Urban Forestry Fundamentals, Part 2: Think like an Urban Forester when Selecting Trees  

Story and photos by Michelle Sutton

In Part I we saw how urban stresses on trees, while particularly grievous along city streets, are present to varying degrees in our parks and home landscapes, too. Urban foresters assess the site on which they intend to plant, identify its challenges and opportunities, and then they select trees with the site conditions in mind. Fortunately, this process of finding “the right tree for the right place” is the same for homeowners and community volunteers as it is for urban foresters. We just need a selection framework.

Getting Started

The best guide to site assessment and tree selection is provided by the Cornell Urban Horticulture Institute and is called Recommended Urban Trees, within which is a terrific “Site Assessment Checklist” with detailed notes on how to complete it.

The process of site assessment has you consider things like sun and shade exposure, USDA Hardiness Zone, microclimates (for instance, the south side of your house versus the north), soil texture, soil pH, and drainage. (This is a useful process for considering all types of plant material, by the way—not just trees.)

The checklist in Recommended Urban Trees includes visual assessment of existing plants—both cultivated and wild. Noting what’s growing well—and what’s not—will give you insights into the site conditions. For instance, if rhododendrons, azaleas, and/or mountain laurels all have lustrous dark green leaves and other signs of vigor, your soil is probably acidic to some degree. But if they are consistently showing pale yellow leaves, your soil may be alkaline.

This is a very thorough checklist. Will you fill out every box for every situation? No. But using the checklist gets you in the habit of thinking systematically about your site, and then you can engage in some informed plant-site matching. Recommended Urban Trees is a great resource for anyone who has site challenges (i.e., everyone.) It includes profiles of more than 90 tough trees—and even more cultivars of those trees.

Sample Scenarios

This is a matching game. What are the site opportunities and restraints? What kind of tree would you like to have? Which one will satiate your need for beauty but also fulfill practical functions like privacy/screening, habitat for wildlife, and/or shading a building? Do the site assessment results look favorable for your intended tree, or do you need to consider a different tree?

Scenario A

• You live in USDA Hardiness Zone 5b.
• Your soil is a desirable clay loam, easy to dig, and drains well, but you can tell by the kinds of plants growing wild there (or by doing a pH test) that you have a soil with higher-than-average pH.
• You have a spot in full sun where you’d like to plant a shade tree that will make your back yard more hospitable in summer.
• There are no overhead wires or underground utilities in the vicinity.
• You would like something that is drought-tolerant, because the spot you have in mind gets dry and is far enough from the house that you don’t realistically see yourself dragging hoses out there. (You’ll have to water it in the first few critical years, though, until the tree gets established).
• You want something with striking fall color.

A good match: Recommended Urban Trees has a Small Tree section and a Medium-to-Large tree section. For shade trees, we’re looking at the latter. The guide shows red maples that make great shade trees and have beautiful fall color, but maples are not especially drought tolerant. Oaks are marvelous shade trees but don’t tend to have showy fall color.

But then, we spy the ginkgo tree! It can take prolonged periods of dry soil, is sufficiently hardy (to Zone 4b), does fine with high pH, and there are cultivars like ‘Autumn Gold’ with excellent golden yellow fall color.

 

Scenario B

• You live in USDA Hardiness Zone 6a.
• You want a small tree in a pocket garden between your driveway and the front porch. It has to be a small tree because the bed is only 15 x 18 feet; there isn’t enough room above or below ground for a bigger tree.
• It’s the south side of the house, and due to the heat radiating from the house, driveway, and sidewalk, its gets hot and the bed dries out quickly, but it’s near the spigot, and you plan to water the tree.
• You would like showy flowers.
• There are no overhead wires or underground utilities in the vicinity.
• This garden bed gets salt runoff from the sidewalk.

A good match: There is a whole page of compact crabapple cultivars in Recommended Urban Trees that would do nicely, ones that are both disease resistant and have extra showy flowers. They can tolerate periods of dry soil and some salt runoff. They are hardy down to Zone 4 or even Zone 3. Just be sure to get the specific cultivar indicated for its beauty and stress and disease tolerance. Avoid the “Assorted Crabapple” at the big box store; call around to the independent nurseries to see who has the desirable cultivars or could order one for you.

Scenario C

• You live in USDA Hardiness Zone 5b.
• This part of your yard is flooded in the spring, but then gets quite dry in late summer. Ergo, you need something that can tolerate extremes of soil hydrology.
• There are no overhead wires or underground utilities in the vicinity. There is plenty of above- and below-ground space.
• Your soil is acidic to neutral (under 7.5).
• You like big trees, but you hate raking leaves.

A good match. You have lots of room, so why not go for something that gets really tall (60 feet or more)? The guide shows that the majestic baldcypress (Taxodium distichum) is adapted to both seasonally wet and dry soils. It is hardy to Zone 5a or higher. The leaflets are tiny and need no raking.

Scenario D or, Not Every Site Can Support a Tree  

Your soil is impenetrably hard to dig in and/or bedrock is close to the surface. You could remediate a discrete area of poor soil with compost and deep tillage, but it will be expensive to create enough hospitable soil volume for tree roots, which grow well beyond the canopy. There is one site problem that we cannot select for—and that is lack of rooting space. Best not to plant a tree here.

Or say you’ve always wanted a weeping cherry but there is only 10 feet clearance between house and sidewalk (weeping cherry trees get much bigger than that over time). Or, you want an oak tree, but there are overhead wires in the vicinity. Find a place in your landscape where these trees will have adequate above- and below-ground room, look for compact cultivars, or consider a different tree so it can enjoy the longevity you wish for it. 🌳