Thursday, February 10, 2011

Integrated & Biodiverse Green Roof Plantings Clean Nitrogen from the Air Providing Plants With Free Fertilizer from the Sky

Easy to see the green roof clover is making fertilizer, fixing nitrogen and making available nutrients for other green roof plants.  Look at how green the green roof clover is, an indication of how much Nitrogen is being pulled from the air and fixed into the soil.

Integrating Nitrogen Fixation Plants on a Green Roof Provides Fertilizer


Smart green roof design integrates all types of plants and avoids monocultures.  The general rule for promoting green roof biodiversity is:

  • Less than 10% of a green roof should be comprised of one Species
  • Less than 20% of a green roof should be comprised of one Genus, and
  • Less than 30% of a green roof should be comprised of one Family
By integrating different plant types and Families (use all C3, C4 and CAM) you;
  • Maximize Biodiversity potential
  • Minimize pest damage risk
  • Provide optimum habitat
  • Allow plants to support one another through structure, nitrogen fixation and other means.
Nitrogen fixation is one important reason to blend C3, C4 and Cam plants with those plant species that can fix Nitrogen into the soil.



Our earth's atmosphere is comprised of approximately 7% nitrogen gas (N2) by volume.

SMOG contains small particles containing carbon and nitrogen too.  Carbon dioxide too can be found in smog.

Most of the nitrogen gas in the atmosphere cannot readily be used by plants as fertilizer. The nitrogen in the air must be converted from N2 to NH3 (Ammonia) or NO3 )Nitrate) for assimilation by plants.

The nitrogen conversion processes are done in a variety of ways.  A flash of lighting can convert N2 gas into NH3 and when you are outside in the rain when lightening hits you can sometimes 'smell' a change in the air.

Bacteria too can convert N2 into NH3 and then into NO3 for plants use.  The complex ecosystem interrelations is discussed with respect to green roof ecosystems in the article located at www.livingroofs.org or by clicking here.

Wikipedia defines the complexity of an ecosystem as a "web, community or network of individuals that arrange into a self-sustaining and complex hierarchy of pattern and process. 

The beauty of the ecosystem web are all the benefits gained through the interrelations.

The photograph above is of an agave, one of my favorite Green roof plants and clover.  The agave provides a windbreak and shade protection to the clover and the clover, through bacteria living in the clover root nodules, provides the agave with nitrate fertilizer.

No need to fertilize.  Clover can add up to 150 lbs of NH3 and NO3 per acre per year ( 68.2 Kilograms per 0.4 hectare).

In fact, adding chemical fertilizers to 'push' or 'rush' a growing process ultimately can do more damage than no additional fertilizer.

Green Roof Design - Stacking


The agave's long leaves filter particulate matter from the air flowing over the roof.  The particulate matter is washed to the green roof surface by rainfall and then used by the clover for nutrients.


The clover is a C3 plant, grows rapidly and prevents erosion around the agave's roots.  The agave is a CAM plant that uses CO2 at night, freeing up daytime CO2 for the clover.


Both the agave and the clover are beautiful.  The agave has white flowers and the clover has bright red flowers.


Both the plants are beneficial to wildlife.  Honey bees love both species.  Other pollinators flock to the green roof's flowers when blooming.


Both the plants are beneficial t humankind.  They sequester carbon, produce oxygen, offer beauty, clean stormwater, and provide habitat.


And they share survival responsibilities.


A perfect coexistence in an complex rooftop ecosystem.

Green Roof Fertilizer from the Sky.

3 comments:

Loret said...

I know you advocate native plants where possible in your green roof applications. Would the native agave and clovers (such as Agave decipiens and Lespedeza capitata) work in this application?

Kevin Songer, J.D. said...

Loret - you comment is correct. Florida's native agaves - I believe there are two, but I admit I am still learning, even after all these years - and the native clover is a good plant also. There are a couple yucca's that are great also - I am doing a blog article tomorrow on your suggestion. Do you know where one could find the Lespedeza for sale? I'd like to try some out on a field trial roof. Tkx for the input - Kevin

Loret said...

Kevin -- I checked the afnn.org site and did a plant search but no luck there. An email to them might lead you in a good direction tho. there is a contact form at afnn.org. The Lespedeza is only up in the panhandle area according to the Atlas of Fl Vascular Plants.