The tops of the hills were continually chosen for idolatrous temples, i.e., “high places.”
Poplar — i.e., the white poplar, not the storax of the LXX., which is a shrub only a few feet high.
Elms should be “terebinth tree” (’çlah).
Related Commentaries of Hosea 4:13
Hosea 4:13
13 They sacrifice upon the tops of the mountains, and burn incense upon the hills, under oaks and poplars and elms, because the shadow thereof is good: therefore your daughters shall commit whoredom, and your spouses shall commit adultery.