If you’re a municipal arborist, you probably got into the field of tree care because you love working with trees and you could make a modest living. You know that that in cities maintenance is the only way that trees get large – and that size matters when we talk about the kind of ecosystem services we want street trees to provide.
Yet maintenance is one of the most mishandled components of creating a sustainable urban forest, and almost every street tree you encounter will not live to maturity (Skiera & Moll). Most street trees are in the process of dying young, yet over 60% of the city’s tree budget is devoted to them (Hauer & Peterson). You’re like a battlefield doctor: watching street trees die before their time, cutting off dead limbs, giving trunk injections to slow, deadly diseases, cleaning up collapsed heart-rot trees, girdling roots or cut roots from sidewalk and roadwork, removing stubs and overcoming old flush cuts.
City trees have to be maintained, just like other infrastructure, over their entire lifetime. The challenge is to give trees the maintenance they need when they need it. If done right, this will reduce overall maintenance costs by huge amounts (Shigo).
When is tree maintenance most important?
The most important part of tree maintenance happens before it is planted.
Like a child, the first years of a tree’s life are the most critical time for maintenance and care, and the next most critical are around the five-to-seven year mark (and then much later, when it enters senescence). Maintenance before the tree is planted, or in the first few years after planting, is likely to be comprised of $5 and $50 problems. Wait until later in life and you may be dealing with $5,000 problems, and quite possibly lots of lawsuits. You get to decide!
These are the maintenance provisions I recommend on a stage-by-stage basis of the life of a street tree.
Before the tree in planted
Write a minimum soil volume ordinance
Demand minimum amounts of loam soil (not rock, sand, or sheetrock) for tree rooting in your city ordinance. These requirements should be written on the plans and added to the project’s tree list spreadsheet. Dr. Kim Coder has shown that two thirds of a tree’s future maintenance needs in temperate regions (such as Atlanta, GA) with more than 30″ (76 cm) annual rainfall can be solved by planting trees in adequate soil volumes.
Diversify your tree portfolio
With Dutch Elm Disease and Emerald Ash Borer wiping millions of large city trees off the map, it’s time to plant more different kinds of trees in our cities – a lot more. Any other response is either willful or ignorant.
How many different kinds? American Forest’s Gary Moll published this rule of thumb recommendation: no more than 10% of any 1 species (e.g. American Elm or Green Ash), no more than 20% of any one genus (e.g. Oaks or Maples), and no more than 30% of any one family (e.g. Legumes: Honey Locusts, Black Locusts, Kentucky Coffee Tree). However easy this is to remember, recent research from John Ball, indicates that no more than 5% of any one genus (Oaks, Maples) is a more reliable defense against catastrophe. In other words, a block full of the same tree (Willow Oaks) will not cause a calamity, but if it’s block after block or neighborhoods full of Willow Oaks or Oaks only that is a problem.
“Contract grow” your city trees
“Contract grown” trees ensure uniform high-quality trees and the diverse species selections you choose, at prices equal to buying them on the open market. All cities of more than 5,000 residents should buy the bulk of their new trees for planting as “Contract grown.” Cities have the bulk buying power to determine tree quality and species diversity; start using it.
Irrigate city trees with stormwater
At least 70% of U.S. cities will have problems with adequate long term water supplies. Globally, the percentage of cities at risk is much higher (Glieck). The idea of cleaning water to a drinking level standard to then irrigate street trees, while millions of gallons of rain fall from the sky in even the driest regions of the Southwest (e.g., Phoenix) only to end up in a pipe to the sea sounds insane. Yet that is precisely what is happening in almost every U.S. city.
It is not realistic to expect that drinking water for irrigating street trees will be available cheaply and in bulk 30 to 40 years from now. Cities need to pro-actively design streets, roofs, parking lots, sidewalks and other impervious parts of the urban landscape to drain to tree openings. Tree boulevards need to be saucers or divots. Cities with over 25” (63.5 cm) annual precipitation should use every 1,000 square feet (93 square meters, or .023 acres) of impervious surface to water large shade trees for long-term health. In arid cities with 8” to 15” (20 cm to 38 cm) annual precipitation, 4,000 square feet (372 square meters, or .09 acres) of impervious can irrigate a large street tree. This means tree planting and stormwater management need to occur simultaneously.
Plant fewer trees better (and further apart)
Enough with the million tree programs; we need trillions of leaves, not millions of trees (adapted from McPherson). Don’t overcrowd them by planting them too class together, either – this just means canopies will begin competing before the trees have even made it to maturity. Use the following as a rule of thumb guideline for planting by species class:
- Large trees: plant >35 feet (10.5 meter) on center
- Medium trees: plant >30 feet (9 meters) on center
- Small trees: plant >20 feet (6 meters) on center
Require tree spacing dimensions on all project plans (Coder, Urban). Consider 8’ (2.4 m) wide boulevards on the outer edge of the city (Johnson).
Don’t choke your trees
I have measured trees with a trunk diameter of 24” (61 cm) requiring a 4’ x 4’ tree opening and a 30” (76 cm) trunk diameter tree requiring a 5’ x 5’ (1.5 m x 1.5 m) tree opening to avoid root and trunk girdling. Large trees are the most important trees in your city; let them grow to maturity without incurring dangerous and expensive trunk girdling problems. Consider codifying in City Ordinance of minimum 5’ x 5’ (1.5 m x 1.5 m) openings for all new street trees.
Watch those roots
Plant tree roots high, look for wide not deep tree planting holes (Watson, Johnson).
If you had to expose the base of the trunk then check for stem girdling roots (weird angles and sizes) and remove immediately. You’re looking for 5-9 even spokes off a wagon wheel (see Gary Johnson and Ed Gilman).
Anchor roots and trunk flare visible
Keep main anchor or first roots visible at the surface. They need to be obvious. (Think of a wagon wheel where the Trunk is the hub and the main roots are 5-9 even spokes of a wheel). If you look down the trunk and see no roots, the trunk is buried too deep, remove soil to expose the trunk flare and primary roots, see Gary Johnson & Gary Watson.
The natural trunk flare needs to be obvious at planting. If the trunk looks like a telephone pole going into the ground, then the tree’s been planted too deeply (Urban). The trunk flare must be made visible before the tree is planted.
Single strong leader
Don’t plant lollipop trees that look like a shrub on a pole, or a coat hanger with all the branches coming from the same place. Always prune out co-dominant branches (>50% trunk diameter at branch attachment, Gilman), and outside the branch collar as per Shigo. Co-dominant branches ensure whole tree loss, if just one co-dominant branch breaks off. The tree leader with side branches needs to look like the branching of a Christmas tree from day one until death (Ed Gilman).
Plant small caliper trees
Many studies have shown (Watson, Green, Ware, Bachtel) that large caliper trees >4”, go into such severe transplant shock that smaller caliper trees under the same conditions outgrow and overtake their larger counterparts. Planting large caliper trees doesn’t help people or trees; they are an expensive design indulgence.
Shovelfuls of mulch
Shredded mulch bark is a great thing for trees – sort of a Forest Floor Starter kit. That doesn’t mean more is necessarily better i.e. mulch volcanoes. Mulch applied thinly (<2”) over a wider area under the tree (3’ to 4’) is better for root establishment (Ed Gilman) and protection from mowers and string trimmers.
During construction + repairs
Never accept tree root cutting for sidewalk or road repairs; unless complete tree removal costs are paid by CIP funds & escrowed – water quality improvement. Johnson found that catastrophic tree failure increased 224% following root cutting activities for sidewalk and road repair.
Require all sidewalk rehabilitation pours within drip line to be poured on top of old existing sidewalks (Johnson). Require all sidewalk trip and fall hazards to grind down concrete edges rather than dig out old slab (Minneapolis Tree Advisory Commission, MTAC).
Require all utility work to be repaired and/or installed with directional boring, not open trench (MTAC).
Require tree root chases be installed under all new sidewalks to eliminate sidewalk heaving.
A robust leaf sweeping program
Tree leaves have high levels of Phosphorous and Nitrogen collected from a huge dispersal area from all kinds of sources (soils, fertilizers, exhaust fumes, lawn clippings). That’s a good thing.
To avoid losing those collection benefits on your MS4 permit and increased TMDL levels in receiving waters (lakes, rivers, bays), sweep and collect those leaves immediately, composting them without allowing nutrient run-off. As leaves begin to fall (Baker & Hobbie) start sweeping and collecting Phosphorous (P) and Nitrogen (N) at less than $100 per pound and re-use these as compost (Bitner). $100 per pound is by far the cheapest collection rate for Phosphorous and Nitrogen out there.
Funding for the long term success of the urban forest
Long-term funding for urban tree care is a constant struggle. Here are some ideas for sustaining your tree maintenance budgets.
Capital Improvement Projects (CIP)
Request that 10% of all capital improvement project stormwater budgets be allocated for new tree infrastructure.
Young tree O&M
Divert 20% of annual tree removal budget and reallocate for young tree care.
Increased soil volumes
Divert 20% of annual tree removal budget and reallocate for increased soil volumes of all new public ROW tree plantings.
Hauer & Peterson found that of more than 600 U.S. cities sampled, over 50 had a claim average of $13,000 per tree incident. Let’s pay that money to trees, not lawyers.
These are just a few of the important factors to consider when setting an operations and maintenance plan and budget for your urban forest. If we can move in this direction, it will pay huge dividends in the benefits we get from our city trees.
In part two of this post, I’ll share select examples of the above recommendations that are being implemented across the United States. We can look to these forward-thinking cities and take lessons from their approach to apply them in our own communities.
L. Peter MacDonagh is the director of science and design for The Kestrel Design Group.