Jesus speaks of the mustard seed comparing it to the kingdom – but he speaks of the mustard seed being planted. A mustard seed that has not been planted does nothing. It will dry out, crack, crumble, and be blow away. But a seed that has been planted will grow into a large, unstoppable plant - to the point where birds will rest in the branches.
When a seed is planted there is the truth of the plant being nurtured and cared for, but there is also the sense that something else ultimately makes it grow. The Kingdom is something we are called to live within, but it is also something that lives within us (Luke 17:21). It will grow too within us … is it possible that there are seeds inside of us that we have not allowed to take root? Seeds that are still not planted?
If we are a part of the Kingdom of Heaven, then we become the soil in which the seed is planted. We are vehicles then for the unruly plant of the Kingdom – and we get to be the places where birds find shelter in our branches.