In Act I scene 4 of Macbeth, when Duncan praises Macbeth and Banquo, he relies almost entirely on imagery related to farming and harvesting.
What conclusion about Duncan's character and attitudes can we thereby draw?
It appears you are refering to the section of this scene in which Duncan responds to Macbeth's words of allegiance and loyalty and then also praises Banquo, expressing the desire to give them the power and position that they deserve for their acts of valour and the way that they have worked to support his position in the battle. Note what he says:
I have begun to plant thee, and will labour
To make thee full of growing.--Noble Banquo,
That hast no less deserv'd, nor must be known
No less to have done so, let me infold thee,
And hold thee to my heart.
The way in which Duncan uses a metaphor to describe his role of King and the way that he helps his faithful Lords to "grow," having been "planted" by him, points towards Duncan's honesty and simple view of the world. We have already been told that he was completely taken in by the former treacherous Thane of Cawdor, as Duncan himself says that he put "an absolute trust" in this traitor, indicating that he is somewhat naive as a character in whom he trusts. This rather simple, agricultural image of how Duncan sees his role as King seems to support our impression of him as a well-meaning leader who is nevertheless unequipped with the Machiavellian understanding that is so essential for his survival, and leaves him open to Macbeth's evil schemes. He, unlike other characters, does not pretend to be anything else than he is: a kind, worthy, noble king.