In a large pan, brown the beef, garlic, and onion over medium meat, until the beef is cooked through. Transfer the meat mixture to a 6-quart slow cooker with the celery, carrots, chili powder, cumin, oregano, salt, cayenne pepper, zucchini, tomato purée, and diced tomatoes.
Cover and cook on high for 4 or 6 hours or low for 7 to 8 hours, until the chili is thickened.