Sidewalks Need to Stop Killing Trees

On February 3rd, the Minneapolis City Council’s Transportation and Public Works committee received a very important, somewhat disturbing report from Professor Gary Johnson in the University of Minnesota’s Department of Forest Resources.  The most striking finding: boulevard trees near recent sidewalk replacement work are more than twice as likely to fail.

The report is based on extensive research Johnson’s team did, with help from City and Park Board staff, after the powerful storm of June 21, 2013.  That storm tipped 367 trees in Minneapolis.  Many of them landed on people’s houses and cars, many of them (somewhat ironically) destroyed nearby sidewalks, and all of them had to be killed and removed.

tree failure

The study found that there were some striking correlations for tree failure.  The most dramatic, again, is that trees near a recently-replaced sidewalk panel were 2.24 times more likely to fail.  Some species were more sensitive – they tend to be the ones that get the biggest, and therefore provide the most canopy.  Bigger trees were more likely to fail.  And trees in narrow boulevards were more likely to fail than trees in wide boulevards, but only where sidewalk replacement work was done.

The takeaway?  The way we repair sidewalks is killing trees.

This can’t continue. Our urban tree canopy is too important to undermine in this way. Canopy has many benefits – to human health, stormwater management, heat island effect, climate, air quality, property values and more. Trees are infrastructure. And urban trees are already facing too many threats and stresses – Emerald Ash Borer, climate change, etc. – for us to do things that make them likelier to die.

This is an issue for pedestrians.  When I walk down a street, I want a reasonably flat sidewalk, but I also want shade from trees in the summer.  I want to walk down a street that is beautiful.  I want to walk down a street that signals to drivers that this is a place where there are people, a place to drive slowly and carefully.  Street trees provide these benefits to pedestrians.

So what do we do?

The staff report lays out some good first steps that staff have already taken.  This mostly includes a commitment to working better with the Park Board’s Forestry division, and their new Forestry Preservation Coordinator.  But more can and should be done.

For example, the City’s Urban Forest Policy, which was adopted in 2004, should possibly be updated.  It includes statements like: “[t]he contractor may remove all roots within the area defined as six and one half (6-1/2) inches below the top of the new finished sidewalk grade, by severing them off cleanly with a sharp axe, or by grinding them off using a root grinding machine, instead of breaking them off with a backhoe or similar equipment.”  Is it really a good idea to remove all roots within 6.5 inches of an entire sidewalk panel?  I think this report says it’s not.

We may have to be a little more flexible on sidewalk grades.  Here’s a good example:

P1160934

The sidewalk panels in front of this house were heaved by the tree on the right.  The homeowner was (justifiably, we now know) worried that the City’s standard practice would end up killing her boulevard tree, so she worked with Public Works to get a very slight grade change built into her sidewalk, rather than cutting or grinding the tree’s roots.  I apologize for the unexciting picture; the grade change is so subtle that it’s really hard to photograph.

We may have to get more comfortable with grinding sidewalks where they’re heaved by roots, rather than grinding roots when they heave sidewalks.  Just as we’ve become more accustomed to including cut-outs for trees that narrow sidewalks a little, for short distances, maybe we need to see our way clear to leaving cracked but still flat panels in place where the replacement work would be disruptive to the roots beneath.  Maybe there are creative new solutions others have tried that we can copy here.  Whatever the solutions, we need to avoid more of this:

tree fail

After the report was presented, the committee voted unanimously for a motion made by Cam Gordon (my boss, for the record), directing Public Works staff to work with both the Tree Advisory Commission and Pedestrian Advisory Committee on recommendations for changes to policy and practice, and report back in July.

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24 Responses to Sidewalks Need to Stop Killing Trees

  1. Peter February 6, 2015 at 10:36 am #

    Go trees!

  2. Bill Lindeke
    Bill Lindeke February 6, 2015 at 10:53 am #

    this is fascinating

  3. Sean Hayford Oleary
    Sean Hayford Oleary February 6, 2015 at 11:43 am #

    Very interesting article, Robin. One criticism I’ve heard in advocating more sidewalks in Richfield is the impact on trees — and indeed, the terrible tree loss in the 2013 storm, and stories like this do emphasize that correlation.

    But it certainly does not have to be this way. Minneapolis’s current practice of completely removing and replacing sidewalk panels (and destroying tree roots below) when they’ve heaved slightly is unnecessary and expensive to homeowners. Not that I would hold Richfield up as an example of great sidewalks generally, but they do grind down edges of heaved panels, and it’s really just fine. I’ve heard of other cities (Stillwater, supposedly) that intentionally ramp sidewalks over tree roots to prevent damage.

    I also think the standard residential sidewalk could be narrower — probably down to 5′ on streets without much pedestrian traffic. 7′ is typical today, 6′ where ROW is tighter — ironically, usually on major streets. So a street like Nicollet through Tangletown, on a high-frequency bus route, has narrower sidewalks than a minor street that sees five pedestrians a day. For those minor streets, 5′ still wide enough to walk abreast (for most people) and wide enough to be able to pass an oncoming person single-file. That itself would reduce impervious surface, reduce impact to trees, and save a bit of shoveling, without impacting the pedestrian experience.

    • Matt Steele
      Matt Steele February 6, 2015 at 1:32 pm #

      There has to be alternatives. Minneapolis charged me $600 a couple years ago for the privilege of having my boulevard trees compromised (they still stand after last year’s storm, thankfully). And the new squares are already heaving a couple inches. Fail.

      • Rosa February 6, 2015 at 9:39 pm #

        Yeah, as a homeowner on a corner lot with gigantic trees, I get to pay for a couple squares a year – it’s really expensive. And as much as I love our boulevard trees (the remaining ones, we lost one to a speeding out of control car a few years ago) I have to admit that when I see they’ve cut out roots as big as a person’s leg and dug down really far for the new sidewalk squares, it’s always made me happy to think that means I won’t have to replace them AGAIN for at least a few years.

        We lost 3 trees in that storm, 2 because they just fell over and their roots ripped out of the ground.

        I didn’t even know “grinding” was an option for heaved sidewalk – it sounds like a good one.

    • Stuart February 6, 2015 at 2:16 pm #

      This type of change to the rules makes sense for a city that doesn’t have a full network (Richfield), but would make little sense for Minneapolis.

      Replacing heaved 7′ sidewalk panels with a 5′ panel would not be functional and it would look terrible.

      The only time that a block long sidewalk is replaced entirely is for full street rebuild that goes all the way up the sidewalk (recently done on the part of Nicollet I walk down often), in which case there apparently aren’t trees in the way anyhow.

      • Sean Hayford Oleary
        Sean Hayford Oleary February 6, 2015 at 2:24 pm #

        Fair point, it was more a theoretical “do we really need 7′ panels everywhere” versus the practical of how to retrofit it.

        That said, I do see differences of about 1′ between old and new panels fairly routinely (possible the standard was 6′ at one time? Or just aberrant panels). And I see sidewalks narrowed as narrow as 4-5′ around existing large trees — which looks fine, IMO, but I will admit, does make shoveling a bit less predictable.

        I think it would be possible to say that 5′ or 6′ is the desirable standard for minor streets, but that replacement panels would only be that wide if, say, 5 or more consecutive panels are being replaced. For added aesthetic ease, you could also do a little triangular transition piece that could be easily taken out if the adjacent sidewalk were also narrowed.

  4. Janne Flisrand
    Janne February 6, 2015 at 11:51 am #

    Thanks for sharing this, Robin. I get angry every summer when I walk past damaged tree roots by new sidewalks. I’m always frustrated to see what the sidewalk repair crews (mostly contractors and not city employees) do to the trees. I’d love to know I could report them and prevent them from getting work with the city in the future.

  5. David Zaffrann February 6, 2015 at 1:38 pm #

    What about different sidewalk materials, as well? Growing tree root balls would be less destructive to more adaptive materials like brick pavers as opposed to 4′ (?) concrete panels. This would also help with runoff. I understand that there would be tradeoffs here, such as a more difficult surface to shovel or plow, and possibly a more uneven surface for pedestrians, wheelchairs, and strollers. However, I have to believe that there are other options out there besides concrete panels.

    • Sean Hayford Oleary
      Sean Hayford Oleary February 6, 2015 at 2:02 pm #

      Hard to best plain concrete, though. Properly installed and undisturbed by tree roots, some sidewalk panels can last 70+ years. I don’t think brick could ever make it that long through MN freeze-thaw cycles.

      I really think a combination of narrower sidewalks (where appropriate), narrower streets (where appropriate), better tree selection for boulevard width (already happening), and less-invasive practices of repair (shaving down edges) could mean a much, much better picture for the urban forest.

      Then again, you could also go the other extreme and ditch sidewalks and conventional street altogether and build tree-lined woonerfs. Tough sell, though, and to actually be pedestrian-friendly they’d have to majorly change the way we drive on minor streets.

  6. Steffanie Musich February 6, 2015 at 2:14 pm #

    The presentation on Tree Failure in Storms by Gary Johnson from the University of Minnesota given to the Park Board was super interesting AND informative. I would imagine it was the same one presented to the City Council. You can watch it on our board meeting recording starting at minute 308: http://v.qwikcast.tv/view.aspx?q=D2HHC

  7. Robin Garwood
    Robin Garwood February 6, 2015 at 3:18 pm #

    Oh, that’s great, Steffanie. I should’ve included the video of the TPW meeting. Folks can find it here: https://www.youtube.com/watch?v=KxLWQAS0z9E. The presentation on trees and sidewalks starts very quickly, at around minute 5.

  8. Alex Cecchini
    Alex Cecchini February 6, 2015 at 4:02 pm #

    How much of the issue could be solved by extending the boulevard into the street area a few feet? There are plenty of streets where, even with cars parked poorly on both sides, cars still have 12++’ of room to drive (particularly one-way neighborhood streets). Cars drive faster than they should/could. Seems like if this would help, along with Sean’s suggestion of slight sidewalk narrowing, we could kill 2 birds with one stone (3 if you count lowering maintenance liabilities)?

    Also, street trees for bucolic streets with mostly single family homes are great. But it seems like we’re willing to sacrifice both pedestrian space and amenities (good tree coverage)on commercial streets because traffic. What should be our best places (that also happen to have residents living above shops oftentimes) are our most barren-feeling.

  9. Robin Garwood
    Robin Garwood February 6, 2015 at 4:41 pm #

    Alex, these are good points. Though sometimes, speaking as a bike advocate as well as a tree advocate, we actually need the space between the curbs, but to rededicate some of it to different uses. I’d also point out two additional things. First, the best trees are the ones we already have (excluding the ash, which are on their way out). While moving curbs into the centerline is a good idea for new construction where trees aren’t present, it might not be the best idea where they are. Second, in some commercial areas it’s not ideal to put in green boulevards at all. That’s where we need new and innovative ways to plant trees, like they’re trying on 4th Street, near the library downtown. What they call “Swedish soil” (it’s mostly rock) with resin-bonded aggregate as the surface. The point is that even in places with constrained pedestrian and planting space, it’s usually possible to find a solution that can support healthy trees and pedestrians.

  10. Reuben Collins
    Reuben Collins February 6, 2015 at 10:56 pm #

    We really do need to start grinding more. Cracked panels should be replaced. Heaved panels should be ground level. Worst case scenario is that If we grind it too thin the panel will crack and we’re right where we started, having lost nothing.

    That being said, I’m pretty squarely in the camp that we shouldn’t let existing trees define the public realm unless we’re talking about some pretty phenomenal trees or something. Nobody is excited about losing mature trees, but i’m also tired of hearing that we can’t have bike lanes in Minneapolis because it would impact trees (e.g. Penn Ave S). Trees can be replaced.

  11. Alex February 7, 2015 at 9:32 am #

    Is this really a new issue? Do you think that maybe sidewalk installation has been affecting boulevard trees ever since concrete walks were first installed, and that Minneapolis has a majestic tree canopy anyway?

    That’s not to say that sidewalk installation procedures shouldn’t be modified to make them less impactful to trees, but the tone of this post (or maybe just the title) makes it seem like the choice is sidewalk or tree. As great as the benefits of trees are, quality pedestrian infrastructure has more important benefits to urbanism and the environment.

    • Robin Garwood
      Robin Garwood February 12, 2015 at 10:15 pm #

      No, this isn’t a new issue. But we’ve never had data this clear and unarguable before. Where once we strongly suspected that root cutting cut trees’ lives short, now we know for a fact that it does, and that the effect is major.

      I am not advocating for removing or degrading sidewalks. I think that there are solutions that get us to a much more positive interaction between sidewalks and trees – like, for example, being willing to have sidewalks rise and fall a little to accommodate tree roots. As I tried to say, I think that both sidewalks and boulevard trees have major benefits for pedestrians, and we can find ways to make the two work better together.

  12. Sean Hayford Oleary
    Sean Hayford Oleary February 7, 2015 at 1:51 pm #

    It’s an issue that is likely continuing to get worse. A hundred years ago, concrete was thinner and they did less to prepare the subgrade. Heaving didn’t matter as much, because there were no ADA standards or similar to ensure safe, trip-tree travel. In the history of Minneapolis sidewalks, my understanding is that the deep shredding of roots is relatively new.

    I don’t believe trees are an argument not to build sidewalks — but they’re definitely an argument to build and maintain differently.

    Plus, things like grinding heaved panels stand to save homeowners hundreds of dollars. And aesthetically, I think it’s kind of a wash, since mismatched-aged concrete doesn’t look that great either. So why not?

  13. David Markle
    David Markle February 7, 2015 at 5:08 pm #

    Trees have been planted very close to sidewalks (or sidewalks installed very close to trees), and it should be no surprise that problems result for both trees and sidewalks. Better planning regarding suitable types of trees for narrow boulevards might help, and Robin seems to have made good suggestions about sidewalk grade adjustments where appropriate. Another obvious possibility might be to move sidwalks closer to residential structures (ie, farther into homeowners’ lawns) and away from boulevard trees, but I’ll bet we’d quickly hear howls of protest!

    I find it interesting that Robin chooses to write about this in a public medium, and I suspect his action may reflect the apparent insularity of the Minneapolis Public Works Department and its resistance to suggestions and pressure, even pressure from the City Couincil. Take as an example the stubborn refusal to install a red, illuminated arrow on the northwest corner of Cedar and Riverside to help dissuade motorists from making improper right turns that endanger pedestrians and cyclists.

    • Rosa February 7, 2015 at 9:46 pm #

      Well, among other things most Minneapolis homeowners have trees on the lot too – you’d just lose different trees if you moved the sidewalks.

    • Monte Castleman
      Monte Castleman February 10, 2015 at 2:47 pm #

      I think the red arrow is less about “being stubborn” and more about not putting up something that violates both state and national standards, and thus potentially exposing them to legal liability, or the extreme expense (tens of thousands of dollars at the very minimum) of completely redoing the signals with possibly a new control cabinet and conduits to put in a legal configuration.

  14. Celeste February 8, 2015 at 3:10 pm #

    I wonder what would happen if the sidewalk panels near trees were scored in shorter segments – would that allow the tree roots to push them up as a gentle curve with no sharp edges that need grinding? I don’t know what the physics of the heaving process is like.

    • Sean Hayford Oleary
      Sean Hayford Oleary February 8, 2015 at 4:53 pm #

      In general when they lay new panels near tree trunks, they do either curve in for the whole panel (narrowing to 4-6′ rather than the 7′), or they score it with control joints in the shape of that curve, so it would work exactly as you describe.

      However, I don’t believe they modify existing panels. At my boyfriend’s place, there were two panels near a medium-sized tree (12″ trunk diameter or so) in a fairly wide (8′-ish) boulevard. Even though they were not cracked nor heaved, the contractors completely removed both panels, ground roots, and replaced with two that formed the aforementioned curve. Cost around $600, and the tree blew down the next summer. (Then again, so did one in the back yard, nowhere close to root cutting.)

      I’m not sure why they can’t just saw-cut the shape out if nothing else is wrong with it. Perhaps the contractors are limited in only being hired to remove and replace whole panels.

  15. David Markle
    David Markle February 11, 2015 at 6:01 pm #

    Minneapolis uses illuminated red arrow “don’t turn now” semaphores on some other intersections. In this case, the engineer’s argument against installling one, as stated at a City Council committee hearing, was that it wouldn’t help enforcement. But behavior is the problem, not enforcement. Bear in mind that the intersection in question is statistically one of the most dangerous in town for both pedestrians and cyclists.

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