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Ask Mr. Smarty Plants is a free service provided by the staff and volunteers at the Lady Bird Johnson Wildflower Center.

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Sunday - November 23, 2008

From: Houston, TX
Region: Southwest
Topic: Soils, Green Roofs
Title: Soil mixes for green roofs
Answered by: Nan Hampton

QUESTION:

We are trying to set up a green roof system on our own. What do you suggest for soil? It seems the soils are proprietary and unavailable to individuals.

ANSWER:

Mark Simmons, Ecologist at the Lady Bird Wildflower Center, suggests the following recipe:  50% expanded clay, shale, or perlite; 30% decomposed granite; and 20% rice husks or another fine, slowly-degradable organic matter.  There are other mixtures that also work. For the Seattle City Hall green roof the "Roof Garden Soil Mix was: "55% PermaTill 5/16" Expanded Slate or equal, 30% USGA Root Zone Sand, and 15% Approved Compost."  Greenroofs.com suggests a mix of "1/3 clean topsoil, 1/3 compost, and 1/3 perlite or other inorganic material"  for an economical option, but this has the potential for being heavier than some other mixes.  

If you visit the Greenroofs.com directory page and choose the category "Suppliers>Individual Components>Growth Media", you will find a list of companies that sell soil mixes for green roofs.  I checked one, ERTH Products, that advertised "ERTH-HydRock Lightweight Structural Soil" described as "an engineered soil designed to be lightweight for use on structural conditions such as roof top gardens." Another company, Permatill, offers "Green Roof Planting Media".  There are more companies listed under the "Growth Media" category.  Also, you can find more information about green roof soils by Googling "green roof soil".

 

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