Green Roof Soil
How to choose?
Soil for green roofs has to have certain characteristics. It must be
light weight, have good water retention, and also good drainage. It also needs to have some nutrient value, because most green roofs are built and then forgotten about.
Recently, it was thought that the best soils for green roofs were
engineered soils, or pasteurized peat based soilless media, with little
emphasis on adapting it to local needs.
The main advantage to this type of soil on a green roof is its
light weight and ease of handling as it comes pre-packaged in bales
from a manufacturer.
Disadvantages are the lack of nutrients, and potential to become
eroded by windy conditions or flooding. It also lacks microflora and
microfauna, the tiny animals and plants that colonize a native soil.
Or Native soil?
There is now evidence that it’s advantageous to use soil native to the area and utilize the micorrhizae
- the tiny animals, yeasts and bacteria - endemic to the locale. This
will enable the soil to function more fully and as an indigenous soil,
make it easier for wild plants, microflora and wildlife to colonize it.
Many green roof professionals recommend adding
and lightweight drainage material such as pumice
to native soils. This is the best of both worlds, as the native soil
now has the ability to drain more fully, (its one drawback) and provides
a symbiotic fungus which enables better root growth.
What kind of Mulch is best?
Regardless of which type of soil you decide to use, a mulch of lava rock or other stone will be beneficial.
Some green roof builders have concerns that the lava rock
could potentially puncture the liner, but if applied correctly, this
shouldn’t be a problem. There will be a consistent layer of soil on the
roof over the liner or root barrier, and once the plants are in place
the mulch can be carefully put around the plants.
It’s not recommended to use wood based mulches such as wood
chips, shavings or bark mulch due to the fire risk and the potential to
be blown away by strong winds.
Some advantages to using lava rock mulch are:
- Colour - Most lava rock is either red or grey and will combine well with the colour of the plants.
- Good drainage around the crown of the plants.
- Water retention at the roots - I know, this seems
to be a contradiction in terms, but the plants will grow much more
uniformly if mulched due to the tiny bit of moisture retained by the
- Prevent the washing away or blowing away of the
soil. This is very important when using an engineered soil with perlite
or other light weight components.
- Low maintenance once installed. A small amount of additional lava rock mulch may be required over time.
- Weed control - most weed seeds require light to trigger germination, so shading the soil with mulch prevents them from growing.
- Small amounts of nutrients are constantly being
released for the use of the plants so little to no additional fertilizer
Fertilizer for Green Roofs
Fertilization of green roofs is very basic. Most of the plants
recommended for use on green roofs prefer a lean soil, only requiring a
small amount of organic material initially. After a few seasons the
system should become self sustaining with the dropping of foliage and
accumulation of dust particles from the air.
The best type of fertilizer is worm castings which are easy to spread and organic. One annual application will be adequate.
It’s unknown at this time if chemical fertilizers may potentially
create an environmentally damaging run-off, but evidence is pointing in
that direction. It would detract from the whole purpose of attempting
to enhance the environment, only to pollute local waterways with
chemical run off from your green roof. This is additionally important
if you plan to capture the rainwater from your roof to use in
irrigation. My preference always is to choose an organic fertilizer for my plantings, and green roofs are no exception.
Green Roof Maintenance
The main care that your green roof will need is to assess the drainage
periodically, and remove any buildup of organic material that may cause a
Plants that potentially will need more care in this respect
are grasses and herbaceous perennials which create debris around the
Most green roof Sedum and other hardy succulent plants such as Sempervivum produce a very small amount of debris so this isn’t a concern.
The Bright Future of Green Roofs
The art of green roof building is still in its infancy, and much more
experimentation will have to take place before it becomes a science.
Every day, new information is released from studies done on
all aspects of green roofs, from tiny sheds to huge office buildings,
giving even more impetus to green roofs.
More green roof professionals as well as home owners and gardeners like us are getting excited by this old yet new technology.
I hope to see many more green roofs of all kinds and sizes in my home town soon.
Green Roof Plants
Green Roof Sedum