Explain why Bud says that "ideas are like seeds." Why does Bud say this?

Bud says that quote in chapter 9:  

It's funny how ideas are, in a lot of ways they're just like seeds. Both of them start real, real small and then ... woop, zoop, sloop ... before you can say Jack Robinson they've gone and grown a lot bigger than you ever thought they could.

He actually carries on with the comparison for the next few pages.  Bud is amazed that something as small as...

Bud says that quote in chapter 9:  

It's funny how ideas are, in a lot of ways they're just like seeds. Both of them start real, real small and then ... woop, zoop, sloop ... before you can say Jack Robinson they've gone and grown a lot bigger than you ever thought they could.

He actually carries on with the comparison for the next few pages.  Bud is amazed that something as small as a seed can grow into a huge giant tree that a person could "drive a car into it and kill yourself" with. Bud tells his readers that is how the idea of Herman Calloway being his father started.  He says that the idea was so small that it could have easily been forgotten and lost like a seed being blown away with the "first good puff of wind."  He admits to readers that the idea has slowly grown, and now it is so big that it is a central focus of his.  He plans to make his way across the entire state of Michigan in search of this man that he believes is his father:

The idea first got started when I was looking in my suitcase at one of the flyers showing Herman E. Calloway and his band. That was like the seed falling out of a tree and getting planted.

It started busting its head out of the dirt when me and the other boys at the Home were getting our nightly teasing from the biggest bully there, Billy Burns.

[...]

That little idea had gone and sneaked itself into being a mighty maple, tall enough that if I looked up at the top of it I'd get a crick in my neck, big enough for me to hang a climbing rope in, strong enough that I made up my mind to walk clean across the state of Michigan.

Bud's initially small idea has grown into a hugely motivating force.  It has grown into an impressive "tree," and that initially small idea is why Bud eventually winds up united with his grandfather.