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Site assessment for busy people

Story and photos by Michelle Sutton

One of my most important mentors, Dr. Nina Bassuk, taught a fabulous urban forestry course that I got to take while in graduate school. Bassuk gave us the opportunity to try out the most professional site assessment tools, protocols, and applications. For instance, to measure soil compaction, we got to use a device called a soil penetrometer that tests the resistance of the soil to pressure, giving you a result in PSI (pounds per square inch).  

The sole surviving box-store wildflower, a lovely, forgiving toad trillium (Trillium sessile)

Bassuk showed us to how to take soil core samples, dry them out in the lab, and then calculate the bulk density of the soil, which is the weight of soil in a given volume. (Bulk density is another indication of compaction and therefore how root-friendly or unfriendly a soil environment is.) She taught us how to systematically take soil samples across a given site and then how to interpret the lab’s findings as to pH, soil texture, and soil nutritional makeup. In her course, we looked at and quantified every possible facet of site assessment.  

That level of testing and documentation is especially important for large-scale projects, like the one that Bassuk and her colleagues Barb Neal, Bryan Denig, and Yoshiki Harada did, at nothing less than … the National Mall. The team was commissioned to analyze the site and the ailing American elm (Ulmus americana) trees and come up with recommendations. You can bet that for something this high profile, they were busting out the penetrometer and all the other gadgetry. Their site findings and their ultimate recommendations are extensively documented in two publications (see Resources). They are fascinating reading—I highly recommend—and spoiler alert: The National Mall has an insupportable American elm monoculture that will continue to be in massive decline. 

To meet this crisis, Bassuk and colleagues came up with a twenty-year, phased plan for systematically diversifying the trees of this iconic landscape. As the failing National Mall elms are removed and a diversity of new tree species is planted, the plan provides for a similar visual coherence that the elms have had, but with the benefit of biological diversity, making the outcome much more sustainable than the elms have proved to be. 

Having worked as an educator in urban forestry for nearly twenty-five years, I’ve found that in everyday practice, the site assessment that urban foresters and tree planting groups do is not always as thorough as the one done for the National Mall project. It’s understandable. Spring or fall planting season comes with a rush; there are sites that need to be filled and trees that need to be planted in a hurry—especially if they’re bare root—and sometimes, because of numerous pressures, not every site assessment box gets checked.  

However, when it comes to matching trees to sites in cities or deciding what to plant in your yard, perfection is not required; in fact, as with all things, perfection can be the enemy of the good. A lot of times, simple observations go a long way, no gadgets required. All this applies to our assessment of our own landscapes, as we work toward creating the gardens we desire that provide the beauty and ecosystem services we hope for.   

I sure as heck didn’t perfect the art of casual or rapid site assessment right away. In fact, I had a lot of failures in my current home gardens when I put them in ten to eleven years ago. I was enamored of plants I had used for clients for many years in Rochester or intrigued by certain plants I saw for the first time here in the nurseries of the Hudson Valley. I didn’t do the most thorough site assessment. I wanted to try stuff, and that’s ok too. I learned from the failures. I present to you a quick tour of some of them.  

In my boney, sandy, overly well-drained fill soil, I planted a rush (Juncus effusus) that needs clay soil and wet conditions, and I put it in near the hot asphalt driveway. I am amazed that it is still alive and has even managed to clump out microscopically over the last eleven years.  

I bought nursery-grown—one hopes that label was truthful—wildflower tubers and other propagules from the big box store down the road for the likes of trillium, wild ginger, and trout lily. I put them on the north side of the house (good), in sandy soil (not so good), and far from the property’s sole outdoor faucet (doom). 

I put in a weeping redbud too darn close to the house. Why did I do that? It had to be moved. 

The currants I planted (one red, one champagne), which prefer soil with the opposite characteristics of mine, produce a good crop if I water like crazy. The chipmunks get to eat most of the fruit before I do, but they are so stinking cute when they scale those vertical branches and stuff their cheeks. I just can’t dwell on how much water it takes to produce every currant.  

I planted a lovely wispy form of coneflower from the native plant nursery, and the woodchuck ate it repeatedly, until I dug it up and gave it away, so it would have a chance at life. 

When I interviewed him for a story in this very publication, the late, great Ted Collins gifted me with two lilacs—one, a ‘Rochester’ lilac (Syringa vulgaris) that is used as a parent in many crosses, and the other, a ‘Mrs. W.E. Marshall’ lilac that has the purple flowers I covet. These poor babies are growing in gravelly fill and are coping by remaining small and not blooming much. (Here you might be wondering why I didn’t amend my soil with massive amounts of organic matter back in 2010. We didn’t think we’d live here more than a year or two. Over the years, I’ve top-dressed with compost I make, but it’s never enough to meaningfully change the essentially challenging nature of the soil here. To do that would be a wholesale remove-and-replace proposition, which would be wildly expensive … and makes me tired just writing this.)

The professional approach to site assessment is best seen in the Site Assessment Checklist and Instructions I’ve listed in the Resources. Meantime, here’s a simpler version for folks in more of an “I-bought-this-and-now-I-have-to-find-a-place-for-it” situation. 

First and always first: Call before you dig! 811. There’s no shortcut around this one. In addition to eliminating any potential safety hazards, you will get helpful information about where your yard’s underground utilities lay, so you can avoid planting a notoriously thirsty tree near the water line—and things of that nature. Seeing where the utilities run will help you determine how much below-ground space is truly available for tree root growth. Above-ground space is simply: how big can this tree get, and is there adequate space here for that to happen gracefully? 

USDA Hardiness Zone: This is super low-tech. Just see where you’re at on the commonly available map.

Microclimate Factors: Might there be heat radiating from nearby surfaces that will bump up the hardiness by half a zone or more? Or is the spot in a low-lying, colder pocket that drives the zone down? Is it super windy in that spot? That will put the tree at more risk of desiccation. 

Sunlight Levels: Is there full sun (6 hours or more), partial sun/filtered light, or shade on the spot you have in mind? Red switch grass (Panicum virgatum ‘Shenandoah’) has been so successful in my south-facing garden that I put a division of it in the backyard, which gets partial shade, to test out its versatility. Poor Shenandoah is just barely hanging in there; it wants back out in the sunny front yard. 

Soil pH: You can take a bunch of samples from all over your yard and send them to the lab or use a quality professional level pH kit … or you can observe what’s already growing and where. On the property where I live, there’s a naturally occurring hedge of blackcaps (Rubus occidentalis) but the fruits don’t get very big, which could be explained by the dry, sandy, excessively well-drained and low-fertility soil they’re growing in. The presence of other intrepid pioneer plants like eastern cedar (Juniperus virginiana) and staghorn sumac (Rhus typhina) also speaks to the challenging site conditions, such that I could see early on that roses (other than rugosas), azaleas, and other ericaceous plants, and thirsty plant species in general were not going to be good matches here.  

Soil Texture: Again, you can take a bunch of samples from all over your yard and send them into a lab that tests for texture or you can try soil texture assessments at home. What plants are growing well on the site? Are they plants known to be tolerant of clayey/poorly drained conditions, or are they ones that require good drainage? The naturally occurring mix of plants will collectively point you in the direction of your soil’s sandy, loamy, or clayey texture. 

Compaction Levels: You could buy or borrow a penetrometer, and they are fun to use. Or you could use a shovel to find out how hard or easy the soil is to dig, a technique that has served me well over the years. 

Drainage: You can test percolation by digging a hole and measuring how many inches drain per hour. Here again, existing plants can tell you a lot, as can your own observation about how long water pools (if it does at all) in a given spot. If water pools there in spring, but the same site gets really dry in the summer—not to worry, there are plants for that! (See resources below) 

Cornell Urban Horticulture Institute (UHI) Team Evaluates Condition of National Mall Elms

UHI Produces Plan for a Sustainable National Mall Treescape

UHI Site Assessment Checklist and Instructions

Woody Shrubs for Stormwater Management

ABOVE: Highly recommended reading about site assessment on a big and hugely important scale, and management recommendations to create a sustainable National Mall landscape going forward

Michelle Sutton is a horticulturist, writer, and editor.

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