The character is two 朿 (cì, 'thorn') side by side — a thicket of thorn-bushes. Modern uses are almost always in compounds: 荆棘 (jīngjí, brambles), 棘手 (jíshǒu, thorny/tricky), 棘皮动物 (jípí dòngwù, echinoderms — 'thorn-skin animals' like sea urchins and starfish).
register
On its own 棘 is literary/written. In speech and modern writing it almost only appears as part of a compound.
Left 朿 is itself a pictograph of a thorny stalk — a tree 木 with a barb on top. Doubled side-by-side it intensifies the thorny meaning, just as 林 doubles 木 for forest. Indexed under Kangxi #75 木 (mù) by tradition, reflecting the tree-base buried inside each 朿. Same phonetic drives 刺.
Right 朿 mirrors the left, completing the doubled-thorn ideograph. Two thorns side-by-side picture a bramble bush bristling with barbs — extended to 棘手 (thorny / difficult). The radical assignment 木 reflects the shared tree-base of both 朿 halves.
Filed under radical 木 (mù, #75) by convention. 木 is not a separate component in 棘, so no strokes are highlighted.
