Wisdom and truths can be found in so many places; they can also have layers of meaning.
I remember hearing about "the faith of a mustard seed" from a friend a long time ago. As I picked this Winter Cress, a Mustard Family member, that phrase ran through my brain. Obviously, investigation was warranted.
In the Christian Bible, Matthew 17:20 says: "He replied, 'Because you have so little faith. Truly I tell you, if you have faith as small as a mustard seed, you can say to this mountain, 'Move from here to there,' and it will move. Nothing will be impossible for you."
In the verses before, it discusses a demon expelled. Jesus then mentions that his disciples could not perform the act of exorcism because they did not have "faith as small as a mustard seed." I find it interesting that Jesus mentions the Mustard plant. This plant is of the Brassica Family, which includes Broccoli, Cauliflower, and Brussel Sprouts, as well as the Cresses. This family is high in sulfur, which is associated with clearing toxins as well as demons. This sulfuric property is also seen as anti-cancer, which is an age-old demon itself. This hints at a very important principle of herbalism: like treats like. Since Mustards are also known for their bite and their heat; mustards irritate. The Greeks saw Mustards as "hot in the 4th degree," which is stimulating to the point of burning. If it gets too hot, this plant will "bolt," or shoot up and sprout flowers. This also supports the statement of like conditions being treated by plants with similar attributes. It also means that you need to use it in just the right amount, or it will cause what it cures.
The flower of this family is cross-shaped, which also loans itself to another name for these fruits of this family, the Cruciferous vegetables, or "those that bear the cross." This is a distinctive characteristic for any flower; many flowers follow a five petal or more pattern. The cross is often used to invoke protection: none shall pass. This invokes the sense of the Warrior. Mustard is plant of Mars, who happens to be a warrior himself. Not only is Jesus hailed as a divine healer, but he is also a warrior, chasing out invaders, or that which doesn't belong. Mustards are spicy and increase circulation, which can help open the pores and release toxins, chasing out that which would do us harm. Many folk practices have used a mustard pack for just this cause.
Mustards are a common plant, but were viewed by the Greeks to be of high importance, so much so that its discovery was attributed to the famed healer Aesculapius. It was common and accessible medicine, which made it important. And Mustards can grow just about anywhere. Perhaps that's another reason that it made its way into Jesus's analogy: it is common and available to all - all you need to do is foster it, and it will bolt and bloom before you know it.