The gardening giant bet $1.7 billion on lighting, nutrients and other pot cultivation tools—and lost. But CEO Jim Hagedorn and his son Chris are plotting a comeback. Here’s how they plan to smoke the competition.

Its a beautiful, chilly November day in Las Vegas and Jim Hagedorn, the CEO, president and chairman of Ohio-based lawn care giant Scotts Miracle-Gro, is pissed off. After nearly a decade of making a …