Left grain-stalk radical - the indexing radical. Pictures a drooping head of grain bowed on its stalk. Anchors 稠 in the crops/density family with 稻 rice, 稀 sparse, 秤 weighing-scale. Here it captures the original sense: a densely packed field of grain plants standing thick on the ground.
Right 周 supplies the sound, drifting from zhōu to chóu - a small zh-/ch- shift along sibilant series. The 'full, complete' sense lends faint flavour: dense crops fill the field completely, packed and surrounding. Same phonetic powers 调 to adjust, 雕 to carve, 绸 silk.
