Symbolism, Imagery, Allegory
Cabbarus is a spymaster extraordinaire, and he confesses to keeping track of everyone and everything in his kingdom—at least, as best as he can. So Cabby's got to get intelligence from what's going on all over Westmark, so he cultivates spies that report to him… and what does he call this collection of loyal eyes and ears? Why his garden, of course. Check it out:
Out of respect for his position, the chief minister allowed himself certain small luxuries. One of these was a private garden that yielded, in all seasons, blossoms of information. Cabbarus fertilized it with generous applications of money. The harvest was always more plentiful and usually more accurate than the labored, vegetablelike reports of provincial constables and police spies. Cabbarus earnestly believed his rank entitled him to this higher quality of produce. Since he cultivated his garden personally, he saw no reason to share it.
As in the most carefully tended gardens, the occasional weed sprang up or plant withered. Cabbarus had his disappointments. The individual he counted on to deal with Torrens had not thrived. This in itself did not trouble the chief minister. As a precaution, the man would have been pruned, in any case. What nettled Cabbarus was that he had no inkling of the doctor's fate. (18.1-2)
The knowledge Cabbarus culls from his spies is essential to—and impossible to parse away from—his power, though Cabbarus prefers to think of it in much prettier terms. His spy network is his garden and, when a good piece of info comes his way, his flowers have bloomed—and, conversely, when he finds himself with a bad seed (to work with his metaphor), he simply prunes it (which is a much nicer thing to say than kill).
Getting information before everyone else is key to Cabbarus's ability to stay ahead of the game and in control of the kingdom, and this is exactly what his garden helps him do. So when we think of his garden, we can immediately think of it as symbolizing information, control, and Cabbarus's investment in power.
On a more subtle level, though, we might also think of the garden as symbolizing deception. Part of this is because it's made up of spies who, you know, deceive people—but a bigger part has to do with how the garden works as a story Cabbarus tells himself. It's almost like he lies to himself about what he's really doing in the world, avoiding the harsh realities of human lives caught up in his power play by pretending it's all one big pretty, flowery picture.