I Need Help Solving A Riddle

Here it is:

I strike you hard,

but you feel no pain.

I grow old with you,

Even after a cane.

I can lock and enforce you with chains,

and you won't even want to get out.

But when the time comes,

and you're set free,

you'll continue to sprout

I am a good thing,

but too much is bad.

So if you use me well,

I can be a magic spell

And you will become glad.

I am associated with a flying insect

I am nothing you can buy.

But like this bug,

you'll never be dug,

And will be able to fly so high.

What Am I?