Top 10 Takeaways: International Society of Arboriculture Conference

Last month we shared a “To 10 Takeaways” blog post about the Soil in the City conference earlier this summer. Today we’re sharing the Top 10s of another conference – the International Society of Arboriculture, held in Milwaukee, WI from August 2nd to August 6th – again compiled by L. Peter MacDonagh, FASLA. This list is a totally subjective assessment of the key things to know about based for the design, planning, and arboricultural communities. We hope these lists will entertain, and also be helpful for people who weren’t able to attend or want a reminder of the key conclusions.  -LM

10) Don’t mess with pH

There were many interesting presentations on urban soils: their characteristics and the best methods for returning soils to health. However, what was most memorable to me was what to NOT change about a soil – it’s pH (acidity or alkalinity). Instead, Jim Urban said, change the tree selection to one that is tolerant of that location’s pH. Trees have evolved for millions of years, adapting to many different levels of soil pH, but not soil compaction. Look around: there’s a big wide world of trees waiting to take their rightful place on the streets of our cities. This is an Alex Shigoism… “Right tree. Right place.” Great insight, Jim.

9) Post-War Trees

Talking about compaction, I learned a very interesting fact about Post World War II soil compaction – the killer of so many trees. Most pre-World War II housing areas did not employ the kind of heavy machinery such as pan scrapers with tires that are so common today. Here in the upper Midwest, basements are very common. Without exception, pre-WW II houses had basements that were hand dug. This loose hand worked soil was spread out over what would become the front and the back yards, with the heavier clay subsoils used for the road base of the streets. Those loose, un-compacted yard soils were the kind that could produce bumper crops in WW II Victory Gardens and also great tree rooting space for the trees that would become the classic leafy streets of American cities and first ring suburbs.

Returning World War II US Corps of Engineers veterans brought back the heavy machine expertise and those pan scrapers, bulldozers and dual-tire trucks they had used in the war, and now turned them to the task of laying out and building of modern day suburbs. The structural stability and compaction of Pre-WW II that was once limited to areas immediately under the streets and buildings now covered entire subdivisions, landscapes and all. Thus, landscapes of the last 60 years have about 4 inches of topsoil, and immediately beneath that, compacted subsoils with enough strength to support large machines, but not enough oxygen and topsoil to grow a decent tree. Let’s get back to landscapes that only need to support foot traffic and grow large trees, for what could be our once and future suburban forests.

8) Four Stories Tall

Between three percent to eight percent of ALL electricity generated in the United States of America is devoted to cooling the lower four floors of buildings – floors that could easily by shaded by trees. It took an Italian arborist to research and model this for us Americans. Landscape architects and planners, let’s get busy. This is a solvable problem.

7) Arborist Lifehacks

Here’s a winning method from Dr. Kim Coder for quickly assessing tree health: cut a tree branch and look at the ratio of heartwood (darker colorful wood in the center) to sapwood (light colored wood that starts under the bark and is the active growing part of the tree). Here goes…

Little Heartwood with Lots of sapwood = Great tree growth.

Both Hardwood and Sapwood are even = Good tree growth.

Lots of Heartwood with Little Sapwood = Tree is dying.

There’s no heartwood in tree roots – so don’t try this method with them!

6) Some things never change – but they should

It’s been over 20 years since I first remember hearing of research on girdling roots, through the first of a series of symposia called The Landscape Below Ground, held at The Morton Arboretum near Chicago, IL. The talks were led by Gary Watson. Decades later, and still we are burying our trees rather than planting them.

This seemingly small act of burying the root collar 6″, 8″, 12″ or worse (“Hiding the Graft” beneath the soil surface, and then further burying with mulch) unleashes a cascade of problems for trees that greatly shortens their life and makes them dangerously unstable. The tapering trunk and 4 to 11 main roots radiating out from the trunk that show up in most children’s drawings of trees disappears, replaced by a telephone pole like trunk and roots crossing over each other and the trunk. These are the adventitious roots – dormant trunk buds, above the trunk/root collar that sprouted and became girdling roots, literally strangling the tree. Of the millions of urban trees that are planted annually,  many, perhaps even most, are still installed in this way. For the thousands of us landscape architects that are responsible, we must reverse and eliminate this problem from our projects.

5) Nursery stock matters. Big time.

We landscape architects are still accepting too many trees that arrive on sites in root packages with damaged root systems. This problem is further compounded by our acceptance of the poor planting practices described above. Whether it’s the encircling roots found in pot-bound trees in plastic pots/containers, or the vibratory, plow-shattered roots inside balled and burlapped (B&B) trees, etc., this problem is pernicious. Roots need to be inspected, and problems mitigated or trees replaced, prior to planting. If not, our profession is doomed to preside over a kingdom of stunted trees that live only 20 or 30 years before dying.

4) The wrong end of the tree

There seems to be an obsession with transplanting large-caliper trees on projects. Four-, five-, and six-inch tree trunks looks impressive on the day of project opening, and they photograph well, but we are spending our client’s money on the wrong end of the tree. Studies and research presented at this conference conclusively showed once again that smaller caliper trees planted in the same growing conditions dramatically outpace their larger compatriots. Additionally, gravel bed nursery trees (finished-off bare root stock with massive undamaged root systems) can be planted for a fraction of the cost of large B&B trees. The tree budget money, previously spent on caliper inches, can instead be used where it will have the most long-term positive impact for trees: the root/soil environment.

3) Landscape architects and arborists: Frenemies?

Arborists are the folks that clean up the bad choices of landscape architects on tree planning, design, selection and installation. I can say that, because I am a Landscape Architect, the same way as I can tell Irish jokes because I am Irish. Whether it’s tree species with bad branch connections that fall apart in storms OR massive monocultures of a single tree species obliterated by a pest OR allowing trees to be planted too deep and then develop girdling roots that strangle the tree’s food and water transport system OR cheating trees with minuscule soil volumes more suitable for a bonsai than a street tree, this is the unhappy gift that landscape architects have delivered to arborists.

Sadly, landscape architects are the laughingstock, and the bane, of the entire tree care industry! Arborists got into their field to take care of large vigorous trees and we’ve saddled them with decades-long disasters. Our projects are filled with trees that need rescuing beginning in their first year. It’s embarrassing and humbling. I would like to know what we landscape architects are going to do to change this situation? I ask that question of our community because we, more than anybody, have the power to do that.

2) Diversify your genera

Dr. John Ball of South Dakota State University gave a rip-roaring presentation on the importance of diversifying the number of genera in our urban forests. In the past we have confined ourselves to selecting trees that are very species rich within their genus and, more importantly, those genera are found on multiple continents in their temperate zones. Here are a list of some of the tree genera that are found in temperate zones on multiple continents. I’m sure most of us who work on landscapes have used some or all of these Genera in our tree plantings. I know I have: Alders (Alnus), Aspens (Populus), Maple (Acer), Birch (Betula), Ash (Fraxinus), Cherries (Prunus), Elms (Ulmus), Mountain Ash (Sorbus) and Oaks (Quercus).

Here’s the good news: there’s a solution.

Here’s the bad news: the solution is that we can’t exceed 5 percent of any one Genus of trees in our urban forests, otherwise we’ll have another Dutch Elm or Emerald Ash Borer catastrophe!

And a drum roll for # 1…

1) Don’t drink the coffee

Dr. John Ball once decided that he wanted to try an old pioneer settler’s coffee recipe ground from the beans of the Kentucky Coffee Tree. John said that drinking Kentucky Coffee Tree coffee from their beans (Gymnocladus diocea), was worse than licking the carpet of the conference room floors. The upside, however, was that no one would ever ask him to make the coffee again!


Peter MacDonagh, FASLA is the Director of Science + Design at The Kestrel Design Group.

Jeramey Jennane / CC BY 2.0


  1. As a Landscape Architect who has spent the last 18 years focused on maintenance and the previous 13 years on design/build I wholeheartedly agree with #3.

  2. Thanks for the comment on #3 my ISA 2014 Blog. In my experience it’s sad, but true. I’ve found that, among LA’s primary peer professionals: Engineers & Architects, there is an almost universal expectation that LA’s know plants & their needs, it’s not all they think LA’s can do, but this expectation overrides all others. Again, here’s my hope, that LA’s will really start to get it right on growing trees, now.

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