Green Roof Growing Medium - What's so special about it!
Extensive green roof. Low weight, cost effective and probably the most popular of the green roofs. But the low weight comes with a cost to the plants - extremely high stress environments.
WATER STORAGE CAPACITY
In general, the bulk of the water storage in a green roof buildup is in the growing medium. The thin ‘soils’ of extensive green roofs severely limit water storage capacity and therefore add greatly to what is already an extreme environment for the plants. This is reflected in the relatively short lists of plants that are suitable for planting extensive green roofs. But this casts the extreme environment as a foe, something we battle with when we design extensive green roofs. It is important to remember that this is a one sided perspective.
WEIGHT & WEEDS
Weight is an important consideration in extensive green roof design. In fact one of the overriding reasons extensive green roofs are chosen is often because of weight limitations. If this is the case, you do not want the green roof to increase in weight as the roof ages. The extreme growing environment of the extensive green roof, due in large part to the low water retention of the growing medium, is exactly what is needed. Robust voluntary weeds may germinate and grow but will be short lived because of the lack of water which helps to maintain the desired extensive plant population in the long term. This is the positive side of having an extreme environment on extensive green roofs. It is often overlooked but extremely important. For that reason high water retention components like rockwool or high precentage of compost should not be added to the growing medium.
Viewed this way, the extensive green roof must be designed as a high stress environment. Failure to do so will result in gradual replacement of the intended plant population, with an increase in biomass or at the very least, greatly increased annual maintenance.
As a sidebar, it is interesting to note that irrigated extensive green roofs often suffer from this problem. The added water from the irrigation offsets this normal growth limiting variable on the roof encouraging invasive ‘weed’ species. The intended species, usually dominated by sedums, loose their competitive edge and are choked out and replaced by coarse weeds like invasive grasses and goldenrod.
From the above it is clear that the growing medium is a critically important component in extensive green roof design. Limiting the available water in the growing medium to a level that meets the needs of desired plant community is important. The question is how can this be done? There are two approaches that are used. The first is designing the growing medium to limit its water holding capacity and the second is to limit the depth of the growing medium.
As you would expect, in practice extensive growing medium design (the recipe) caters to the needs of the drought tolerant plants that populate these roofs. This includes light weight, high percolation rates, relatively low nutrient levels, high porosity and longevity, all the conditions you would expect for growing drought tolerant plants. In practice this group of characteristics is best met with a recipe high in mineral aggregates, low in organics as specified by the FLL.
GROWING MEDIUM DEPTH
The second way to limit the available water is sort of obvious, especially in a situation requiring light weight; reduce the depth of the growing medium. The ‘right’ depth for the individual green roof will be determined by site conditions such as local precipitation patterns, sun and wind exposure as well as the composition of the desired plant community. Growing medium depths greater than required to match the water needs of the plant community will result in higher maintenance costs than necessary removing unwanted volunteer plants. The type and depth of the growing medium should be matched to the water needs of the intended plant community. Finally, this raises another interesting point. The different objectives that different parties in the green roof planning process bring to the table can work at cross purposes. Consider this case; a green roof intended to retain maximal water to meet site hydrology objectives, paired with low budgets and light loads which drive the design toward the low water demands of the extensive green roof plant community.
There is, of course, more to tell about growing media. This includes but is not limited to; designing for light weight, nutrient availability, important physical characteristics etc. all intended to create a system that is sustainable with minimal maintenance for the life of the building.
Author: Jelle Vonk, B. Sc - Account Manager ZinCo Canada Inc.