ES&C Spotlight
New! Internal Curing Video
Video: Internal Curing Presentation on Improved Service Life Jason Weiss, Purdue University Watch now!
Lightweight Aggregate for Rooftop Gardens
Case Study:
Lightweight Aggregate for Rooftop Gardens
By James Hames
Irrigation and Green Industry Magazine
Peter Lassig had a dilemma.
He wasn't satisfied with the consistency of the topsoil mixtures he had been getting for the 35 acres of garden he was managing at the Church of Jesus Christ of Latterday Saints (LDS) facilities in downtown Salt Lake City, Utah.
Mormon Temple Square has "about 3.5 acres planted with nothing but flowers of every variety imaginable," Lassig says. "We were planting about 165,050 flowers every six months with 200 different kinds sf flowers and 30 to 55 varieties in any one single flower bed." Lassig served as head gardener and grounds services manager for the LDS site until his retirement three years ago.
"Our reputation depended en hew well these gardens looked," Lassig says. "With five million visitors a year coming to visit the Temple, I felt a lot of pressure to produce continuous blooms."
Some of that pressure obviously come from his underlying premise: "Our vision was to call attention tn the creator, not the designer," he says. Some pressure indeed - the Bible has a garden story and it includes making life from soil. The Bible doesn't report any consistency troubles with the soil in that garden story, but Lassig had problems of his own.
If Lassig was to make a glorious garden, he had to tackle his inconsistent topsoil dilemma. "I would analyze and approve a shipment of topsoil and then halfway through the deliveries, the quality would drop. The problem was that the suppliers weren't topsoil vendors, they were truckers," Lassig says.
He was already aware of various expanded shale, clay and slate products. These are produced by putting the raw material into a rotary kiln, heated to 2,000 degrees Fahrenheit. The minerals puff up and, once cooled, become a high-strength lightweight aggregate (LWA) that is sterile, durable, stable and free draining.
Lassig had used the material in sealed-bottom containers; he was aware that the greenhouse industry had abandoned topsoil for LWA mediums. With that knowledge in mind, and while still "agonizing" over his search for a dependable growing medium, Lassig took a summer class at Harvard University with highly esteemed landscape architect Theodore Osmundson. Osmundson is a long time proponent of lightweight aggregate, using it successfully in the rooftop gardens for which he is renowned.
With Osmundson endorsement, Lassig was convinced. "It was a no-brainer because this material is better than dirt." He contacted the trade association for LWA producers - the material is primarily created fur concrete applications — to find an economical supplier for a growing medium that was dependable throughout the delivery. He was connected with a nearby company affiliated with the Expanded Shale, Clay and Slate Institute (ESCSI).
About the same time, WR Grace & Company had developed a balanced liquid fertilizer for light-weight soils. Lassig‘s experiment was underway. He used LWA for a few small gardens, for rooftops and growing vegetables, and "it worked just dandy."
Before Lassig could get LWA into widespread use on the grounds, he had to mollify the landscape design firm's concern that they could be sued. Lassig says they came onboard only after he reassured them that he would take full responsibility himself.
Last unto the bandwagon were Lassig's own landscaping teams. "You need employees to support your efforts or they will become non-compliant, even hostile. We had problems with the sand in our area and everyone was tired of dealing with weeds and other soil issues. Eventually, they agreed to try lightweight aggregate," he says.
Plants were set into a 1-to-1 mix of expanded shale and Canadian sphagnum peat moss spread six inches deep.
"We had such good success, when the LDS conference center was built, we insisted the landscape architect specify the materiel throughout the 1o-acre site plan," Lassig says.
The key to the success of the growing medium is that LWA is 50% air by volume. The expanded particles, Lassig says, hold moisture and nutrients not only on the outside surface of the particle but also in the interior pore spaces. The medium "can lay at full compaction but still be 50% air," allowing suitable flow of water, nutrients and oxygen to plants' roots, he says.
Lassig has created what he calls a "modular soil mix," a matrix that can be replicated from material available throughout the country.
It's been about 10 years since LWA has been used for the Temple's grounds and the gardens have received national awards from the American Nurserymen's Association for their beauty. In 1999, the American Society cf Landscape Architects designated the gardens as a "National Landmark for Landscape Architecture." That bronze plaque hangs en Lassig's wall at home and is a reflection of his 46 years of work at Temple Square.
Click here to download the full article
