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torquillI read an article a little while back about why street trees no longer grow huge and magnificent -- they get to a certain size and stay there, and you don't get the look of the old streets in Chicago and England and Paris. The brief rundown is this: the way houses are built these days, all the dirt is bulldozed off the top, the rest is packed into almost concrete, then a thin layer of topsoil is spread over everything after the houses are built. Four to six inches of imported dirt, then hardpan -- no roots can easily penetrate that packed earth, and water has trouble.
Trees need at least a foot of soil to really root well, and biiiiiig trees need to be able to put taproots down twenty feet and feeder roots down about a yard. So what do you do when all you have is six inches, max? You plant trees that are okay with that. This explains why every city I see plants loads of London Plane, which is otherwise a hideously-behaved street tree (it gets aphids which rain honeydew on cars, it lifts sidewalks and pavement like mad, and it litters the ground constantly with huge leaves, to name a few problems). The reason being, London Plane can thrive in a foot or less of depth.
So the thought I had today was...
1) There's a layer of hardpan a few inches down
2) Very few roots can penetrate the hardpan
3) London Plane trees still do very well in those conditions
4) Doing "very well" equates to becoming relatively large trees
5) which have roots only going six inches down
6) Big trees are heavy, with lots of wind resistance...
How many of these trees are going to topple straight over as soon as an unusually strong storm comes through?
What a nasty thought.
Trees need at least a foot of soil to really root well, and biiiiiig trees need to be able to put taproots down twenty feet and feeder roots down about a yard. So what do you do when all you have is six inches, max? You plant trees that are okay with that. This explains why every city I see plants loads of London Plane, which is otherwise a hideously-behaved street tree (it gets aphids which rain honeydew on cars, it lifts sidewalks and pavement like mad, and it litters the ground constantly with huge leaves, to name a few problems). The reason being, London Plane can thrive in a foot or less of depth.
So the thought I had today was...
1) There's a layer of hardpan a few inches down
2) Very few roots can penetrate the hardpan
3) London Plane trees still do very well in those conditions
4) Doing "very well" equates to becoming relatively large trees
5) which have roots only going six inches down
6) Big trees are heavy, with lots of wind resistance...
How many of these trees are going to topple straight over as soon as an unusually strong storm comes through?
What a nasty thought.
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Date: 2006-07-14 02:47 (UTC)no subject
Date: 2006-07-14 05:13 (UTC)no subject
Date: 2006-07-14 07:07 (UTC)July 29th
Date: 2006-07-14 23:30 (UTC)The shallow rooting is a big problem. Surprisingly less trees are blown over than you might expect, but even the old giants, with better root space go occasionally. Often there has been root injury, fungus, or dieback before one goes. However, if the canopy gets two large and top-heavy (easy if your root space is tiny) any tree will fall. One of the other major problems for urban trees is that they are usually planted in little tree wells. Even if the soil in a 4'x4' planting area is thoroughly ripped to a couple feet deep. It's still just not a large enough volume for the tree to get really big. No nutrients, no water, etc. I've seen some studies and recommendations on a minimum volume of soil and something in the 80-100 CF is coming to mind, but I don't guarantee that number.
~M