C3, C4 and CAM plants all have different metabolisms so they grow and add biomass at different rates.
Why is this maxim important?
Because no matter how much fertilizer you add to a C4 or CAM plant they may not grow as fast as a C3 plant grows.
In haste to make the green roof plants grow quickly and provide roof coverage, additional fertilizers are sometimes added (the magic potent) to encourage more biomass growth.
If the green roof is planted with C4 and CAM plants and heavily fertilized, the green roof plant growth rate will probably not be as fast as an unfertilized planting of C3 plants. Adding more and more fertilizers may even kill the plants.
Some may feel there is comfort in adding fertilizers to a green roof. The small, round little pellets impart a sense of 'doing more than one has to' to make the green roof grow. In fact, adding fertilizers is truly 'doing more than one has to'.
|Green Roof Fertilizer Runoff, Algae on Concrete|
However we need to remember these nutrients may run off the green roof during storm events and feed algae in local waterways. Here in Jacksonville the St. Johns River becomes green with algae blooms as a result of fertilizer runoff.
Dead fish can litter the banks of the river due to the river's algae blooms.
The included photos here show a tale-tell sign of excess fertilizer or nutrients running of the green roof, encouraging algae growth across the concrete walls.
Like infra-red signatures for water leaks, the green stains of algae are a 'spot-on' indicator of the presence of fertilizer and other nutrients.
Remembering C3, C4 and CAM plants all have different metabolisms and grow and add biomass at different rates will help reduce nutrient and fertilizer loading into our waterways.
Work with your local agriculture information staff to pick the right fertilizer for your green roof plants. Spot fertilizer application is best and can be customized to the types of plants and where they are planted on the green roof.
Broadcast fertilizer applications are not recommended because no matter how much fertilizer you add to a C4 or CAM plant they may not grow as fast as a C3 plant grows.
Unless the green roof is a permaculture or food roof I recommend designing a nature irrigated habitat-type green roof and specifying the vegetation to take advantage of the natural nitrogen and phosphorous content in rainfall, supplemented by biomass from the roof leaf litter, bird feces and other natural sources. These natural sources still contain nutrients for algae to feed on, but may be more manageable.
The simple message is this, do not broadcast fertilize a green roof. One size does not fit all. Finally, be sure to scrub the algae off the concrete if you do fertilize using an eco-friendly cleanser and not bleach.
As always, email your questions and comments here.
Happy Green Roofing!