Species diversity in the urban forest is an extremely important topic and – unlike so many other design choices that are hindered by lengthy planning processes, tightening budgets, and maintenance considerations – it’s something we can actually have an impact on in the short term as part of the ongoing cycle of tree replacement.

I’ve written before about how our impulses to plant “clean and simple” monocultures leads to “dead and gone” swaths of the urban forest, and the high cost that we bear when we take that approach. I’ve written about the “naughty” list, of what not to plant. But what about what we should be planting? That’s what I’m going to tell you about today. The trees listed below have what it takes to grow in cities and also dramatically increase the diversity of the urban forest population.

On the tree selection process

You’ll find a number of species and genera on this list that are not been commonly found in the nursery trade; however all plants listed can be found in nursery catalogs.

You will find no (or modest) representation of many common tree and shrub species or genera because they are overrepresented in the urban forest and are already plagued with debilitating, sometimes fatal, diseases. They strain municipal resources because of the disproportionate care needed to keep them alive and safe for people, relative to their contribution to the entire urban forest.

Trees that are susceptible to a devastating disease such American Ash and Emerald Ash Borer (EAB) are not represented; only resistant species of that genera are included.

There are some trees listed here which have weak branch structures but do superbly in wet, saturated conditions; consider using them away from buildings and vehicles. There are other trees and shrubs that, while very tolerant of wet/dry conditions, are not included because they are too sensitive to high pH, e.g. almost all Rhododendrons.

Banned

Cherries (Prunus), Apples (Malus), Pears (Pyrus), Junipers (Juniperus), Roses (Rosa), Mountain Ash (Sorbus), Russian Olives (Eleagnus), and the Red Oak Group of North America (Quercus Sect. Lobatae).

Allowed in moderation

Willows (Salix), Dogwoods (Cornus), Maples (Acer) and Hawthorns (Crataegus).

Resistant species only

Elms (Ulmus) killed by Dutch Elm Disease (DED); Ash (Fraxinus) killed by Emerald Ash Borer (EAB); White or Yellow Birches (Betula) killed by Bronze Birch Borer (BBB).

What the terms mean

Stormwater = trees that can remain healthy, even with occasional flooding and prolonged wet soils.
Street = trees that are durable and the right shape for planting along streets, plazas, etc.
sp. = species
‘subsp.’ = subspecies, determined by geographic location
‘CV’ = cultivars, specially bred types, which are then propagated by cuttings, not by seeds
‘x’ = hybrid, a cross between species, which is reproduced via seeds
var. = a rare but naturally occurring variety, depending on species grown by both seed or cutting.
MALE = males are recommended because they will not produce the large fruit, nuts or pods that the species is known for and bears on female plants of the species.
Hardiness Zone = USDA Hardiness Zone, the general guideline to can determine which plants are most likely to thrive where. For reference, USDA Zone 1 is the Tundra in Northern Canada, and Zone 10 is the Sub-Tropical Mangroves of Texas’ South Padre Island and the Everglades. Most of the trees fall in the 5-7 Hardiness. Whatever USDA Hardiness Zone is listed for a tree, move it one zone south for superior performance. I did not subdivide numbered zones by A (colder) or B (warmer). If a tree did well in 3A, then Zone 3 was selected; if a tree did well in 3B then Zone 4 was selected. You may disagree with some of my assignments; I chose conservatively, based on experience observed growing in the field; or if not by his experience, then by Michael Dirr’s experience (Michael Dirr, 1999; Manual of Woody Plants).

Note: my recommended species appear in bold.

The “Notes” column

Make sure to check the “notes” column for information about whether the tree has weak branch structure, harvestable fruits and/or nuts, or any other items of interest.

Urban trees take more abuse and require more maintenance than any other kind of trees. The note to select only standard forms no multi-stems or co-dominants is to address a huge cost burying our cities Urban Forest Departments. To reduce long term maintenance costs, it is imperative to select only standard forms, no multi-stems or co-dominants.

I am not concerned with nuisance levels of fruits, nuts, twigs; these are problems which can be solved with a broom. Of all the real challenges facing our cities, some errant pods on the ground are the very least of our worries. Remember that it was the search for (and over-use of) a perfect, no maintenance, and “clean” tree species that got us into three major tree plagues in the last century.

This tree list is not comprehensive, but is based on my almost 30 years of experience in horticulture, arboriculture and landscape architecture. There are many individuals with more and better experience than me, and you’ll see my biases clearly expressed. Whether you agree, disagree, or have questions – please write in so that we can keep this important conversation alive.

Famous forester Alex Shigo coined the phrase “Right tree, right place.” As the 1,000 year old Knight guardian of the Holy Grail in Raiders of the Lost Ark said, “Choose wisely.”

This is part four in a series about species diversity in the urban forest. 

Part one: How “clean and simple” becomes “dead and gone”

Part two: The high cost of urban monocultures

Part three: What not to plant

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