Landscape Architect & Specifier News

JAN 2019

LASN is a photographically oriented, professional journal featuring topics of concern and state-of-the-art projects designed or influenced by registered Landscape Architects.

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Page 32 of 117

Above: This is Winterberry holly in Plot 3 after a storm. Plot 3 ended up with 3" of standing water, while Plot 1 and 2 had no standing water after the storm. Left: The trouble with a high sand soil is the window between field capacity, and wilting point is the absolute smallest of all soil types. While there are a number of ways to measure this, a simple measure is the percent of water in the soil matrix. In a high sand soil, say a Sandy Loam, like most rain garden recipes out there, we must manage a soil water content between about 8-10% and 20%. Above 20% and gravity pulls it out due to sand's inability to hold onto water. Below 10% and the plant cannot physically pull the water off the soil matrix. January 2019 33 periods of extreme flooding where the plant must endure up to 48 hours of saturation (anaerobic soil). The soil quickly drains to field capacity, then depending on the season and location, these plants must then survive a period of time where the soil rapidly reaches, and remains at, or below, the wilting point. That happy time in between field capacity (when gravity has pulled all it can out of the soil) and wilting point (plant cannot pull any more water out of the soil) is called Plant Available Water (PAW). Horticulturists spend their whole life trying to maintain landscapes within this PAW window. Too much water, the plant can't breathe; too little water, the plant can't drink. Knowing that I couldn't walk into the conversation and say, "hey, clay is awesome at holding water and nutrients unlike sand, so let's make a rain garden soil with a bunch of clay!" (if you see figure 1. The PAW represented by the orange arrow, of Sandy Loam is the same amount as pure clay). I knew I had to meet the engineer requirements for infiltration rates, filtration capacities, and load bearing capacity. I also wanted to meet the horticultural needs of keeping as much PAW in the soil as possible. Findings Well, three years later, here I am analyzing my results and we've found a winner. Utilizing the impressive capability of lightweight expanded shale, combined with some compost and pine fines, there is a soil recipe for stormwater bioremediation that is able to keep the "bio" in bioremediation and meet all engineering specifications for chemical and physical filtration. For my experiments, we utilized

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