We can determine the general time period spoken of here because this is the time right before Israel begins to be scattered. The scattering of Israel begins with the sacking of the northern kingdom by the Assyrians. Therefore, this verse likely refers to the state of affairs before 721 BC. At that time, the northern kingdom was completely wicked. They were practicing idolatry. They had a long line of wicked, idolatrous kings, and had altogether turned from the Lord:
’And they left all the commandments of the Lord their God, and made them molten images, even two calves, and made a grove, and worshipped all the host of heaven, and served Baal.
And they caused their sons and their daughters to pass through the fire, and used divination and enchantments, and sold themselves to do evil in the sight of the Lord, to provoke him to anger.
Therefore the Lord was very angry with Israel’ (2 Kings 17:16-18)
This was when the main top thereof began to perish. The other part of this prophecy refers to the young and tender branches which were still viable. This would refer to the same time period—when the smaller kingdom of Judah was still faithful to the Lord. Probably the most righteous king at this time was king Hezekiah. Of him the scriptures record:
’And he did that which was right in the sight of the Lord, according to all that David his father did.
He removed the high places, and brake the images, and cut down the groves, and brake in pieces the brasen serpent that Moses had made: for unto those days the children of Israel did burn incense to it: and he called it Nehushtan.
He trusted in the Lord God of Israel; so that after him was none like him among all the kings of Judah, nor any that were before him.’ (2 Kings 18:3-5)
However, the interpretation that the wicked kingdom of Israel is the “main top” and that the kingdom of Judah represents “the young and tender branches” cannot be too strictly adhered to because there were those of the northern kingdom who were preserved by being scattered, and there were those of the southern kingdom whose wickedness ripened until they were destroyed. The example above is given to the reader for a frame of reference.