During a talk with Collider, he said that Swamp Thing would involve a ton of creature effects, since they wouldn't have enough of a budget to make the CG look good.
"If we were to do it, it's a hard character to pull off, in terms of visual effects make-up. We don't have $10 million to do the CG. We definitely wouldn't see him in the first season. If we're fortunate enough to keep going and we feel like we can pull it off, that one requires a lot of creature design, so we need to make sure it doesn't look dumb and is organic to the story."
..."Organic." Get it?
Constantine airs Friday nights at 10 p.m. ET/PT.