Green Roof and Leaves, Cation Exchange and Anti-Allelopathy Issues for Design Consideration

I had a couple of comments yesterday about my post on Allelopathy and Green Roof design.  As we focused on the potential negative impacts adjacent trees and their sap drip, leaves, seeds and shade can have on green roof plants we should also remember certain trees can bring distinct advantages to a green roof or nearby permaculture garden.

Leaves on the Green Roof can exert both Allelopathic and/or anti-Allelopathic influences

One comment suggested we should take into account the addition of organic matter from leaves, replacing lost or broken down compost.


Trees or adjacent shrubs that stimulate growth through their presence, leaf or sap drop or other factors are commonly referred to as possessing 'Anti-Allelopathic' properties.  An anti-allelopath encourages nearby plant growth.  Actually this is a big complicated name describing a common simple practice - composting.


Chinese tallow tree, Triadica sebifera, a terribly invasive species here in Florida and across tropical areas is rated as one of the best anti-Allelopaths, her leaves and sap encouraging adjacent plants to grow as though fertilizer has just been aded to their base.


Leaves can be very useful on the rooftop garden, green roof or in the ground level permaculture garden!  Leaves cool the soil, promote soil moisture content, provide organic matter (good for bacterial breakdown of pollutants) and supply micronutrients, can adjust  pH and cation exchange potential.


No leaves?  May not be too glamorous yet very practical ( remember Louis Sullivan and functionalist architecture) collecting your neighbor's bagged up yard leaves can benefit your green roof or permaculture garden and benefit your wallet.


As mentioned, cation exchange capacity and pH are two important variables of a green roof. Additionally, leaf compost can add valuable trace minerals needed by the rooftop plants.  Some trees, including certain maples containing high sugar content in their sap and leaves possess an extremely low pH, in the range of approximately 4.5 to 5.5.  Other leaves such as white ash and popular may have quite high pH's, approaching 8.0 or above.

American sycamore leaves shown here can exert Allelopathic influences

Leaf compost typically contains twice the amount of trace minerals by weight as does horse manure.  


Interestingly, leaves from different species of trees offer varying characteristics. While ash leaves are relative neutral in pH, some maple species leaves are documented to possess a pH of around 4.5. 


Research also shows use of properly composted leaves greatly increases the cation exchange capacity of soils. One of the important functions needed in green roof soil media is cation exchange capacity.


Though varying opinions of organic compost value to green roofs exist throughout the industry, many believe organic material in the soil media is highly beneficial to green roof plants.


Massachusetts' state DEP has published an informative paper on the value of leaves for compost, including discussing issues and benefits, nitrogen to carbon ratios, composting processes and more.  This information is helpful to those just starting composting or those interested in studying potential allelopathic or anti-allelopathic effects on their green roof, living wall or ground level permaculture bed.


Fossa alterna technology also relies heavily on leaf use and in turn has produced informative leaf nutrient information.

Remember, green roofs are individual ecosystems, intricate webs of life with complex interactions.

Flat roofs with poor drainage may or may not require lower organic content to prevent water saturation and facilitate drainage.

Sloped roofs may function appropriately with high organic content.

Again, if you lack adjacent anti-allelopathic trees then go out about the cityand find leaf waste destined for the landfill.  Collecting heavy duty garbage bags filled with fresh raked leaves (we avoid those lawns heavily treated with fertilizers and herbicides/pesticides - those lawns are easy to spot here in the US due to the small advertising signs the lawn companies stick in the lawn after a fertilizer application - and speaking of lawns and fertilizers - a short must-see video of the history of the American Lawn will have you rolling in your chair and scratching your head at the same time can be viewed here) is a positive step for the environment.

Other benefits include;

• Free highly effective cation exchange capacity supplements from the leaf compost
• Free organic matter from the leaf compost
• Free trace minerals from the leaf compost
• Free pH adjustment material from the leaf compost (this is especially important when using higher pH soil media or media high in calcium)
• Free garbage bags
• And a lesson to your children riding with you to scavenge that recycling is more important than pride. :)