustard seeds scattered like dust. Think of a plant whose small parts become trash when discarded.
For カイ, imagine a kite (カイト) flying over a field of mustard plants, scattering seeds like dust: kite -> カイ.
limbless wooden doll; kokeshi doll
opium poppy (Papaver somniferum)
Akutagawa Prize (literary award)
trash; rubbish; garbage
rubbish; trash; garbage; refuse; dirt
rubbish; trash; garbage; refuse
kitchen waste
watercress (Nasturtium officinale)
garbage truck; dustcart
sow thistle (Sonchus oleraceus); milk thistle
mustard
variableleaf yellowcress (Rorippa indica)
celery-leaved buttercup (Ranunculus sceleratus); cursed buttercup
white mustard (Sinapis alba)
Indian lettuce (Lactuca indica)
bombardier beetle (any insect of subfamily Brachininae)
mustard plaster
poppy seed
Keya Golf Club (Fukuoka)
kai-lan; gai-lan; Chinese broccoli