This passage is found in Narrative Lectionary, Epiphany Season, Year B (Most recently: January 17, 2016)
Here is my post for Mark 4:26-34
http://lectionarygreek.blogspot.com/2012/06/mark-426-34.html
I suggest three possible directions for a sermon:
1) Reflection on church growth and how a congregation can open themselves to this
2) Reflection on daily dying and rising and what growth looks like in a Lutheran context
3) Reflection on Christ as the mustard seed
Also, for those doing the parable of the sower, here is my commentary on Matthew's version of this parable; while not the exact same, perhaps some helpful insights into the passage:
http://lectionarygreek.blogspot.com/2011/07/matthew-1318-23.html