Jesus was very specific in his choice of the mustard seed, as we have seen, for several reasons. And he was also careful in his choice of agricultural examples for several reasons. The most prevalent reasons for any of his choices for topics was that it was first and foremost an easy lesson for simple people. Even the most rudimentary examination will surmise that Jesus was telling the people that the kingdom of God would grow, and that it would grow quickly and impressively. To finish with such a superficial exegesis is to miss the point Jesus hoped to leave the listeners to ponder.
Growth is often unseen.
It is a miracle that humans have little power over. Those who come to faith in Jesus and his disciples and their message of salvation will see the kingdom of God growing and providing shelter. News from behind the former Iron Curtain and lingering Bamboo Curtain testifies to the kingdom ability to reproduce itself, even where it is not wanted or sought, just like the mustard seed.