Something simple and extremely fundamental must underpin any effort to increase the biodiversity of the urban canopy: adequate quantity and quality soil and water. That’s it. If a guy wanted to grow a great tree in a typical 4 x 4 urban tree opening, he could add macronutrients (NPK), micronutrients (Mn, Mg, etc.), compost tea, mulch, foliar sprays, even have the tree’s nails done – none of that will matter. That tree is headed for an early death. That’s how unimportant all those other things are compared to adequate quantity of soil, adequate quality soil, and water.
Once good growing conditions are established, then tree selection becomes extremely important. We’ve come to over-rely on a very small selection of trees in our cities. Today I’m going to go through a list of all the trees not to plant (I’ll go over trees that we should plant in a future post).
What tree where?
Probably 5 percent of all tree species can grow just about anywhere. We know them well, and while we’re comfortable with these species, this is exactly what we should stop planting.
Why? There are a few genera that are vastly overrepresented in the North American urban forest. These trees are vulnerable to four catastrophic failures that are all human health and safety factors:
- an epidemic;
- a fall tree/branch hazard to people, cars and buildings;
- invasive species that destroy wild & natural areas;
- roots that enter sewer pipes and other underground utilities.
The lists that follow are not comprehensive, the idea is to eliminate from consideration the 5 to 10 percent of trees that cause severe problems and don’t belong in urban areas. I’m telling you which city trees not to plant because I think it’s easier for people to stop doing an existing thing, rather than start doing a new thing. On to the lists.
Vulnerable to an epidemic
(Insect, Virus, Bacteria, Fungi)
- Ash (Fraxinus): vulnerable to Emerald Ash Borer (EAB); except Manchurian (mandshurica)
- Cherries (Prunus): vulnerable to many insects and diseases; most species & cultivars
- Colorado Blue Spruce (Picea pungens): Cytospora Canker eventually will kill all needles leading to defoliation and death
- Crabapples (Malus): vulnerable to Cedar Apple Rust and Fire Blight which completely defoliates trees; Most species & cultivars
- EAST Section Lobatae: Black Oak (Q. velutina); Water Oak (Q. nigra); Northern Red Oak (Q. rubra/borealis); Pin Oak (Q. palustris); Northern Pin Oak (Q. ellipsodalis); Willow Oak (Q. phellos),
- Elms (Ulmus): vulnerable to Dutch Elm Disease (DED); except DED resistant ‘Accolade’; ‘Homestead’; Lacebark or Chinese (U. chinensis ‘Allee’); Cedar Elm (U. crassifolia)
- Leyland Cypress (Cupressus leylandii): Canker Disease, West of the Rockies
- Lombardy Poplars (Populus nigra’Italica’): vulnerable to Fungal Leaf Spot
- Maples (Acer): vulnerable to Asian Long Horned Beetle, especially NOT these Eurasian species: Hedge (A. campestre); Norway (A. platanoides); Amur (A. ginnala); Tatar (A. tatarica); 3 North American Natives that are seriously overplanted and are thus extremely vulnerable to an overseas epidemic: Silver (A. saccharinum); Red (A. rubra); Sugar (A. saccharum)
- Pines (Pinus): especially Austrian (P. nigra); Ponderosa (P. ponderosa): Dothistroma Needle Blight eventually will kill all but current year’s needles leading to defoliation and often death
- Planetree (Platanus): Anthracnose; except 2 London Plane tree cultivars (P. x acerifolia ‘Bloodgood’ & ‘Columbia’)
- Red Oaks (Quercus…Section Lobatae Eastern North America; Section Protoblanus of Western North America): the Red Oak group (with bristles on their leaves); have an extreme susceptibility to Oak Wilt. Additionally the Red Oak group has a huge number of species and is found on 4 of the 5 continents that grow trees, this increases the odds of an epidemic dramatically.
- WEST Section Protoblanus: e.g. Canyon Live Oak (Q. chrysolepis)
- White Birches (Betula): vulnerable to Bronze Birch Borer; most species and cultivars, except River (nigra and nigra ‘Heritage’)
A note on maples
I’m afraid that the coming story for maples is not a happy one. I believe an epidemic similar to Dutch Elm Disease, Emerald Ash Borer, or Chestnut Blight awaits them. In U.S. cities east of the Mississippi River, Maples now make up over 30 percent of the urban forest canopy – tens of millions of trees. Perhaps it will be Asian Long Horned Beetle, which is already in some states in the United States; perhaps it will be something else we don’t even know about yet. It is a matter of when, not if, and it’s safe to say the devastation will be significant. So, I appeal to all responsible professionals in the business of planting trees to please stop planting maples.
(soft or weak wood; included branch attachments; unstable root plates)
- Cottonwoods (Populus): Eastern (P. deltoidia); Big Tooth or Western (P. grandidentata): Soft wood; Weak branch attachments
- Eucalyptus (Eucalyptus): Weak branch attachments; Most species & cultivars
- Maples (Acer): Boxelder (A. negundo), Silver (A. saccharinum): Soft wood; Weak branch attachments
- Pears (Pyrus): Weak branch attachments; Most species & cultivars
- Siberian Elm (Ulmus sinensis): Weak branch attachments, DED suspectible
- Water Oaks (Quercus nigra): Soft wood; Weak branch attachments
- Willows (Salix): especially Weeping (S. babylonica); Crack (S. fragilis): Soft wood; Weak branch attachments; Main trunk and Root Plate failure is common
Severely Invasive: The Naughty List
(Self-seeds and overruns native trees, shrubs, grasses and forbs in wild and natural areas, think Kudzu Vine)
Depending on your point of view, trees on this list ascended or descended onto the list because in over 200 United States counties they are listed as invasive, according to the 2014 USDA’s Invasive Species Group. This list is comprehensive for these purposes. But this list does not include aggressive natives growing outside their pre-European Contact range such as Black Locust, Hedge Apple, Eastern Red Cedar, etc. or commercial fruit and nut trees that may have escaped from cultivation and become problematic.
- Buckthorn (Rhamnus): Especially European & Asian species: Glossy (R. cathartica)
- Callery Pear (Pyrus calleryana): This is NOT a commercial fruit
- Chinaberry (Melia azedarach)
- Chinese Tallow (Triadica sebiferum)
- Eucalyptus (Eucalyptus): Most species & cultivars
- European Mountain Ash (Sorbus aucuparia)
- Honeysuckles (Lonicera): Especially European & Asian species: Tatarian (L. tatarica) Amur (L. maackii); Bells (L. bella); Morrow’s (L. morrowii)
- Lombardy Poplar (Populus nigra’Italica’); White Poplar (P. alba)
- Maples (Acer): especially Eurasian species and cultivars: Amur (A. ginnala); Norway (A. pseudoplatanus)
- Mimosa (Albizzia julibrizum)
- Mulberry (Morus): Especially the Asian/White/Paper (M. alba)
- Paradise Apple (Malus pumila): This is NOT a commercial fruit
- Princess Tree (Paulownia tomentosa)
- Privets (Ligustrum): especially Chinese (L. sinense); Common (L. vulgare)
- Russian Olives (Eleagnus): Russian (E. angustifolia); Thorny (E. pungens); Autumn (E. umbellata)
- Tamarisk (Tamarix): Salt Cedar (T. ramosissima); Chinese (T. chinensis)
- Tree of Heaven (Ailanthus altissima)
- Trifoliate Orange (Poncirus trifoliate): This is NOT a commercial fruit
- Willows (Salix): especially Eurasian species and cultivars: Weeping (S. x ’Babylonica); White (S. alba); Crack (S. fragilis)
- Siberian Elm (Ulmus pumila)
The trees below have very vigorous root systems, they are phriatiphytes growing where constant supplies of water are available: along streams and lakes. These trees roots, in their search for water and nutrients, occasion on waste water pipes (sewer pipes). Because this water and nutrients supply is perpetual, these trees’ roots will intrude and fill these pipes with roots, eventually coming to rely on this artificial source, and completely obstructing them. This does not seem to be a problem with stormwater pipes.
- Cottonwood (Populus): Eastern (P. deltoidea); Dogtooth (P. grandentata)
- Quaking Aspen (Populus tremuloides)
- Silver Maple (Acer saccharinum)
- Willows (Salix): Most species & cultivars
The Right Tree for the Wrong Place
There are also a number of tree species that can cause problems in some places and contexts while being great choices for others. Consider the species on these lists based on the factors present on your site.
Minor Human Health Hazard
(massive cones that can hurt people, asthmatic inducing levels of pollen)
- Bristly Locust (R. hispida): Extreme Thorns
- Common Locust (Robinia): Extreme Thorns: Black (pseudoaccacia): Except ‘Chicago Blues’:
- Cottonwood (Populus): Eastern (P. deltoidea); Dogtooth (P. grandentata): Prodigous pollen levels and ‘Cotton’ can block HVAC intakes
- Honey Locust (Gledistisia): Extreme Thorns: Except (‘inermis’) (‘Skyline’)
- Mountain Cedar (Juniperus asheii): Prodigous Pollinator
- Pines (Pinus): Especially the Gigantic Cones of: Coulter’s (P. coulteri): World’s Heaviest; Sugar (P. lambertiana): World’s Longest Pine Cone
- Prickly Ash (Zanthoxylum americanum): Thorns
These are trees that can be planted in the city and will have no serious issues with pests, invasiveness, invading roots, structural problems, or people getting hit hard on the head. Almost all of these problems can be solved using a broom. Except for their perceived messiness, these are very, very sound trees, and some of them will show up on our recommended planting list.
(large pods, fruits, nuts or abundant airborne seeds)
- Black Walnut (Juglans nigra): Nuts
- Catalpa (Catalpa): Southern (C. bignoides) Northern (C. speciosa): Pods
- Common Locust (Robinia): Black (R. pseudoaccacia): Pods except ‘Chicago Blues’; Bristly (R. hispida)
- Hackberry (Celtis occidentalis): Ball Bearing Sized Nuts
- Hickories (Carya): Nuts
- Honey Locust (Gledistisia): Pods except ‘inermis’ ‘Skyline”
- Horse Chestnut (Aesculus hippocastanum): Nuts
- Mimosa (Albizzia julibrissin): Pods
- Pecan (Carya illinoisensis): Nuts
- Southern Magnolia (Magnolia grandiflora): Seed Heads
- Sweet Gum (Liquidambar stryraciflua): Gum Pods
- Chinese Chestnut (Castinea mollissma): Husk of Nuts
- Gingko’s Females (Ginkgo biloba Female): Fruits; Use males only
Fruit and Sap
- American Linden (Tilia americana): Sap on Cars]
- Asian Mulberry (Morus alba): Pavement Staining Fruit
- Ginkgo Females (Ginkgo biloba Female): Slippery Fruits
- Sweet Gum (Liquidamber strycaflua): Gum Pods
So what should we plant?
Credit for these lists goes also to the following from their writings and conversations: Dirr, Coder, Urban, Johnson, McPherson, Gilman, Harris, Shigo. The next and final article in this series will list the trees that are proven contenders over the years in cities. This is a large selection that will grow well in cities given reasonable care (adequate soil quality, adequate soil quantity, and water); use the list widely with no genus exceeding 5 percent of the city’s Urban Tree Canopy.
Go forth and prosper, young trees.
L. Peter MacDonagh is the head of Science + Design for The Kestrel Design Group.