How deep should the soil be for planting ferns in a rooftop garden?

Answer 1

Depends on how often you want to water, or how much water will just show up.

Most ferns do not need very deep soil. But, generally soil is about 8" deep. That way you have enough root depth and soil to hold water for a while.

Just use normal soil for thngs.

Soil can be made lighter by using perlite or vermiculite in larges percentages. It holds water and nutrients well but is much lighter than sand of soil.

Answer 2

Depends on the fern.

There are some epiphitic ferns that need almost no soil. A fern native to the Pacific Northwest, Polypodium glycyrrhiza or Licorice fern, can often be found growing in the moss layer that forms on tree branches or large rocks with essential no real soil.

Some people in the Portland area are using them as on of the plants used on green roofs (eco-roofs).