Jamieson, Fausset and Brown on Deuteronomy 16:21

21. Thou shalt not plant thee a grove--A grove has in Scripture a variety of significations--a group of overshadowing trees, or a grove adorned with altars dedicated to a particular deity, or a wooden image in a grove (Jud 6:25; 2Ki 23:4-6). They might be placed near the earthen and temporary altars erected in the wilderness, but they could not exist either at the tabernacle or temples. They were places, which, with their usual accompaniments, presented strong allurements to idolatry; and therefore the Israelites were prohibited from planting them.