luxurious grass, a smartweed
蓼 centers on a specific kind of plant: a smartweed, often described as a luxurious or lush grass. The two given meanings point to the same weedy plant rather than separate concepts.
蓼 combines the grass radical 艹 with 翏, which likely provides the sound. The character refers to a specific plant, smartweed, and the historical development of the phonetic component is uncertain.
The top 艹 shows it's a plant, and the bottom 翏 looks like feathers (羽) with extra strokes, like a lush, feathery weed. Picture a field of smartweed with feathery plumes.
For シン, imagine a thin, luxurious grass that looks like a shin-high carpet: shin -> シン, and the smartweed grows in a dense, lush patch.
knotweed (Polygonaceae); jointweed; smartweed
kiss-me-over-the-garden-gate (Persicaria orientalis); prince's feather
water pepper (Persicaria hydropiper f. aquaticum)
silver vine (used in traditional Chinese medicine)
water pepper (Persicaria hydropiper)
water pepper (Persicaria hydropiper)
water pepper (Persicaria hydropiper f. purpurascens)
dyer's knotweed (Persicaria tinctoria, used to produce indigo dye)
water pepper vinegar; vinegar infused and mixed with water pepper leaves, esp. served with salt-grilled sweetfish
some prefer nettles; there's no accounting for taste; every man to his taste
dyer's knotweed (Persicaria tinctoria, used to produce indigo dye)