Rashi: This is Tzidkiyah, [Yehoyakim's] son. Radak - Nebuchadnetzar did not appoint a king from another nation who did not know the law of the land, like one who seeds foreign seed in the land. It will not succeed, for that land does not make that vegetation sprout. It was proper that Tzidkiyah succeed, if he would go in the good path.

Malbim: It is what is proper to graft onto another tree (a branch); regarding trees, this is called seed.

Rashi: Yerushalayim, the capitol.

Radak: It is a field in which that seed should succeed. So Yerushalayim was proper to have good, truthful people who serve Hashem. The opposite is Nochri, like it says "Elohei Nechar ha'Aretz" (Devarim 31:16); "v'Lo Saki ha'Aretz Eschem Ka'asher Ka'ah Es ha'Goy Asher Lifneichem" (Vayikra 18:28).

Malbim: He grafted it on another seed.

Radak: It is past tense, like Lakach. It has a Kamatz, like "Arye Sha'ag Mi Lo Yira" (Amos 3:8). It is not a command, like "Kach Es ha'Leviyim" (Bamidbar 8:6)

Rashi: It is rooted on much water. I.e. he gave to him grandeur and authority over what is around him.

Radak: When he took from Zera ha'Aretz, he took a field proper to sprout quickly, for it was near much water. I.e. where Tzidkiyah was, he would have succeeded if he wanted, like a sapling planted near much water.

Malbim: He took mi'Zera ha'Aretz, which initially grew by much water, and grafted it onto Tzaftzefah, which grows on mountains without water.

Malbim (according to the Nimshal): [Nebuchadnetzar] took Tzidkiyah, from the seed of the kingship, and made him king under himself. He grafted him into a seeded field, to rule under him, like a grafted branch that is subordinate to the tree from which it nurtures. Even though Aravos grew in Bavel - "Al Aravim b'Sochah Talinu Kinoroseinu" (Tehilim 137:2) - he grafted onto an inferior Tzaftzefah. I.e. in place of the great influence in Eretz Yisrael, he made him dwell in a way that he will be desolate and dry, and dwell without good and Brachah.

Rashi: It is a kind of willow with many branches.

Radak citing Sukah 34a: It is an inferior kind of willow. I made them like a willow that grows on much water, and they made themselves like Tzaftzefah that grows between mountains! Radak - according to this, "Samo" means that Yisrael made themselves so.

Radak citing Targum Yonasan: He made it like a standing vine. Radak - even though he planted a seed, he made it like a planted vine - it grew quickly like a vine, and not like a seed, which does not sprout quickly.