The main color of your Mother Nature Halloween costume should be green. You can easily wear a green dress or skirt to create this visual effect.
Depending on how sexy you want to make the costume, you can choose how much of your leg that you want to show. A full dress down to your ankles is certainly more traditional and fashionable but you can were a shorter skirt to make it fun too.
For example, if you decide to go with a short-skirt make sure it hangs just above the knees and wrap ivy garland around your waist. It should droop down over your skirt. For the remainder, try making a few bracelets and anklets out of them. Some women will even wrap the ivy garland a few times around their upper torso.
For your head, you can wear a straw hat that has flowers in it, they can be real or fake. Just be sure to diversify by using different types of flowers and colors.
If you have long hair that falls past your shoulders, try putting some colorful flower petals or leaves in your hair. If you don't have long hair, maybe consider wearing a green wig and then attach flower pedals to it. As for your footwear, you can wear green high-heel slippers with maybe a small flower sticking out of the front.