The Romans have been using it since the days of Julius Caesar, but not to get high. Both Washington and Jefferson grew it.

Now that several states have legalized the use of marijuana for some recreational and medical purposes, one of the biggest untapped markets for the cannabis plant itself — at least one variety — could be as a building tool.

The most sustainable building material isn’t concrete or steel — it’s fast-growing hemp. Hemp structures date to Roman times. A hemp mortar bridge was constructed back in the 6th century, when France was still Gaul.

Now a wave of builders and botanists are working to renew this market. Mixing hemp’s woody fibers with lime produces a natural, light concrete that retains thermal mass and is highly insulating. No pests, no mold, good acoustics, low humidity, no pesticide. It grows from seed to harvest in about four months.