Florida Extensive Green Roof Systems, Green Roof Depth Considerations and Root Architecture

I ran across an interesting article in the American Journal of Botany entitled 

Root deployment and shoot growth for two desert species in response to soil rockiness by Gretchen B. North, Edward G. Bobich and Park S. Nobel.

The article confirmed interesting data we've been seeing with respect to root formation in coarse and shallow green roof soils.

MetroVerde Green Roof system, shallow, extensive and well-drained

Shallow green roof soils work well for us  we believe, for a number of reasons, including;

1. Except for the rare lengthy, monsoon-like downpours that occur annually and deliver the false impression Florida is more rain forest like than desert, water penetration into green roof soil media is minimal and lengthy periods of drought are frequent.  Therefore growing systems that force shallow root growth also promote stronger rainwater contact for those light drizzles we may experience.

2. Shallow growing systems combined with appropriate soil media enhance root-oxygen contact.  Root-oxygen contact importance has been especially shown as important through agrinomy studies in hydroponics.  Shallow root systems on extensive green roofs allow for increased oxygen uptake leading to significantly more biomass production.

Agaves and Yuccas are some of our standard green roof plants for use here in Florida.  There are two agave species native to Florida (and the Southeastern U.S.) - they are false sisal (Agave decipiens) and wild century plant (Agave neglecta).  Native Yuccas include:  Spanish bayonet (Yucca aloifolia), Adam's needle (Yucca filamentosa), and moundlily yucca (Yucca gloriosa), and curve-leaf yucca, Yucca recurvifolia.

Though the paper looks at Agave deserti and bunchgrass, Pleuraphis rigida, the concepts and conclusions are important.

Observations included:

1.  Water potential was higher in sandy soils than in rocky soils.

2. Agave deserti preferred rocky soils and was absent in pure sandy soils.

3. Root growth was greatest in sandy soils 

4. Agave deserti had twice the biomass and root surface area in sandy soils over rocky soils (example of environmentally induced acclimination)

5. Agave deserti leaf's water potential was the same at rocky and sandy sites but the transpiration rate was twice as high in rocky soils.

Conclusions and assumptions Green Roof Designers can use are:

Certain agaves (and yuccas) growing in well drained soils (rocky instead of sandy) exhibit acclimation characteristics of slowed growth rates and biomass production.  Slow growing plants on the green roof may be  more likely to survive long periods of drought.

Agave root architecture will develop slower and more shallow in rocky soils than in sandy soils.  Again, on an extensive green roof - one with relatively shallow soils - a plant with a shallow root system does better than a plant requiring a deep root system.  Agaves can acclimate to the coarse soils. Again, a shallow root system plant may be preferable over the deep rot system species.  A deep root species may have the tendency to assert agressive root barrier or roofing membrane attack.

Agaves do not want wet feet.  Where there is an over-abundance of moisture (very sandy soils) agaves may not be present.  Keep the green roof well drained.  This concept is in general disagreement with the school of thought that green roofs should serve as stormwater retention volumes or basins in conjunction with cisterns.  We've made the suggestion many times that due to Florida's water crisis, an irrigated green roof may not be considered sustainable - as one third to one half of the year it will end up irrigated with potable (during times of drought).  Within rainy season periods one will being irrigating the roof more frequently to empty the ever filling cistern - thus exposing the agaves, or any slow growing and green roof tolerant plant to 'wet feet' conditions.  Keep your green roof plants in conditions that do not generally comprise long term saturation.

Rocky soils may have a tendency to absorb and capture more dew than sandy soils.  This assumption/conclusion has many important implications for green roof plants water needs.  Because air can flow more easier through rocky soils, the dew has a tendency to penetrate to rockier soils.  Sandy soils may block moisture laden air.

I like to compare the ability of rocky or coarse soils to capture dew to the summer morning wet grass in your front yard.  There will always be more visible dew on the high surface area grass blades than say, on flat concrete.

Capturing all the water  from dew, fog and slight drizzles is important for our approach to green roof design based on non-potable irrigation.

Nature is always the best teacher.