Dirty Thoughts

by Sam Ferro – Turf Diagnostics & Design

Portions of this article were published in the Spring 2014 issues of Living Architecture Monitor.

Green roof growing media is not your every day garden variety soil. Sure it has to retain water and nutrients, and provide a hospitable environment for plant growth, but there is so much more required. Growing media must be lightweight, provide effective drainage, and in many cases retain significant amounts of water or at least slow runoff.

With the intense performance requirements for green roof growing media, it should come as no surprise that native soils do not provide a proper growing media. To ensure a quality growing media, specialized soils must be manufactured to meet project requirements. These specialized soils are typically manufactured from multiple components that may include sand, compost, light weight shale or clay, perlite, or other specialty components.

Production, delivery, and maintenance of quality green roof growing media is critical for green roof success. For this article, I spoke with green roof media producers and specifiers to get the inside scoop on what it takes to produce quality growing media. Kevin Anderson (Missouri Organic), Scott Jenson (Utelite), Charles Duprey (WeCare Organics), Dick Hayden (American Hydrotech) and Jeff Bruce (Jeffrey L Bruce & Co.) were contacted to provide their thoughts on growing media production.

These individuals represent the full spectrum of the production process including the designer, roofing contractor, component suppliers, and soil blenders. These gentlemen come from different backgrounds and are from different parts of the country. In spite of differences in background, there were many issues where there was nearly complete agreement.

There was total agreement that locally sourced materials are the most cost effective components for the growing media. Specifications that call for materials that can’t be sourced locally can incur delays and very high shipping costs. Jeff Bruce indicated that he will consult with local suppliers or consultants familiar with an area to assess potential materials prior to preparing a media specification. He also mentioned that green roof sales are very small for most aggregate suppliers, so we can’t expect these suppliers to create custom materials to meet our needs. We usually have to adapt our mixes to work with available materials.

The mix producers indicated that they, of course, use their own products in the mix. The other required components are sourced locally. Duprey indicated that sourcing locally is typically lower cost, and is also the green choice due to reduced trucking needs. Jenson added that they only use local sources that have test data to verify quality, but price is a key factor. Sometimes the materials that are preferred by the supplier, may be rejected for the project because cost is too high.

Once components have been sourced, they have to be blended to create the desired growing media. Sourcing quality materials isn’t enough, these components needed to be blended to create a consistent mix that meets project specifications and client needs. Hayden indicated that suppliers/blenders should have resources and knowledge of local business conditions and requirements. He also suggested that experience is a key, as he has a long relationship with most of the soil blenders he works with.

The desired green roof growing media can vary widely depending upon usage. Storm water control requires something different than what a wetlands would require. Intensive mix blends with their deeper depths are typically not the same as extensive mixes. Thus, proper components and component ratios are crucial to achieve desired performance. How do suppliers come up with mixes to meet project needs? It depends on the specifications, and a well written specification is critical to project success.

Growing media specifications can be divided into two broad types. “Material Specifications” usually provide for what components to use and how much of each component goes into the mix. “Performance Specifications” will typically focus less on specifying particular materials, and more on detailing what the performance of the blend should be (permeability, wet weight, water retention, etc.). Material specifications may be appropriate for proprietary blends or when there is intimate knowledge of the available local materials. Performance specifications are often preferred because they allow for some flexibility in materials and focus on the characteristics of the final blended material.

The suppliers/blenders were somewhat split on which type of specifications are preferred. Anderson has a preference for performance specs, indicating that these types of specs allow for the best mixes to be prepared from locally available materials. He indicated that his company will initially prepare small batches of media, have them tested for performance, then adjust mix based on test results. Duprey likes specs that provide an exact blend recipe, but he also uses routine testing to verify mixes meet specifications. All agreed that specifications must take into account regional differences in sands, composts, and other components, as well as, budgetary limitiations.

Regardless of the specification type, there should be a component detailing how the blended green roof growing media is expected to perform. The blended material is, after all, what’s going up on the roof. Both Hayden and Bruce indicated that it is imperative to know what type of performance is expected from the roof before a proper specification and resultant growing media can be prepared. Unfortunately some designers and roofing professionals don’t have a firm grasp on what is meant by performance. Bruce indicated that factors such as, type of vegetation, water management, material depth, load limits, and intensity of use must be considered.

From mix design and development to verifying blended materials meet project specifications, testing is a critical component of growing media production. Each of the producers indicated that testing is integral to both the mix development and verification stage. An experienced testing lab can help make the mix development stage go faster and produce a better mix.

Hayden uses testing to confirm blenders are producing the correct materials, but also relies on on-site observations to evaluate materials. He says that soils are living environments and must be treated as such. Timing of media installation and proper handling of materials are crucial. If media is on the roof and there will be planting delays, the media should be protected to help prevent infiltration of weed species.

Bruce believes that specifications are a guideline to achieve desired function in the field. He uses testing as a critical component of his specifications. He equates media production to water coming from a faucet, “you have to expect fluctuations”. Testing, when performed by an experienced green roof lab, can tell us what those fluctuations are. It is then incumbent upon the designer or other professionals to determine whether they are significant. An understanding of what parameters are important and what tolerances are appropriate is key to evaluating laboratory data.

Green roof growing media are complex mixes, requiring multiple components blended at precise ratios. A team of professionals with diverse expertise is needed to design and create these mixes. When quality materials are combined with knowledgeable people, the resultant growing media should perform well on the roof and provide optimal conditions for plant growth.

Sam Ferro is the president of Turf Diagnostics & Design.