Top grass radical — the indexing component, marking 蕾 as a plant feature. A bud is the unopened beginning of a flower, so the plant family is the right home: same radical in 花 (flower), 苗 (sprout), 茎 (stem), 蕊 (stamen).
Bottom 雷 supplies the sound lěi (drifted in tone from léi). 雷 itself shows rain over a field — thunder rumbling above plowed ground. A faint poetic echo: spring storms are the cue for buds to burst, so the phonetic doubles as a quiet semantic hint.
