Isaiah 17:9-11. “In that day shall his strong cities be as a forsaken bough, and an uppermost branch, which they left because of the children of Israel: and there shall be desolation. Because thou hast forgotten the God of thy salvation, and hast not been mindful of the Rock of thy strength, therefore shalt thou plant pleasant plants, and shall set it with strange slips: In the day shalt thou make thy plant to grow, and in the morning shalt thou make thy seed to flourish: but the harvest shall be a heap in the day of grief and of desperate sorrow.”
In that day: This is one of the few places where this phrase does not refer to the Millenium. In this passage, it refers to the destruction Assyria creates in Syria and Ephraim. The strong cities were destroyed, mainly because of the sins of Israel.
Verse 10 enumerates the specific sins that ended in such destruction: They had forgotten God; they had neglected their salvation; they had not remembered that true help and protection come from God.
So great would be the destruction that even newly-planted crops would fail. All their efforts to plant new and even unusual plants from distant places would fail. The harvest was not going to give them sustenance. It would be nothing but a heap of dead plants.