On my street the city has newly planted Gleditsia triacanthos var. inermis 'Skyline'. They are not so unusual. But the planting method is something I've never seen before. The workers who planted the trees very carefully and thoroughly tamped gravel in the planting opening after the plant was set. They used 3 different sizes and colors of gravel. On the top most surface they used a clay colored gravel and created a very level and fairly smooth surface where the gravel is bound together almost like a porous resin.The material comes right up to the trunk covering the base completely. ??? I know.... so many words when just one pic would explain. I won't be able to post a pic until my pc is repaired.
If it lets water and gases through it could be quite a good idea. Here the unprotected soil around street trees gets trodden upon and compacted, sprouts weeds etc. And I think maybe Seattle has been spraying herbicide on the tree wells lately instead of covering the soil. People might not litter on a covering like the one shown here as readily as they do a patch of bare ground.
My concern is lack of organic matter. How much space does it really have to grow roots out underneath the concrete?
Actually the area of non-paved surface around the trunk shown here is quite a bit more than many street trees get.
that depends on the sub-terranean method implemented at planting. Normal tree pits are about the size of the area obviously not paved around its base. Some cities use structural soil, some use structural soil with radiating and connecting trenches and the next thing on the radar is Silva soil cells. The more soil volume, the longer the tree will grow in that space before it runs out of room and resources.
Guys, I did learn that the planting was directed or at last partially managed by the Pennsylvania Horticultural Society. I sent a message to learn more about the technique but haven't heard back yet. My friend who owns a coffee shop on that street told me that a memo was sent out describing the material, how it was porous and expanded and contracted with the weather and the growth of the tree. She had misplaced it. Inside those holes was a compacted clay soil and the trench was dug to about 3 feet. In Philly we have massive trees in tiny holes all around the city. Mostly sycamores but plenty of linden and plenty of ginkgoes, there are some oaks, cherries, pears, hawthorns etc all in my own neighborhood. Mostly all are planted in modest sized sidewalk plots. Some of the trees are in the 60' range!
