Left tree radical - the indexing radical. Marks 植 as a botanical action: planting a tree. The natural anchor for any cultivation word: 树 (tree), 果 (fruit), 林 (forest), 桃 (peach), 栽 (plant/cultivate). Tells you immediately: this involves a plant.
Right 直 supplies the sound (zhí, identical) and a perfect semantic hint: trees should be planted straight and upright. Wood + straight = setting a tree firmly upright in the soil. One of the cleanest phono-semantic compounds in the script, where phonetic and meaning reinforce each other exactly.
