Thou preparedst room before it - The Hebrew word used here means properly to turn; to turn the back; then, to turn in order to look at anything; to look upon; to see; then, in Piel, to cause to turn away; to remove. Then it comes to mean to remove, or to clear from impediments so as to prepare a way Isaiah 40:3; Isaiah 57:14; Isaiah 62:10; Malachi 3:1, and hence, to remove the impediments to planting a vine, etc.; to wit, by clearing away the trees, brush, stones, etc. Compare Isaiah 5:2. Here it means that the hindrances in planting the vine were taken out of the way; that is, God removed the pagan so that there was room then to establish his own people.
And didst cause it to take deep root - Hebrew, "And didst cause it to root roots;" that is, Its roots struck deep into the soil, and the plant became firm.
And it filled the land - Its branches ran everywhere, so as to fill the whole land. See the notes at Isaiah 16:8.

Thou preparedst - before it -
3. When the ground is properly cleared, then it is well digged and manured, and the vines are placed in the ground at proper distances, etc. So when God had cast out the heathen, he caused the land to be divided by lot to the different tribes, and then to the several families of which these tribes were composed.
And didst cause it to take deep root -
4. By sheltering, propping up, and loosening the ground about the tender plants, they are caused to take a deep and firm rooting in the ground. Thus did God, by especial manifestations of his kind providence, support and protect the Israelites in Canaan; and by various religious ordinances, and civil institutions, he established them in the land; and, by the ministry of priests and prophets, did every thing necessary to make them morally fruitful.
It filled the land -
5. To multiply vines, the gardener cuts off a shoot from the old tree, leaving a joint or knob both at top and bottom; then plants it in proper soil; the lower knob furnishes the roots, and the upper the shoot, which should be carefully trained as it grows, in order to form another vine. By these means one tree will soon form a complete vineyard, and multiply itself to any given quantity. Thus God so carefully, tenderly, and abundantly blessed the Israelites, that they increased and multiplied; and, in process of time, filled the whole land of Canaan. Vines are propagated, not only by cuttings, but by layers, seed, grafting, and inoculation.

Thou preparedst room before it,.... By sending the hornet before the Israelites, and driving the Canaanites out of the land, Exodus 23:28 and so the Targum,
"thou didst remove from before thee the Canaanites;''
which made way and room for them: and thus the Lord prepared room for his interest, church, and people, in the Gentile world, in the first times of Christianity, by sending the Gospel into all parts of it, and making it successful, and still there is room, Luke 14:22.
and didst cause it to take deep root; which denotes the settlement of the people of Israel in Canaan, in church and state, as a body ecclesiastic and politic; so believers, being rooted in Christ, are grounded, settled, and established in him, and in a Gospel church state, and so become fruitful; see Colossians 2:7.
and it filled the land; with people, who, in the days of Solomon, were as the sand of the sea, 1-Kings 4:20 and so the Gentile world was filled with Christian converts in the first times of the Gospel; and the interest and church of Christ will fill the whole world another day, Isaiah 11:9.

Preparedst - Thou didst root out the idolatrous nations. Deep root - Thou gavest them a firm settlement.