"What shall we say the kindom of God is like?....It is like a mustard seed.....when it is planted, it grows and becoms the largest of all the garden plants, with such big branches that the birds of the air can perch in its shade."
People in farm country know that the mustard is a weed. It is from the family Cruciferae, so named because their 4 petals form a cross. There are 138 species in our area.
We don't know which species Jesus had in mind, but we don't have one here that grows into a bush.
Our pastor pointed out that Jesus often turned conventional wisdom up side down, and in this case, we could conclude from the context of the previous section of the Gospel of Mark that Jesus may have been saying, "The kingdom of God is like a weed." The seed sprouts and grows even though the farmer doesn't know how.
We need to plant the good seed, but God through the Spirit grows it, and then we need to be ready for the harvest.