Hemp
Concrete Used for Carbon-Negative Construction
The building sector is responsible for a significant of all
greenhouse gas emissions in the U.S. annually, but carbon-negative construction is possible, using durable, super-insulating, carbon-rich materials like organically-grown industrial hemp.
CO2 from the atmosphere is trapped in the hemp plants and the soil as they grow, and remains there after the plants are harvested. Even when combined with lime binder to form concrete, the overall product takes more CO2 out of the atmosphere than it puts into it.
http://www.cnn.com/video/data/2.0/video/tech/2010/08/25/dnt.nc.hemp.house.wlos.html 
