Jamieson, Fausset and Brown on Ezekiel 17:5

5. seed of the land--not a foreign production, but one native in the region; a son of the soil, not a foreigner: Zedekiah, uncle of Jehoiachin, of David's family.

in a fruitful field--literally, a "field of seed"; that is, fit for propagating and continuing the seed of the royal family.

as a willow--derived from a Hebrew root, "to overflow," from its fondness for water (Is 44:4). Judea was "a land of brooks of water and fountains" (De 8:7-9; compare Joh 3:23).