Notes: 

The Mother Nature costume above is quite perfect, but you could wear anything green, attach silk (or even real) flowers or leaves to it, and wear a crown of flowers, and you're all set! 

Gardeners and landscapers could wear a flower costume or green suit, yoga attire, or other comfortable outdoor attire to get in the garden. Top it off with a flower hat and garden gloves. Don't forget your sun hat! Or, if you want to be cheeky, be a garden gnome. Or, the watering can costume above is hysterical! It's always good to bring extra laughs to the party with you. 

Any other nature lover, such as an environmentalist role, could wear a hippie costume, a green suit, or basically anything nature-related - a flower or plant costume, Mother Nature costume (as long as there's no Mother Nature character on the guest list - but I can't imagine I'd put an environmentalist and Mother Nature in the same game), or anything green is great. 

You could also wear a Poison Ivy costume for any plant-loving or environmentalist role. You wouldn't have to wear the bright red wig with it - you could make it your own to fit your character.  And if you already have bright red hair - you're genetically lucky, and that's great, too- it will be okay if you look like the character Poison Ivy, as she was a plant lover!