Throughout the entire play, Shakespeare uses plant or agricultural imagery to develop characterization and plot. If you remember, King Duncan tells Macbeth, when he first sees him after the victorious battle, that he has
begun to plant [Macbeth] and will labor
To make [him] full of growing (I. iv. 28-29)
And when Duncan promises to do the same for Banquo, Banquo's responses is full of agricultural imagery, for he states that
There if I grow,
The harvest is your own (I. iv. 31-32).
This agricultural imagery reveals just what kind of character Duncan is; it reveals that he is a humble and generous king, and it certainly reveals that Banquo is a most loyal servant. All this occurs while Malcolm is listening.
So, when Malcolm uses the analogy of the restoration of Scotland as a new planting, we not only see the teachings of his father, but we see the new beginning for Scotland, for like a newly planted crop, Malcolm has hopes for a bountiful harvest.