If the Earth was once all rock and fire, how do flowers grow?

The Earth is no longer all rock and fire. That helps flowers grow. 

Comets brought water.

Then things started growing.

Rock erodes to become soil.

After the fire went out rocks broke down to dirt and THEN flowers evolved from plants that weren’t flowers

 6 I planted the seed, Apollos watered it, but God makes it grow. 

The climate changed and rock was broken into tiny fragments by the weather. As plants and animals appeared, the earth got more varied and easy to grow in and on.

Ponder

If we eat inert things, how can we think?

It still is all rock and fire underneath. The arth's crust is relatively thin (it's about the same thickness of apple peel scaled up). There are nutrients in the soil and that combined with sunlight and water allows plants to grow.

The key phrase here is "was once". That is, it no longer is all rock and fire, hasn't been so for billions of years.