I don't know anything about your setup, or even what kind of king you have, but I would bet money that its not big enough.
People here would be quick to tell you that plants and kingsnakes won't work, but they can if the environment is spacious enough that the king isn't directly on top of one poor plant 24/7.
I kept a small male call king, maybe 30 inches, in a 75 gal naturalistic enclosure with several type of succulents. The plants were all potted, but you could hardly see the pots since I used a deep sort of rocky substrate
2 bits of advice, only fill pots halfway with dirt and then around the base of the plants put a few layers of river rock or other large pebbles to discourage constant digging in the roots. 2nd, if possible, set up the plants and give them a few weeks to establish before adding the snake. If you just throw a pot into the snakes tank, the snake will be curious about the new object and dig around in it. Combined with the stress of a new environment, most wont make it.
I have seen a custom vivarium (about the size of a 180 gal tank) for a milk snake and it used a fully dirt bottom with plants directly in the soil. The snake killed a few of the more delicate ones but many survived just fine.