If you go to one of the excellent classes provided by Green Roof for Healthy Cities, a point that is emphasized is that green roof plants are planted in planting media and not soil or dirt. Why? Most green roof applications are concerned about water retention and weight. Organic matter has a tendency to hold onto water and weigh more, depending on its actual composition. Therefore it is of greatest importance that the planting media is carefully thought out, specified and installed.
The factors that affect media selection are climate, weight, fertility, depth, location, microclimate and irrigation or lack of it.
The products used are typically a mixture of organic and inorganic matter with significantly more inorganic matter in extensive systems. Inorganic matter used includes vermiculite, expanded slate, expanded clay, lava rock, coarse sands, pumice stone, zeolite, diatomaceous earth, perlite and rock wool.
Planting media is critical to long term plant survival, stormwater retention and heat mass transfer. The composition of the media determines the system’s saturated weight, evaporation, drainage capacity and compactability. Plants need water, air and space for roots and nutrients. How well these needs are addressed depends on the planting media.
An area of controversy within the industry is how much organic vs. inorganic material should be in the planting media. The Winter 2008 Issue of Living Architecture Monitor addressed this issue. The inorganic supporter, Chuck Friedrich defines planting media as the particulate matter or substrate that anchors plant roots to sustain plant growth. His recommended mix is:
- a lightweight aggregate, 3/8” or finer to provide space to retain water and nutrients. The lightweight aggregate will lighten the load and be permanent.
- Quality sand to provide a filler and stability
- Organic compost for microbial activity. He points out that currently there are no standards for organic compost.
He gives the following plugs for inorganic planting media:
Inorganic materials provide good drainage
Inorganic materials do not change over time like organic matter does. Organic matter can break down and wash away. Replacement of organic material can be labor intensive. Organic material may produce fine particles which clog filter fabric.
Inorganic media ensures that enough air can be provided to encourage roots to go down instead of up because of oversaturation. If roots are down, the plant can sustain itself during periods of extreme temperature.
He instructs NOT to mix fertilizer into the planting media but to apply it directly to the plants after planting. He also recommends the availability of irrigation for droughts.
The Organic supporter, Rick Buist, bases his support for organic media on over 80 successful installations using organic based planting medias. He feels that the push for non organic planting media stems from proprietary blends. However, the “Organic based planting medias” used in the 80 successful installations are not defined, so it is difficult to tell the degree of difference between Rick and Chuck’s arguments.
Many current practices regarding planting media rely on a set of standards developed by a not for profit group in Germany, Forschungsgesellschaft Landschaftsenwicklung Landsschaftsbau. It seems that there is also confusion regarding what organic material is and how to measure it from Rick’s review of the FLL standards. Differences can occur if one considers the standards for mass or volume of organic material. Furthermore, the standard focuses on material specifications rather than performance specifications, which doesn’t allow room for specific applications or creative solutions.
Chuck does make some good points for organics in the following excerpts:
“The performance which organic matter brings to stormwater retention, pollutant degradation, plant variety, cooling benefits, sustainable materials, etc. is too great to ignore.
“Organic-based growing media can hold far more water than mineral based growing mediums while maintaining porosity: this is because of the way in which they hold water through particles swelling instead of just void filling and capillary forces. Biology can be customized to degrade specific and non-specific pollutants. Plants that provide evaporative cooling can be used more frequently with success. Fertilization can occur naturally through nutrient cycling. Materials can be sourced locally.”
“Practically every argument I have heard against the use of organics comes with a relatively easy solution:
Lost depth because of organic cycling is easily addressed through inputs such as biomass created by the plant choice through roots or refuse or annual (if required at all) top-dressing with a pelletized product such as compost or alfalfa (readily available in a dry, bagged form)
Fines clogging drains or water logging can be averted by careful selection of organic materials. Organics with crystalline structures such as certain bark-based products will behave much like sand for free drainage while organics with strand characteristics will hold structure together. Careful selection and installation of components such as filter cloth are also helpful.
Wind erosion is averted by using biodegradable netting until plants are established, thereby providing continuous cover.
Fire prevention can be improved by avoiding certain substances (i.e. peat moss – a limited resource) and by using organics with a high ignition-thresholds and large moisture-retention capacities. Although any biomass on a roof can burn, it is easy to mitigate the risk by paying attention to the details.”
Source: The Organic Question, Living Architecture Monitor, Winter 2008 Volume 10, No.1.
How can we apply these observations into a planting mix? Please share any thoughts, recommendations or experiences.
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