Him growing weed would be potentially hilarious. Its called weed for a reason its relatively easy to grow. He'd hyper fixate on it and somehow fuck it up.
Growing inside has all sorts of areas he could fail here's a few:
-light intensity: depending on the light source it's super easy to burn plants to a crisp by putting lights too close "the extra light is gonna help these plants real good toobs, I put them right next to the bulb"
-light source: if he uses an old school HPS bulb it could explode and burn down his trailer if he gets oil from his skin on the bulb, but he'll probably get a modern LED grow light, or just regular ass bulbs that are too weak. He could get eye damage from some grow lights without wearing protective glasses.
-light timing: when growing regular plants you need to start them with 18 hours of light and 6 hours of darkness, then switch to 12 light / 12 dark to initiate flowering. He'll probably leave the lights on 24 hours a day.
-soil quality: the boglim is totally gonna dump the cheapest shitiest soil, probably from his yard or a random ditch, into a regular plastic pot and then over or under water the hell out of it. He'll either add no fertilizer or way too much and burn the plants "these plants are gonna get huge toobs, I'm giving them extra fertilizer daily"
-there's no way in hell he could comprehend a hydroponics system so thats out of the question
-pests: he'll almost definitely get some sort of mites or thrips on his plants right away. You're supposed to have very high levels of hygiene in your grow room to segregate your plants from a plethora of very common pests that exist outside. If he gets spider mites his plants will be covered in literal cobwebs. No way in hell he would keep his grow room clean and pest free, and they would eat the plants until they're dead. He might use some sort of crazy pesticide and smoke it anyway knowing him.
-humidity control: I'm not sure how humid Casper is but weed is extremely vulnerable to budrot (a mold that kills the flowers you smoke and spreads extremely fast) if conditions are cold and damp often. Indoors it shouldn't be a problem with any sort of ventilation and air flow, but I bet he could fuck it up.
-harvest: he would either dry the plants way too fast and make the resulting product the harshest driest garbage ever or leave it damp and have it mold in the jar.
I can't wait to see if he actually tries this some day, depending on rules at the park he might even be able to grow outdoors which is his best bet (easiest way to grow but less control on quality/conditions) but we'll have to wait patiently and see what happens.