The mustard seed is very small and insignificant. Pliny the Elder, a contemporary of Jesus, wrote a great big book called Natural History, in which he describes all the plants that were known in the Mediterranean world. He says only two things about the mustard plant: It’s medicinal, so it did have some value. But he said not to plant it because it tends to take over the entire garden. It is a weed that cannot be stopped. Those would have been the two images that Jesus was clearly building on: What I’m describing for you is therapeutic—it’s life, it’s healing, it’s medicinal—but it’s like a weed. What a shocking image, like a virus: I’m planting a weed in the world. I’m going to talk about stupid things like nonviolence and simple life, but they’re planted and they’re going to take over; the old world is over. I don’t know if it’s going to take two thousand years or four thousand years before you get the point, but I’ve planted what I know is eternal truth in the world and it’s going to take over.
—from Jesus' Plan for a New World: The Sermon on the Mount by Richard Rohr, OFM