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Archive for the ‘Questions’ Category

Or, ‘This is not a tree’.

Thinking again about, and then past the pollen issue, I wonder if humans had such strong allergic reactions in pre-industrial times. In much the same way that we have been using the world’s oceans as a dumping ground for every substance we don’t want to deal with, we have been pumping fine particulates into the atmosphere in ever-increasing quantities since the start of the Industrial Revolution.

Maybe pollen's not the real problem. Photo courtesy of The Library Of Congress, via Flickr.

We breathe oxygen. Oxygen shares atmospheric space with pollen and with myriad other particulates; our bodies work to filter out the particulates as we draw in oxygen.

Certainly, pollen poses challenges to the smooth operation of the human body. And planting fewer pollen-abundant trees might help breathing conditions to some extent. But really, now — shouldn’t we also look at the overload of particulates continuously (not only seasonally) streaming into our breathing space from coal-fired power plants, miscellaneous smokestacks, vents, trains, trucks, buses, and cars? And act to place more stringent limits on emissions from all those sources?

Making the natural world the culprit is easy. Calling ourselves to account for the consequences of our much more harmful actions may, as painful as it is, may be the more responsible and fruitful response.

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One consideration in how we define slope — the audience to whom we’re communicating. When I’m discussing the pitch of a walk with another landscape architect, I’ll talk about it as a percentage (“From the wall to the driveway the walk slopes at 3 percent, with a consistent 2 percent wash”) When a contractor looks at a slope, he tends to see it as a ratio (“So I’ll set the stones to slope a quarter of an inch per foot, then”). This switching back and forth was confusing to me at first, but then I realized that the language (slope) is the same; it’s simply a matter of mastering the various dialects (ratios, percentages, decimals).

(Several years ago I had a meeting with one of the chief engineers on Boston’s Big Dig, about some grades one of the downtown tunnels. We chatted for a while, and he batted around elevation numbers with amazing facility. For me, physical slope is easier to visualize than numbers, and I still work at making the conversion from physical form to numeric form. I asked the engineer how he understood slope — as form, or as numbers — and he told me that the numbers were an easier way for him to conceive of it. He could read the slope simply from seeing the sequence of station numbers. It was remarkable to me that he could envision the nature of the grade this way, and showed yet another way of conceiving of slope.)

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One reader wrote in with this comment to my last post:

“It would be no good to specify bare root unless you were thoroughly acquainted with the land – soil, ledge, utility lines, for example – and spreading roots of other trees.”

And my answer, because there’s a lot to it:

Actually, bare root is good just for the reasons you enumerate; it’s much easier to plant roots alone than it is to plant a big slug of soil encasing a plant’s roots. And landscape architects, contractors, and arborists have ways of dealing with the issues you mention.

Contractors are required to call Dig Safe (http://www.digsafe.com/) to locate underground utilities on site before any excavation begins. The mis-location of utilities has been known to happen, but excavators are (ideally) careful about how they dig and about stopping when they hit something. Accidents can and do happen, but safeguards have been worked out to minimize their occurrence. (We had a little excitement at last week’s transplanting site over a gas line — apparently DigSafe found one gas line and marked it, but didn’t realize there was another several feet away. The mini-excavator found it — without breaking it — and DigSafe was called out to mark its course immediately.)

Irrigation lines, visible in some of the photos from last week’s posts, are considered expendable/fixable during a construction project. They are relatively flimsy and they run everywhere under many projects, so it is understood that they may be broken (even a shovel can break one), and will be fixed after construction and planting have been completed.

Bare-rooting a tree or shrub for planting — regardless of the surrounding soil type — often is better for the plant than planting it in a soil root ball. When one type of soil is introduced to another, as when a clayey soil root ball is placed in a sand/loam soil, the interface between those two types of soil resists the movement of water from one to the other. That means that if a clay root ball gets watered in thoroughly, water may not move so readily into the sandy loam. What incentive does the root mass have to move beyond that interface and thus into the sandy loam? Not much. Opening up a root ball and mixing some of its clay with the surrounding soil in the wall of the hole will help, but still — with a bare-root plant that issue is a non-issue. Even with a poor soil, it’s easy to mix some planting loam in with the surrounding soil (again, you want to mix, not simply dump a pile that will give you that same resistant interface) and plant the bare-root tree or shrub in the mix; doing so will make it possible for the plant’s roots to reach as far as they have to for the moisture they need.

it is a tendency, unfortunately, of many planting crews (especially on very large jobs where speed is of the essence and there may be little job training for laborers) simply to push the burlap on a root ball down just below the surface, or in some instances to leave it tied in place before backfilling. Natural burlap eventually will rot, but it can take years, especially given the subsurface soil environment, where the burlap is protected from the atmospheric oxygen and UV light that breaks it down so readily in the nursery. In the meantime, that burlap constrains root extension into the surrounding soil, and can contribute to the roots turning back in to the root ball, which affects the growth of the whole tree. So — another reason bare-root is a good approach: no burlap to fool around with and to constrain root growth.

As for ledge: You’re unlikely to know the location and profile of subsurface ledge until you start digging. That’s just the way it is. Again, though, the presence of high ledge (that is, ledge just below the soil surface) argues for using a bare-root planting method. Since tree roots typically live in the top 12″ of soil (sometimes 18″, and sometimes deeper, given the plant genus and the depth of good soil), and tree root balls can be as deep as 36″, planting a tree with soil around its roots means that you have to accommodate that root ball. Sometimes you can slice off its bottom with little ill effect on the roots. Sometimes you can’t. With a bare-root plant, you don’t have to jimmy around so much with adjusting the height of the root ball. Certainly, you’ll have to be sure you have adequate soil depth to plant the roots themselves (spreading them out radially, as they typically need to grow), but bare-root planting gives you much more flexibility in this regard.

OK. That’s it for this post, because I have to hit today’s design deadline, and this was a digression from working on it. Sorry about the lack of photos on this post; next one will have a set of really good ones, courtesy of Matt Foti.

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