Man the tredges! We must protect children from air pollution!

Every once in a while, I like to talk about the benefits of incorporating plant life in and our buildings and cities. In addition to the mental health benefits, which I think are reason enough, there’s ample evidence that more plant life can reduce the harmful effects of air pollution. This is another one of those times when it’s so obvious something’s a good idea, I find it perplexing that more cities aren’t investing more heavily in urban vegetation. I know a great many cities around the world have been doing just that, but as ever, it’s not enough to satisfy me.

So, here’s some more evidence that we should have more plants around us:

A team of researchers led by Barbara Maher, Emeritus Professor at Lancaster University, and supported by Groundwork Greater Manchester, installed ‘tredges’ (trees managed as a head-high hedge) at three Manchester primary schools during the summer school holidays of 2019.

One school had an ivy screen installed, another had western red cedar and the third school had a mixture of western red cedar, Swedish birch and an inner juniper hedge. A fourth school, with no planting, was used as a control.

The school with the ivy screen saw a substantial reduction in playground particulate matter concentrations, but an increase in black carbon. The playground with the mixture of planting saw lower reductions in air pollution to that of the western red cedar.

The biggest overall reductions in particulate matter and black carbon were shown at the school with western red cedar planted. The results showed almost half (49%) of black carbon and around 46% and 26% of the fine particulates, PM2.5 and PM1 emitted by passing traffic were captured by the western red cedar tredges.

The tredges also significantly reduced the magnitude and frequency of acute ‘spikes’ in air pollution reaching the playgrounds.

Professor Maher said: “Our findings show that we can protect school playgrounds, with carefully chosen and managed tredges, which capture air pollution particulates on their leaves. This helps to prevent at least some of the health hazards imposed on young children at schools next to busy roads where the localised air quality is damagingly poor, and it can be done quickly and cost-effectively.”

I could never have predicted that becoming a writer would one day lead me to learning the word, “tredges”. That said, I’m not surprised that I’m learning about this from researchers in England.

It seems pretty clear that this is a good investment. It also seems like the kind of thing that parents interested in direct action could band together to demand, or even just do. I’m not speaking from experience, but I expect this would be hard for local politicians to oppose, and easy to unite parents around, regardless of ideology. There may be parents out there who wouldn’t support better health for their children, but I doubt there are many.

There are a lot of small things that most communities are capable of doing for themselves, but that don’t get done. I think at least some of that is people just not realizing that they have the resources they need, but a lot of it is the likelihood that any good will be undone by the system that’s supposed to be working for us. Where that system is working against us, it may be worth doing something like – in this case – putting up a hedge, without waiting for permission. If one were to do that, I imagine it would be best to have signs attached to it, explaining what it’s for, and encouraging people to fight to keep it. Even if one lost such a fight in the short term – if the hedge (or tredge) is destroyed because the right paperwork wasn’t filled out, then that would become something around which you could rally support.

There are a lot of problems in the United States caused by activist parents making noise at school board meetings and other such local political events. This seems like a way to activate people in a more constructive direction by using similar tactics (assuming that getting your tredges isn’t as easy as I think it ought to be). Getting a hedge put between a play area and a road is a small enough change that I think most people will believe that it’s within reach, which will make them more likely to put in the effort to make it happen. I also believe it that the conversation about the benefits of greenery for children could fairly easily be turned to conversations about air pollution and greenery in general, waging a campaign like that could well make people think about what other things they could accomplish by working together. I don’t know whether good fences really make good neighbors, but good tredges definitely make better neighborhoods.

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  1. says

    ohh i thought for a minute you’d pulled a “all-intensive purposes” and died from second-hand embarrassment. imma go hide in a tredge now while i catch my breadge.

  2. Pierce R. Butler says

    … ‘tredges’ (trees managed as a head-high hedge) …

    Why not taller?

  3. says

    @Pierce – no good reason I can see? I think trimming them to be “tall fences” rather than “lines of trees” is traditional. I suppose one reason would be so your hedge doesn’t shade out your neighbor’s garden.

  4. wearsbellsonlegs says

    @ Pearce – it might be that leaves present a larger surface area than trunks. The concentration of most particulate pollutants would be greater lower down. With most trees, as they get taller the lower parts lose their limbs – which bear the leaves.

  5. Pierce R. Butler says

    Abe Drayton @ # 3 & wearsbellsonlegs @ # 5 – good points.

    I’d really like to know why “the mixture of planting saw lower reductions in air pollution to that of the western red cedar” – seems like the combination would produce the most leafage and thus the most protection. (Of course, having only one example of each type of tredge, and one control, limits how much we can conclude from that barely-an-experiment in the first place…)

  6. says

    It got caught in the filter for some reason.

    I just liberated the longer one, unless you feel a need for the others to be visible as well?

  7. Pierce R. Butler says

    Abe Drayton @ # 8 – Thanks for taking care of that; the rest is redundant.

    I wondered at the time whether the word “[7-letter string beginning with “l”]” triggered some rule a naïve country boy like me would miss…

    [Note – if this shows up, that l-word must have something to do with the previous attempt not getting through.]

  8. Pierce R. Butler says

    Yeah, that must’ve been it. Does the nanny here have a vendetta against, say, the whacky terbacky?

  9. Alan G. Humphrey says

    From the list of plants used the less effective screens included broad-leafed plants, the mixed with birch and the ivy. The cypress family plants, juniper in the mixed screen and redcedar, have scaly leaves that grow in a tighter configuration than broad-leafed ones. This gives a better resistance to airflow and probably more surface area to catch pollutants.

    Also, trimming hedges is much more difficult if they are much above head high.

  10. M. Currie says

    Is there some reason why trees make better hedges than good old-fashioned hedges? Why not revive the dense, leafy boxwood hedge, or barberry.?

    Or if you want it to serve the function of a protective fence and to harbor all sorts of wildlife, with some nice flowers thrown in, rosa multiflora. It’s pretty impenetrable. I recall some years ago there was a big planting of rosa multiflora in the median of the old Merritt Parkway in Connecticut, because it grows with such density that it can actually stop cars. And birds love it.

  11. Alan G. Humphrey says

    M. Currie @ #13
    The plants you describe have two problems in screens for blocking pollution near high traffic roadways. First, they’re deciduous and since in the cited study the screens were for schools, they would have no leaves to effectively block the pollutants for much of the school term. Second, those flowers attract pollinating insects and being next to a high traffic road would lead to pollinator loss that can be avoided by using wind pollinated plants such as the cypresses.

  12. M. Currie says

    Point taken about rosa and barberry, but last I knew, boxwood was evergreen.

  13. Alan G. Humphrey says

    @ 15
    Yes, boxwoods are evergreen. They also produce small aromatic blossoms in early spring, the worst time for emerging pollinators to be attracted to a screen between a busy thoroughfare and a school.

  14. M. Currie says

    Well, I stand corrected. Good thing I’m not in charge of any school hedges or tredges, then.

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