Or place some rocks of crack under the soily bit inside the pot, then ring HR (put on a false voice so they don’t know it’s you) telling them your colleague has a drug problem and an intervention is necessary as you’ve seen them sprinkling crack grains on their cornflakes in the kitchen.
The next day your colleague will be confronted about their drug problem, obviously they come out of the meeting upset, or ranting and raving about that the allegations are false, at that moment try to comfort them anyway you can (don’t touch them in an inappropriate place though) and tell them “of course the allegations are false” while accidentally knocking their plant off the desk onto the floor, pick the plant up from its plant trunk revealing the crack rocks in the pot! shout “of my god its true look everybody crack!”
I think after that they won’t be allowed to have plants on their desk.