We should first note that Jesus is telling a parable. A parable is a form of comparison to help us understand something that is difficult or abstract. The word parable means "to cast alongside." You "cast alongside" something known to help the audience understand something unknown.
In this case, the unknown is the Kingdom of God (Mark 2:26)
"The kingdom of God is like...
Particularly, this parable is addressing the mysterious nature in which the kingdom of God advances or expands. You can't see it happening. It happens while you sleep.
Ultimately, we know that plants grow - but most people don't fully understand how it happens. I suppose if you are a biologist you could teach the mechanics but even then there is still a wonderful mystery to how plants grow.
The sower is anyone who advances the kingdom of God.
When we plant seeds of the kingdom of God we can't see how growth is happening and we can't fully understand how the kingdom expands, but we can have faith that it does. We do the work in very small steps and those steps mysteriously bring about the kingdom.
Our job is not to figure out when someone will recognize Jesus as Lord. Rather, our job is to sow the seeds for the Holy Spirit to work and bring about God's desired end.