{{image2-a:r-w:300}}Recently my wife Pam and I went to a big box home centre – Pam for some plants and yours truly for a piece of moulding.
The store has a pretty nice parking lot with trees scattered about in parking lot islands. Actually, there is a rhythm to the planting – a long, skinny island the length of two cars occurs every tenth parking space. Midway between them is a small diamond shaped island at the junction of four parking spaces. And every so often there is a very big area running the whole depth of the parking lot with a sidewalk, shopping cart stalls and lots of trees.
Here in Arizona’s Valley of the Sun, those shady places are the most sought after parking spots.
I noticed something while we were looking for our shady spot. I pointed out to Pam that the biggest trees tended to be in the sidewalk areas and long skinny islands. The smallest trees tended to be in the little diamond islands. Her reply: “Well, duh! The big trees have more room to spread out their roots!”
Shucks! I knew that – I was just leading up to a short discussion about a special soil designed to support pavement while allowing for tree root growth.
Before I could say a word, Pam told me that a major impediment to tree growth in paved urban areas is the lack of an adequate volume of soil for root growth.
A general rule of thumb is that a tree needs a planting area about the size of its canopy. Its roots try to spread out even further than that. But the pavement limits root growth to a very small area without adequate water, nutrients or oxygen. The result? Undernourished, small trees.
Here was my chance. I fired back that it is the compacted soil under the pavement – not the pavement itself – that confines the roots.
A parking lot is constructed to maintain a stable pavement surface. Construction starts by compacting the subgrade. Above the subgrade there usually is a structural base made from dense graded aggregate that is well compacted and possesses high bearing strength.
The asphalt surface goes on top of the structural base. Roots don’t stand a chance of getting through the structural base.
Before Pam could respond, I blurted out: “Aggregate to the rescue!”
A relatively new material, referred to as structural soil, has been developed that can be compacted to meet engineering requirements for paved surfaces, yet possess qualities that allow roots to grow freely under the pavement.
A structural soil is a two-part system comprised of a stone lattice for strength and soil for horticultural needs. The lattice is made from 38mm to 50mm (1½ to two-inch) crushed stone, which provides strength through stone to stone contacts.
The voids in the lattice are only partially filled with loam, thus providing interconnected voids for root penetration, air and water movement. A hydrogel is added to some structural soils, which acts as a tackifier that prevents separation of the stone and soil during mixing and installation.
On the way back to the car Pam had three plants in the cart, so I asked her if she had any crushed stone for the structural soil. It is amazing how much she can communicate just by rolling her eyes!
Thanks to my colleague and friend Steve Stokowski for bringing this topic to my attention. If structural soil catches your fancy, you can read all about it at:
hort.cornell.edu/uhi/outreach/pdfs/CU-Structural%20Soil%20-%20A%20Comprehensive%20Guide.pdf