Single 蔗 is mostly bound - in everyday speech you say 甘蔗 for the plant. 蔗 alone shows up in compounds: 蔗糖 (cane sugar), 蔗田 (sugar-cane field), 甘蔗渣 (bagasse).
艹 is the grass radical 艸 in compressed top form. It tags 蔗 as a plant, specifically the tall jointed cane that yields cane sugar. It groups with 草 (grass), 苗 (seedling), and 茎 (stalk).
庶 supplies the reading, drifting from shù to zhè along an old retroflex alternation. Its sense of numerous masses also fits faintly: a sugarcane field is row after row of identical thick stalks growing in a dense crowd.
