Legal marijuana is on the brink of explosive growth. But there are two key hurdles to overcome… and marijuana REITs like Innovative Industrial Properties (NYSE: IIPR ) are solving them both. For growers, it’s not easy to produce high-grade marijuana. Nor is it cheap — and getting that funding is tricky. Until the U.S. government removes marijuana from its list of Schedule I drugs — the ones considered most dangerous, like heroin — most banks want no part of it.