To grow the population of your forest you will need to expand and clear spaces. This is done by chopping expansion foliage. Use coins to “Clear” away leaves and expand your environment. The new spots you unlock can either be “Grow Spots” for raising new moths, or “Flower Spots”, which contain a new Pollen Flower.
Before choosing which spot to go for next, take note of the icon above the spot’s price, this shows whether it's a Grow Spot or Flower Spot. You can fit two moths in each grow spot but flower spots will not increase your forest population.