But walked therein - Rather, “he walked therein,” meaning Joash, the “saviour” of the preceding verse.
There remained the grove also in Samaria - It seems strange that Jehu had not destroyed this when he put down the worship of Baal 2 Kings 10:26-28. Perhaps the “grove” or “Asherah” worship was too closely connected with the old worship in high places to be set aside with the same ease as the rites newly introduced from Phoenicia.