What Urban Horticulture Has to Teach Us

By Michelle Sutton

Whenever I visit a new client site, I put on my urban horticulturist hat. In this context, “urban” means significantly altered by human activity — and what landscapes have not been greatly affected by people? Not just city landscapes, but suburban and exurban landscapes too — especially in terms of soils.

Even in the country, where there may be welcome pockets of remnant cow compost, there will also be places on the property where the soil is compacted because of foot or machinery traffic. All home landscapes have conditions that mimic what goes on in cities. For example, my suburban garden along the driveway endures reflected heat from the asphalt, just as any city garden would. Another of my gardens along the main sidewalk gets deicing salts runoff in the winter. The key as landscapers is to identify these challenges (and opportunities) at the outset, and we need a framework for that.

The best guide to site assessment is provided by Cornell’s Urban Horticulture Institute (UHI) within their free publication called “Recommended Urban Trees” (RUT) (see Resources). It contains a terrific “Site Assessment Checklist” with detailed instructions on how to complete it, and it’s applicable to all landscape plant material, not just trees. The process of site assessment has you consider things like sun and shade exposure, USDA Hardiness Zone, microclimates, underground and overhead utilities, available rooting volume, soil texture, pH, and drainage.

The checklist in RUT 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 example, if rhododendrons, azaleas and/or mountain laurels all have lustrous dark green leaves and other signs of vigor, the client’s soil is probably acidic to some degree. But if they are consistently showing pale yellow leaves, the 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. This will bring your client so much more success and gratification than if you use the cookie-cutter design approach, and this will enhance your reputation as a landscape professional who knows what they are doing.

Assess for stress

Here are some “urban” stresses that we can be on the lookout for on our clients’ properties. I learned about these and all facets of urban horticulture from Dr. Nina Bassuk, who directs the Cornell UHI:

Compacted soils (reduces oxygen, water, and nutrient availability to plant roots)
“Boney” native soils (inadequate fertility and perhaps limited room for roots to grow)
Junk fill soil (low fertility and low water-holding capacity)
Presence of construction debris or other buried trash (concrete, for instance, can leach, causing soil pH in the vicinity to go up)
Runoff from deicing salts (toxic and desiccating to plant roots)
Heat reflected from buildings, paved surfaces, and cars (puts more water stress on plants)
Inadequate soil volume, for trees especially (plants can’t get their moisture and nutrient requirements met)

Many of these urban stresses are related to soils. Jamie Blackburn was schooled in these as one of Dr. Bassuk’s UHI graduates and now works as a consulting arborist. He did his master’s research on woody groundcovers for urban conditions and difficult situations like slopes and produced a very useful bulletin with Dr. Bassuk with suggestions based on his findings (see Resources).

Said Blackburn, “Soil assessment and management is something that is routinely neglected in home landscape projects, especially if the home is new construction and the topsoil has been scraped off and the exposed subsoil heavily compacted. Too often, budgets are absolute minimal for things like spreading compost, tilling, ripping, soil mixing, etc., as well as proper mulching. Homeowners typically are fine with spending money on plants, even at large install sizes, but getting them to realize the necessity of paying for good soil amending is a harder sell.”

Landscapers could use the resources available on the UHI site to educate homeowners about the critical need to remediate poor soils.

Urban horticulturists, especially municipal horticulturists, often have to deal with budget constraints for both materials and labor. They have to think long-range, and ask themselves, “What can we afford not just to install, but to maintain?” said Blackburn. “So often I see situations where a homeowner spends tons of money on a nice new landscape, but does not take into account future maintenance costs, and then the long-term design intent is never realized, and the homeowner then sees the whole project as a waste of money, which is frustrating for everyone involved. Consideration of future maintenance is, or should be, one of the primary factors driving the design of a landscape.”

Plant material that quickly outgrows its space is a maintenance hassle for busy urban horticulturists. With regard to plant size and spacing, Blackburn said that knowing the true mature sizes and habits of landscape plants in one’s climate — and working with those realities — is key in order to avoid the need for annual corrective over-pruning.

It’s not all challenges, right? What of the “opportunities” mentioned earlier? Some site opportunities include a big open space free of overhead or underground utilities, where a large-maturing tree could be planted; loamy soil that is fertile and well-drained and needs little amendment; full sun, shade, dappled shade or part-day shade (depending on what one hopes to plant); acidic soil (if one wants to plant rhododendrons, etc.); or a microclimate that is warmer (or colder), allowing for the planting of things not normally hardy in the region.

Case study: my garden

Now, having extolled the virtues of soil remediation, I’m going to tell you how I got around it by using my site assessment and plant-matching skills. I don’t recommend this (not doing the proper soil amendment) but this is what urban horticulturists on a budget, realistically, sometimes have to do. Sometimes the soil preparation is less than ideal, so plant selection becomes extra important.

When I moved in with my husband in 2010, I was faced with one major challenging site condition around our house — the soil is almost universally the junkiest of junk fill — sandy, stony mayhem, with no fertility, lots of gravel throughout, and impenetrable hardpan within the first 7 inches.

At the time, I thought we weren’t going to live here more than a few years, so I didn’t want to spend hundreds of dollars on truckloads of compost. Yet I wanted to have a big garden on the south side that would provide a visual buffer and sense of privacy between us and our neighbors.

The issue with this terrible soil is that it doesn’t hold moisture or fertility well, so I gathered up some of the loveliest, most rugged, full-sun, drought-tolerant plants I know; I bought them all affordably in small sizes since the planting depth here is so limited. In some cases I had to plant higher than I’d like, but I made sure those plants were extra well mulched. I planted things farther apart than is my natural wont, because I had had a tendency in the past to plant too densely.

Along with good plant selection and planting small sizes, I committed to good maintenance. I watered the heck out of those plants the first two years, I keep the garden mulched, and I fertilize to provide the nutrients the soil does not.

Listed below are some of the tough, site-appropriate plants I amassed; they created a full beautiful garden within two seasons. To be fair, I had junky sandy soil. If I’d had junky compacted clay soil, remediation/amendment would have been essential. Urban horticulture is all about the particulars of the site.

‘Royal Purple’ smokebush (Cotinus coggygria). A trio of them forms a mass of beautiful foliage that is both backdrop and focal point. Smokebushes can tolerate a range of poor soils.
Rugosa roses (Rosa rugosa) are so tough, they have been successfully used by Bassuk and collaborators in highway medians in Ithaca.
A Scotch broom (Cytisus scoparius) with speckled pink flowers.
‘The Blues’ little bluestem grass (Schizachyrium scoparium) — blue in summer; bronze-red in fall
Big bluestem grass (Andropogon gerardii)
‘Rotstrahlbusch’ red switch grass (Panicum virgatum): gorgeous burgundy in the fall
Dwarf eulalia grass (Miscanthus sinensis ‘Adagio’)
Salvia ‘Carradonna’

For annual interest, I add some super tough characters from seed, such as burgundy okra. Okra? Yes, okra. It’s in the hibiscus family, and has a striking flower, leaves and fruit. It does amazingly well in my junky sandy soil and lends a tropical flair to the whole scene.

Michelle Sutton (michellejudysutton.com) is a horticulturist, writer, and editor.

Resources

Urban Horticulture Institute (including Site Assessment Checklist): http://tinyurl.com/kqp5m97
Deciduous Woody Groundcovers for Urban Situations: http://tinyurl.com/otk56m8