For ye shall be as an oak whose leaf fadeth, and as a garden that hath no water.

Jamieson-Fausset-Brown Bible Commentary

30. oak-Ye shall be like the "oaks," the object of your "desire" (Isa 1:29). People become like the gods they worship; they never rise above their level (Ps 135:18). So men's sins become their own scourges (Jer 2:9). The leaf of the idol oak fades by a law of necessary consequence, having no living sap or "water" from God. So "garden" answers to "gardens" (Isa 1:29).

Matthew Henry's Concise Commentary

1:21-31 Neither holy cities nor royal ones are faithful to their trust, if religion does not dwell in them. Dross may shine like silver, and the wine that is mixed with water may still have the colour of wine. Those have a great deal to answer for, who do not help the oppressed, but oppress them. Men may do much by outward restraints; but only God works effectually by the influences of his Spirit, as a Spirit of Judgment. Sin is the worst captivity, the worst slavery. The redemption of the spiritual Zion, by the righteousness and death of Christ, and by his powerful grace, most fully accord with what is here meant. Utter ruin is threatened. The Jews should become as a tree when blasted by heat; as a garden without water, which in those hot countries would soon be burned up. Thus shall they be that trust in idols, or in an arm of flesh. Even the strong man shall be as tow; not only soon broken, and pulled to pieces, but easily catching fire. When the sinner has made himself as tow and stubble, and God makes himself as a consuming fire, what can prevent the utter ruin of the sinner?