Gardens are unique and understudied systems that can have multi-faceted and positive impacts on the environment and public health.
The positive impact of gardens is an important thing to consider when planning, installing and maintaining individual gardens as well as neighborhood-wide landscaping installations. It turns out that gardeners and landscapers manage and maintain a significant amount of land — approximately 2 percent of the land area in the continental United States.
Gardens and other ornamental landscapes are special because of their unique levels of plant abundance and diversity. In some cases they can be considered “biodiversity hot spots.”
In New York, my lab group documented the important role that plant abundance and diversity in urban and suburban gardens can play in conserving pollinator biodiversity. We want to do the same thing in Oregon.
Recently, there have been calls for conservation plans to better harness the positive environmental benefits of gardens and landscapes. But before we can get there, we need to answer some basic questions. This is where the Garden Ecology Lab comes in.
Our group works to understand the benefits of gardens to the environment and to human health and well being. We want to document the biodiversity of plants, pollinators and other organisms in Oregon gardens, and analyze what factors constrain or promote garden biodiversity. We are studying the physical, chemical, and biological characteristics of urban garden soils, to understand the challenges that plants may encounter when installed there.
Ultimately, our goal is to understand how gardens — and the decisions we make in our gardens — either promote or constrain ecosystem services such as pollination or pest control.
Currently, we are working on three projects: garden pollinators (Gail’s project), native plants (Aaron’s project), and garden soils (Michael’s project). In this article, we provide an update from the first season of field work.
In this study, we aim to document the diversity of bees that can be found in Portland-area gardens. We also want to know how landscape context (e.g., urban, suburban, forested), garden design, and garden management impact garden bee diversity.
We selected 24 gardens to study, with sites (see Figure 1) ranging from Woodburn to the St. Johns neighborhood of Portland, but largely hugging major roadways. They were chosen to facilitate efficient travel and sampling.
Gardens were categorized according to their landscape context:
Urban: gardens located within Portland, in highly populated neighborhoods
Forest: gardens located within Portland, located adjacent to Forest Park
Suburbs: gardens located outside of Portland, on the edges of the metropolis.
We used a modification of a standardized method for monitoring bee populations to sample pollinator diversity. We used colored bowl traps to passively sample bees from home gardens (Figure 2). We hand-collected bees, allocating 10 minutes of collection time within each garden.
Bees were brought back to the lab, where they could be pinned and identified (Figure 3). Butterflies were identified on the wing, when we saw them in the garden (Figure 4).
At this writing, we are still pinning and identifying bees, and results are forthcoming. But, even at this early stage, we were impressed with the diversity of bumble bees in Portland-area gardens. So far, we think we have five species!
Anecdotally, we also noticed that bee diversity and abundance might be lower in gardens that use sprinkler systems, versus those that use drip irrigation or no irrigation. However, this observation awaits confirmation from more data.
Finally, we were impressed with the diversity and abundance of bees from a garden near Portland’s city center. This garden was surrounded by concrete, yet the intentional plant choices (e.g. big leaf maple, borage, California poppy, coreopsis, anise hyssop, spirea, Douglas aster, yarrow, phacelia, meadow foam, and more) were noticeably successful at attracting a diverse and abundant pollinator assemblage.
This summer, we conducted our first field season of a study screening native plants for their attractiveness to pollinators and natural enemies.
We selected 23 native Willamette Valley wildflower species, as well as four exotic garden species known to be attractive to bees: Nepeta cataria ‘Catnip’, Salvia elegans ‘Pineapple Sage’, Origanum vulgare ‘Italian’ and Lavandula intermedia ‘Grosso’. For a full list of native plants, please visit https://tinyurl.com/ya8orsgm.
All plants were installed in 1-meter squared plots at OSU’s North Willamette Research Center (Figure 6). Between April and October, we monitored floral visitation, sampled visiting insects using an “insect vacuum,” and tracked floral bloom.
Our early impressions of which wildflower species are the most attractive to bees include Goldenrod (Solidago canadensis) and Douglas aster (Symphyotrichum subspicatum). These flowers were also attractive to a wide diversity of beetles, bugs, and syrphid flies.
As an added bonus, both these species had long bloom durations, providing habitat and colorful displays for significant portions of the summer. Annual flowers Clarkia amoena (Figure 7) and Gilia capitata also attracted a wide range of native bees.
Next summer, we will ask gardeners to rate the attractiveness of each of our flower species. Eventually, the results of this study will be a pollinator planting list for home gardeners, as well as a pollinator and natural enemy friendly plant list for agricultural areas. These will help inform deliberate plantings that increase the habitat value of planted areas.
We sampled soils from 27 gardens across Corvallis and Portland. These gardens used raised beds and in-ground gardens to grow a variety of vegetables, herbs, and fruits (Figure 8).
We want to study urban garden soils, and soils in raised beds versus in ground beds, to address a few questions. For example, do raised bed gardens offer greater protection from soil contaminants than in-ground gardens?
Raised beds are recommended as a way to work around soils that may have heavy metal contaminants. However, heavy metals can become airborne and deposited on soils from industrial emissions, traffic and re-suspension of road dust. If this is the case, then gardening in raised beds might offer a false sense of comfort. We thus chose to sample gardens that are close to, versus further from, major roadways and traffic.
We also want to better understand the general state of urban garden soils in Oregon. If we had to grade soil health, by looking at soil structure, tilth, nutrients, and other biological, chemical, and physical characteristics of soils, what would that grade look like? We are now processing the soil samples in the Central Analytical Laboratory of OSU (Figure 9).
Immediately, we noticed that sampled soils had an abundance of organic matter. The high organic matter content complicated lab analyses.
The first step taken when a lab receives a soil sample is to pass the sample media through a sieve. The larger pieces are lightly ground and sieved again. The aim is to isolate the soil from non-soil matter in order to restrict laboratory tests to just the soil content itself.
Garden organic matter is often shredded by this process, which can alter the results of the laboratory tests. The primary problem here is that the organic material in our sampled garden soils is mostly forest by-products: timber waste. This material is generally inert in the garden setting and not accessible to plants. When this organic matter is included in a soils analysis, it may skew the results away from the actual state of the garden’s soil.
The initial results align with our hypotheses: urban garden soils are extremely heterogeneous. Soils from a raised bed often have very different characteristics than soils in adjacent in-ground beds. Soil sampling methods recommended for commercial agriculture don’t seamlessly translate to small-scale, diverse urban gardens.
We launched three garden-focused research projects in the summer of 2017. At this point, results are limited to initial observations, but we look forward to sharing more with you, in the future. To follow our progress, please visit http://blogs.oregonstate.edu/gardenecologylab, or search for ‘Garden Ecology Lab Oregon’ in an online search engine.
Gail Langellotto is an associate professor and Extension specialist in the Department of Horticulture at Oregon State University. Contact her at
Michael Nelson is a graduate student in the Department of Horticulture at Oregon State University. Contact him at email@example.com.
Aaron Anderson is a graduate student in the Department of Horticulture at Oregon State University. Contact him at firstname.lastname@example.org.