Book Review: Good Weed Bad Weed

Distinguishing weeds from desirable plants is a skill that takes years of experience. If you’re not an avid gardener or a practiced naturalist, the distinction between the two groups may be blurry. There are weed identification guides aplenty, but even those aren’t always the most user-friendly and can often leave a person with more questions than answers. One of those questions may be, “Why is this plant considered a weed and not that one?” Through her book, Good Weed Bad Weed, Nancy Gift attempts to answer that question, offering much needed nuance to a regularly vilified group of plants.

In the introduction, Gift acknowledges that the term “good weed” sounds like an oxymoron. A weed, by definition, is an unwanted plant, an interloper and a troublemaker, without value or merit. What could be good about that? Gift, on the other hand, asserts that “it is a weakness of the English language that weeds are universally unwanted.” We need a word that describes plants that may have weedy characteristics but some redeeming qualities as well. For now, Gift uses “volunteer” – “a plant that comes up without being planted or encouraged” – suspending judgement until its performance can be fairly assessed.

Good Weed Bad Weed is a weed identification guide designed for beginners, for those wondering if their yard is “infested or blessed.” It is specifically concerned with weeds commonly found in lawns and garden beds, and “not meant to apply to farm fields or any other landscape.” It sets itself apart from other identification guides by organizing weeds into three categories: Bad Weeds, Not-So-Bad Weeds, and Good Weeds. Each plant profile includes a description, notes about benefits as well as problems, and some recommendations for control. Assigning good/bad designations to these plants is bound to cause some heated debate and outright disagreement, and Gift acknowledges that; however, we all have our “unique judgement” about the plants we encounter in our landscapes, so as “weed-lovers-in-training,” Gift hopes that we can “make a few new friends in the plant kingdom” and, perhaps, a few less enemies.

For the ten plants that make the Bad Weeds list, the reasoning is pretty clear. They are highly competitive and difficult to control [foxtail (Setaria spp.), garlic mustard (Alliaria petiolata), and Canada thistle (Cirsium arvense)], they are poisonous to humans despite being beneficial to wildlife [poison ivy (Toxicodendron radicans ) and poison hemlock (Conium maculatum)], they are known allergens and otherwise unattractive [common ragweed (Ambrosia artemisiifolia)], or, like Japanese knotweed (Fallopia japonica), they are on the list of top 100 worst invasive species.

The other two categories are where more personal judgement comes into play. The twelve plants considered Not-So-Bad Weeds are said to have “admirable qualities despite some negatives.” Prostrate knotweed (Polygonum aviculare) provides excellent erosion control. Orange hawkweed (Hieracium aurantiacum), bull thistle (Cirsium vulgare), and musk thistle (Carduus nutans) are quite beautiful and highly beneficial to pollinators and other wildlife. Nutsedge (Cyperus spp.) is edible and easy to keep in check if you stay on top of it. Bindweeds (Convolvulus arvensis and Calystegia sepium) avoid the Bad Weeds list because their flowers are so appealing. Aesthetics really matter to Gift, which is made clear with the entry for common fleabane (Erigeron philadelphicus), which could have made the Good Weeds list were it not for its disappointing and forgettable floral display.

field bindweed (Convolvulus arvensis)

As for the Goods Weeds list, more plant species find themselves in this category than the other two categories combined. That being said, those who have strong, negative opinions about weeds should probably avoid this section of the book, lest they experience an unsafe rise in blood pressure upon reading it. But be advised that making the Good Weeds list doesn’t mean that there are no negatives associated with having these plants in your yard; it’s just that the positive qualities tend to overshadow the negatives.

Positive qualities include edible, medicinal, low growing, slow growing, easy to control, beneficial to wildlife, not a bully, hardly noticeable, uncommon, and soil building. Certain weeds are desirable in lawns because they are soft to walk on, like ground ivy (Glechoma hederacea), yarrow (Achillea millefolium), and moss. Other weeds, like self-heal (Prunella vulgaris), stay green year-round and don’t leave ugly, brown patches when they die or go dormant. Still others, like bird’s-foot trefoil (Lotus corniculatus), black medic (Medicago lupulina), and clovers (Trifolium spp.) fix nitrogen, providing free fertilizer. Gift notes that, for those who keep chickens, weeds like common sorrel (Rumex acetosa) and cuckooflower (Cardamine pratensis) are great chicken feed.

Speaking of eating weeds, Gift concludes her book with four pages of recipes. The “Weedy Foxtail Tabouli” is particularly intriguing to me. Reading this book definitely requires an open mind, and some people simply won’t agree that any weed should ever be called “good.” Gift seems okay with that. She calls herself a “heretical weed scientist,” insisting that “a weed is in the eye of the beholder.” As “beholders,” I hope we can all be a little more like Nancy Gift.

A weedy lawn (photo credit: wikimedia commons)

More Book Reviews on Awkward Botany:


What Bugs Can Tell Us About the Value of Vacant Urban Land

Back in October 2017, we discussed some potential benefits of spontaneous urban vegetation (commonly referred to as weeds) and the abandoned or undeveloped urban spaces they inhabit. There is much to learn about the role these plant communities play in the ecology of cities and their contribution to vital ecosystem services. In a review published in the December 2013 issue of Environmental Entomology, researchers from Ohio State University discuss how studying arthropod communities on vacant lands can help “advance our ecological understanding of the functional role” these habitats may have in our cities.

Arthropods were selected as the subject of study because their “populations respond quickly to changes in the urban environment, making them key indicators of how land use change influences biodiversity.” Urban-dwelling arthropods “are diverse and occupy multiple trophic levels” and are “easy to sample.” Additionally, many of the services that vacant, unmanaged land offers are “arthropod-mediated,” including “pollination, decomposition, nutrient cycling, and biological pest control.”

photo credit: wikimedia commons

Vacant land was selected as the study site because “understanding [its] ecological value is important to the advancement of urban ecology and ecosystem management,” and even though such areas are often overlooked in conservation planning, studies have shown that they “have the potential to be valuable reservoirs of biodiversity.” In order to determine just how valuable vacant land might be, more research is needed comparing these spaces to other parts of the city. In addition, vacant lots are generally ephemeral and in due time may be developed. Whether this means that a building or parking lot takes their place or that they are converted into a park, garden, or urban farm, it is important to understand what these land use changes mean for urban biodiversity and ecological functions.

Urbanization is often measured by comparing the amount of built area to the remaining green space. Where there is a high degree of urbanization, there is a low degree of green space comparatively. As urbanization increases, so does habitat fragmentation, pollution, and the urban heat island. In the meantime, biodiversity suffers. The authors cite a number of studies demonstrating that increased urbanization negatively impacted beneficial insect populations. For example, a study in the United Kingdom found that bumblebee diversity in gardens “decreased with increasing urbanization of the surrounding landscapes.” Similar results were found in a study we wrote about.

photo credit: wikimedia commons

Together with remnant natural areas, parks, private and public gardens, greenways, and commercial landscapes, vacant lots are part of a mosaic of urban green space. Each of these areas “experience different levels of disturbance and harbor varying plant species,” which, in turn, “influence arthropods and the services they can supply within and between patches.” Because vacant lots can remain undisturbed and virtually unmanaged for long periods of time, they help provide a contrast to the homogeneous, highly managed green spaces that are common in cities. By their very nature, they “have the potential to aid conservation and enhance green space quality and connectivity within city centers.”

It’s one thing to recognize the value of vacant lots; it’s another thing to change the negative perception of them. Aesthetics are important, and to many people vacant lots are an eyesore and a sign of neglect. Some management may be necessary in order to retain their important ecological value and assuage the feelings of the public. The authors present a number of ways that vacant lots can be and have been managed in order to achieve this goal. They also consider how certain management strategies (mowing, removing and/or introducing plant species) can impact arthropod populations for better or worse. Yet, where vacant lots are left alone and allowed to advance in the stages of ecological succession, changes in arthropod diversity and ecosystem function also occur. For this reason, “the regional species pool of a city requires a mosaic of all successional stages of vacant land patches.”

photo credit: wikimedia commons

Finally, the authors discuss the conversion of vacant land to urban agriculture. Even this land use change can have dramatic effects on the arthropod community. For example, undisturbed or unmanaged areas are a habitat requirement for cavity and soil nesting bees, and regular disturbance associated with farming may interfere with this. Also where pesticides are used or plant diversity is minimized, the arthropod community will be affected.

Thus, “the study of vacant land ecology necessitates a transdisciplinary approach” in order to determine how changes in vacant, urban land “will affect diverse ecosystem functions and services.” A variety of management strategies are required, and land managers must “determine the most appropriate strategies for improving the sustainability of cities from a connected landscape perspective.” It is clear that vacant lots have a role to play. The extent of their role and our approaches to managing them requires careful investigation.

One thing is certain – for biodiversity’s sake – don’t pave over vacant lots to put up parking lots.

Weeds and Winter Interest

In climates where winter sucks the garden inside itself and into quiet dormancy, it is often dead stalks and seed heads that provide the most visual interest. They also become, in some respects, a reminder of a garden that once was and what will be again.” — Gayla Trail, Grow Curious

If, like me, it is during the growing season that you really thrive, winters can be brutal. Color has practically been stripped from the landscape. Death and slumber abound. Nights are long and days are cold. It’s a lengthy wait until spring returns. Yet, my love of plants does not rest. And so, I look for beauty in a frozen landscape.

In evergreens, it is obvious. They maintain their color year-round. Large bunchgrasses, shrubs and trees with interesting bark or branching habits, dried fruits and unique seed heads – all of these things are easy to spot and visually interesting.

Beyond that, there are things that we are not accustomed to finding beauty in. Such things require a keen eye, close observation, and the cultivation of greater understanding and appreciation. For most people, weeds fall into this category. What is there to love or find beautiful?

I am of the opinion that there is plenty there to intrigue us. From their spent flowers to their seed heads and dried-up leaves, they can be just as interesting as the plants we deem more desirable. The winter-long green of winter annuals alone is evidence enough. So, here is my attempt to redeem some of these plants by nominating them as candidates for winter interest.

common mallow (Malva neglecta)

field bindweed (Convolvulus arvensis)

common mullein (Verbascum thapsus)

common dandelion (Taraxacum officinale)

Russian thistle (Kali tragus, syn. Salsola tragus)

Russian thistle (Kali tragus, syn. Salsola tragus)

redstem filaree (Erodium cicutarium)

curly dock (Rumex crispus)

curly dock (Rumex crispus)

prickly lettuce (Lactuca serriola)

salsify (Tragopogon dubius)

wood avens (Geum urbanum)

yellow evening primrose (Oenothera biennis)

annual honesty, a.k.a. money plant (Lunaria annua)

white clover (Trifolium repens)


A friendly reminder: Refrain from being overly ambitious with your fall cleanup and, instead, leave certain plants in place. This not only provides winter interest but can also be beneficial to the wild creatures we share space with.


When Urban Pollinator Gardens Meet Native Plant Communities

Public concern about the state of bees and other pollinating insects has led to increased interest in pollinator gardens. Planting a pollinator garden is often promoted as an excellent way for the average person to help protect pollinators. And it is! However, as with anything in life, there can be downsides.

In many urban areas, populations of native plants remain on undeveloped or abandoned land, in parks or reserves, or simply as part of the developed landscape. Urban areas may also share borders with natural areas, the edges of which are particularly prone to invasions by non-native plants. Due to human activity and habitat fragmentation, many native plant populations are now threatened. Urban areas are home to the last remaining populations of some of these plants.

Concern for native plant populations in and around urban areas prompted researchers at University of Pittsburgh to review some of the impacts that urban pollinator gardens may have and to develop a “roadmap for research” going forward. Their report was published earlier this year in New Phytologist.

Planting a wildflower seed mix is a simple way to establish a pollinator garden, and such mixes are sold commercially for this purpose. Governmental and non-governmental organizations also issue recommendations for wildflower, pollinator, or meadow seed mixes. With this in mind, the researchers selected 30 seed mixes “targeted for urban settings in the northeastern or mid-Atlantic USA” to determine what species are being recommended for or commonly planted in pollinator gardens in this region. They also developed a “species impact index” to assess “the likelihood a species would impact remnant wild urban plant populations.”

A total of 230 species were represented in the 30 seed mixes. The researchers selected the 45 most common species for evaluation. Most of these species (75%) have generalized pollination systems, suggesting that there is potential for sharing pollinators with remnant native plants. Two-thirds of the species had native ranges that overlapped with the targeted region; however, the remaining one-third originated from Europe or western North America. The native species all had “generalized pollination systems, strong dispersal and colonization ability, and broad environmental tolerances,” all traits that could have “high impacts” either directly or indirectly on remnant native plants. Other species were found to have either high dispersal ability but low chance of survival or low dispersal ability but high chance of survival.

This led the researchers to conclude that “the majority of planted wildflower species have a high potential to interact with native species via pollinators but also have the ability to disperse and survive outside of the garden.” Sharing pollinators is especially likely due to super-generalists like the honeybee, which “utilizes flowers from many habitat types.” Considering this, the researchers outlined “four pollinator-mediated interactions that can affect remnant native plants and their communities,” including how these interactions can be exacerbated when wildflower species escape gardens and invade remnant plant communities.

photo credit: wikimedia commons

The first interaction involves the quantity of pollinator visits. The concern is that native plants may be “outcompeted for pollinators” due to the “dense, high-resource displays” of pollinator gardens. Whether pollinator visits will increase or decrease depends on many things, including the location of the gardens and their proximity to native plant communities. Pollinator sharing between the two has been observed; however, “the consequences of this for effective pollination of natives are not yet understood.”

The second interaction involves the quality of pollinator visits. Because pollinators are shared between native plant communities and pollinator gardens, there is a risk that the pollen from one species will be transferred to another species. High quantities of this “heterospecific pollen” can result in reduced seed production. “Low-quality pollination in terms of heterospecific pollen from wildflower plantings may be especially detrimental for wild remnant species.”

The third interaction involves gene flow between pollinator gardens and native plant communities. Pollen that is transferred from closely related species (or even individuals of the same species but from a different location) can have undesired consequences. In some cases, it can increase genetic variation and help address problems associated with inbreeding depression. In other cases, it can introduce traits that are detrimental to native plant populations, particularly traits that disrupt adaptations that are beneficial to surviving in urban environments, like seed dispersal and flowering time. Whether gene flow between the two groups will be positive or negative is difficult to predict, and “the likelihood of genetic extinction versus genetic rescue will depend on remnant population size, genetic diversity, and degree of urban adaptation relative to the planted wildflowers.”

The fourth interaction involves pathogen transmission via shared pollinators. “Both bacterial and viral pathogens can be transmitted via pollen, and bacterial pathogens can be passed from one pollinator to another.” In this way, pollinators can act as “hubs for pathogen exchange,” which is especially concerning when the diseases being transmitted are ones for which the native plants have not adapted defenses.

photo credit: wikimedia commons

All of these interactions become more direct once wildflowers escape gardens and establish themselves among the native plants. And because the species in wildflower seed mixes are selected for their tolerance of urban conditions, “they may be particularly strong competitors with wild remnant populations,” outcompeting them for space and resources. On the other hand, the authors note that, depending on the species, they may also “provide biotic resistance to more noxious invaders.”

All of these interactions require further investigation. In their conclusion, the authors affirm, “While there is a clear potential for positive effects of urban wildflower plantings on remnant plant biodiversity, there is also a strong likelihood for unintended consequences.” They then suggest future research topics that will help us answer many of these questions. In the meantime, pollinator gardens should not be discouraged, but the plants (and their origins) should be carefully considered. One place to start is with wildflower seed mixes, which can be ‘fine-tuned’ so that they benefit our urban pollinators as well as our remnant native plants. Read more about plant selection for pollinators here.

Bumblebees and Urbanization

Urban areas bear little resemblance to the natural areas that once stood in their place. Concrete and asphalt stretch out for miles, buildings of all types tower above trees and shrubs, and turfgrass appears to dominate whatever open space there is. Understandably, it may be hard to imagine places like this being havens for biodiversity. In many ways they are not, but for certain forms of life they can be.

An essay published earlier this year in Conservation Biology highlights the ways in which cities “can become a refuge for insect pollinators.” In fact, urban areas may be more inviting than their rural surroundings, which are often dominated by industrial agriculture where pesticides are regularly used, the ground is routinely disturbed, and monocultures reign supreme. Even though suitable habitat can be patchy and unpredictable in the built environment, cities may have more to offer than we once thought.

Yet, studies about bee abundance and diversity in urban areas show mixed results, largely because all bee species are not created equal (they have varying habitat requirements and life histories) and because urban areas differ wildly in the quality and quantity of habitat they provide both spatially and temporally. For this reason, it is important for studies to focus on groups of bees with similar traits and to observe them across various states of urbanization. This is precisely what researchers at University of Michigan set out to do when they sampled bumblebee populations in various cities in southeastern Michigan. Their results were published earlier this year by Royal Society Open Science.

common eastern bumble bee (Bombus impatiens) – photo credit: wikimedia commons

The researchers selected 30 sites located in Dexter, Ann Arbor, Ypsilanti, Dearborn, and Detroit. Most of the sites were gardens or farms in urban centers. They collected bumblebees from May to September using pan traps and nets. The species and sex of each individual bumblebee was identified and recorded for each site. The percentage of impervious surface that surrounded each site was used as a measurement of urban development. Other measurements included the abundance of flowers and average daily temperatures for each location.

Bumblebees were selected as a study organism because the genus, Bombus, “represents a distinct, well-studied set of traits that make it feasible to incorporate natural history into analysis.” Bumblebees live in colonies – eusocial structures that include “a single reproductive queen, variable numbers of non-reproductive female workers, and male reproductive drones.” They are generalist foragers, visiting a wide variety of flowering species for pollen and nectar, and they nest in holes in the ground, inside tree stumps, or at the bases of large clumps of grass. The authors believe that their nesting behavior makes them “a good candidate for testing the effects of urban land development,” and the fact that members of the colony have “distinct roles, [behaviors], and movement patterns” allows researchers to make inferences regarding “the effects of urbanization on specific components of bumblebee dynamics.”

Across all locations, 520 individual bumblebees were collected. Nearly three quarters of them were common eastern bumblebees (Bombus impatiens). Among the remaining nine species collected, brown-belted bumblebees (Bombus griseocollis) and two-spotted bumblebees (Bombus bimaculatus) were the most abundant.

brown-belted bumblebee (Bombus griseocollis) – photo credit: wikimedia commons

Because bumblebees are strong fliers with an extensive foraging range, impervious surface calculations for each site had to cover an area large enough to reflect this. Results indicated that as the percentage of impervious surfaces increased, bumblebee abundance and diversity declined. When male and female bumblebee data was analyzed separately, the decline was only seen in females; males were unaffected.

Female workers do most of their foraging close to home, whereas males venture further out. The researchers found it “reasonable to hypothesize that worker abundance is proportional to bumblebee colony density.” Thus, the decline in female bumblebees observed in this study suggests that as urban development increases (i.e. percent coverage of impervious surface), available nesting sites decline and the number of viable bumblebee colonies shrinks. Because male bumblebees responded differently to this trend, future studies should consider the responses of both sexes in order to get a more complete picture of the effects that urbanization has on this genus.

Interestingly, results obtained from the study locations in Detroit did not conform to the results found elsewhere. Bumblebee abundance and diversity was not decreasing with urbanization. Unlike other cities in the study, “Detroit has experienced decades of economic hardship and declining human populations.” It has a high proportion of impervious surfaces, but it also has an abundance of vacant lots and abandoned yards. These areas are left unmaintained and are less likely to be mowed regularly or treated with pesticides. Reducing disturbance can create more suitable habitat for bumblebees, resulting in healthy populations regardless of the level of urbanization. Thus, future studies should examine the state of insect pollinators in all types of cities – shrinking and non-shrinking – and should consider not just the amount of available habitat but also its suitability.

two-spotted bumblebee (Bombus bimaculatus) – photo credit: wikimedia commons

Managing Spontaneous Urban Plants for Improved Aesthetics

As discussed last week, our wild, urban flora is a cosmopolitan mixture of plants that were either native to the area before it was developed, introduced from all corners of the world on purpose or by accident, or brought in by migrating wildlife. These are plants capable of establishing and sustaining themselves outside of human cultivation and management, and are found in abundance beyond the borders of our tidy gardens and manicured landscapes. They vegetate sectors of our city that have been abandoned, overlooked, or routinely neglected. Given enough time – and prolonged lack of intervention – such vegetation will proceed along the process of ecological succession in the same way that plant communities in natural areas do. And just like other plant communities within their respective ecosystems, these wild, urban plant communities provide a suite of ecological services vital to the health of our urban ecosystems.

Peter Del Tredici writes in Wild Urban Plants of the Northeast, “landscapes that include spontaneous vegetation fit the technical definition of sustainable in the sense that they are adapted to the site, require minimal maintenance, and are ecologically functional.” In an interview with Scenario Journal, Del Tredici goes on to define sustainability as “the value of the services provided by the ecosystem divided by the cost required to maintain that ecosystem.” Spontaneous urban landscapes offer “substantial ecological services at relatively low cost, or in some cases no cost,” and thus, by Del Tredici’s definition, they are “highly sustainable.”

There is one unfortunate downside – “weedy” landscapes like this are, by popular opinion, thoroughly unattractive and a sign of urban decay. This belief is held in spite of the fact that many of the plants found therein would be cherished or admired in other settings. Among deteriorating infrastructure, litter, and less attractive plants, some of our favorite plants are rendered guilty by association.

Despite their ecological benefits, abandoned areas vegetated with wild, urban plants are not favored by the public. So, to appease our aesthetic standards, sites like this can be enhanced through minimal intervention to be more attractive while retaining their ecological functions. In a paper published in a 2006 issue of Journal of Landscape Architecture, Norbert Kühn asserts that “to use spontaneous vegetation for ornamental purposes, a kind of enhancement or design work is necessary.” Species can be added and removed, and simple, infrequent maintenance measures can be implemented. Examples include extending the flowering season with spring flowering bulbs and mowing the area once or twice annually to maintain and improve the composition of the stand.

Wild bergamot (Monarda fistulosa) – one of the plants that Norbert Kühn included in his study as a candidate for improving the aesthetics of spontaneous, urban plant communities.

Favoring attractive weeds over less attractive ones and using minimal maintenance to improve aesthetics and function are the principles behind Del Tredici’s “cosmopolitan urban meadow.” In his book, he lists some criteria for plants that would be suitable for “this novel landscape form,” including: erosion control (long-lived; vegetatively spreading), stress tolerance (full sun; drought; compacted and polluted soil), aesthetic value (ornamental characteristics; not “weedy” looking), wildlife friendly (attractive to pollinators; edible seeds), and commercially available.

In an article in Harvard Design Magazine, Del Tredici and Michael Luegering describe the cosmopolitan urban meadow as “a stable assemblage of stress-tolerant, low-maintenance herbaceous perennial plants that are preadapted to harsh urban conditions and that will provide an attractive vegetation cover on vacant land.” Whether it is a “long-term landscape feature” or a placeholder until future development, it will have “the capacity to increase the aesthetic and ecological value of vacant land without the investment of large sums of money typically required for the installation and maintenance of traditional managed landscapes.”

Abandoned or undeveloped, urban lots like this one are ideal sites for “cosmopolitan urban meadows.”

In an urban context, some plant species are particularly noxious and may need to be removed from urban meadows, such as ragweed (Ambrosia spp.) for its allergens and poison ivy (Toxicodendron radicans) for its Urushiol-induced contact dermatitis. Species with a history of being invasive should also be avoided and contained, particularly in sites that are adjacent to or within a short distance from natural areas. Despite this and other minor concerns, spontaneous vegetation has great potential. In Kühn’s words it is “authentic” and a “reminder of the history of the site,” it is part of “the natural dynamic” with potential to bring us “closer to nature,” and finally, “it can be maintained for a long time [with] less care and low costs.”

Finding beauty in these urban, wild landscapes might even cause a shift in what we find appropriate for cultivated landscapes. In her book, Grow Curious, Gayla Trail reminds us that, despite all of our efforts, wildness persists even in our most earnest attempts to subdue it. Perhaps we should embrace it:

‘Wild’ and ‘cultivated’ are social constructs that we place in opposition to each other, when in reality there is a knotty labyrinth between them. We subjugate our cities and our gardens with chemicals and artifice because we are unable to see that wild and cultivated can be entwined, can be all at once tended, lyrical, surprising, domesticated, irrational, functional, and free.


See Also: Arnold Arboretum’s Cosmopolitan Meadow at Weld Hill

Is There a Place for Weeds in Urban Ecosystems?

Highly urbanized areas have a long history of disturbance. They are a far cry from the natural areas they once displaced, bearing little resemblance to what was there before. In this sense, they are a brand new thing. During the urbanization process, virtually everything is altered – temperatures, soils, wind patterns, hydrology, carbon dioxide levels, humidity, light availability, nutrients. Add to that a changing climate and increased levels of pollution, and the hope of ever seeing such a site return to its original state – whatever that might mean – is crushed.

What then should we consider the natural flora of an ecosystem like this? Certainly it is not the native flora that once stood on the site before it was developed; virtually none of the conditions are the same anymore. If we are defining “natural” as existing with minimal human intervention, then the natural urban flora would be whatever grows wild outside of our manicured landscapes and managed, remnant natural areas. It would be a cosmopolitan mixture of plants that have joined us in our migrations with and without our permission, along with a collection of species that are either extant to the site or have been brought in by wildlife. In many ways it would mirror the human populations of our modern cities – an assortment of residents from around the globe with diverse backgrounds and cultural histories.

In Wild Urban Plants of the Northeast, Peter Del Tredici classifies urban land into three general categories based on their ecological functions: native, remnant landscapes; managed, constructed landscapes; and ruderal, adaptive landscapes. Native, remnant landscapes are generally small areas within city limits that have never been developed. They contain a portion of the native plants that once populated the area, and they require vigilant and regular maintenance to keep non-native plants from invading and to control those that already have. Managed, constructed landscapes include all of the parks and gardens that have been designed and intentionally planted. They require regular maintenance of varying intensity in order to keep them looking the way they are intended to look. Ruderal, adaptive landscapes are abandoned or neglected sites that are populated by plants that have arrived on their own and that maintain themselves with virtually no human intervention. This is where the true, wild urban flora resides.

Prickly lettuce (Lactuca serriola) growing in an abandoned lot.

Many of the plants that make up our wild urban flora are what we commonly refer to as weeds. These weedy plants appear in landscapes throughout our cities, but are generally removed or controlled in all landscapes except the abandoned ones. It is in these neglected sites that weeds have the greatest potential to provide vital ecosystem services, performing ecological functions that are beneficial to urban life.

Not all plants are suited for this role. Spontaneous urban vegetation is particularly suited due to its ability to thrive in highly modified, urban environments without external management. Regardless of provenance, this suite of plants, as Del Tredici points out, seem to be “preadapted” to urban conditions and “are among the toughest on the planet.” A long list of traits has been identified for plants in this category, ranging from seed dispersal and viability to speed of growth and reproduction to tolerance of harsh conditions. Del Tredici summarizes by stating, “a successful urban plant needs to be flexible in all aspects of its life history from seed germination through flowering and fruiting, opportunistic in its ability to take advantage of locally abundant resources that may be available for only a short time, and tolerant of the stressful growing conditions caused by an abundance of pavement and a paucity of soil.”

Abandoned lots flush with weeds, overgrown roadsides and railways, and neglected alleyways colonized by enterprising plants are generally seen as ugly, unsightly eyesores – products of neglect and decline. Some of the plants found in such locations are valued in a garden setting or prized as part of the native landscape in a natural area, but growing wildly among trash and decaying urban infrastructure they, too, are refuse. As Richard Mabey has written: “If plants sprout through garbage they become a kind of litter themselves. Vegetable trash.”

Abandoned chicken coop overtaken by tree of heaven saplings (Ailanthus altissima).

Despite how we feel about these plants or the aesthetics of the locations they find themselves in, they are performing valuable services. Apart from adding to the biodiversity on the site as well as producing oxygen and sequestering carbon – services that virtually all plants offer – they may be preventing soil erosion, stabilizing waterways, absorbing excess nutrients, reducing the urban heat island effect, mitigating pollution, building soil, and/or providing food and habitat for urban wildlife. While cultivated and managed landscapes can achieve similar things, these neglected sites are doing so without resource or labor inputs. They are sustainable in the sense that their ability to provide these services is ongoing without reliance on outside maintenance.

Sites like these should be further investigated to determine the full extent of the services that they may or may not be offering, and in the event that they are doing more good than harm, they should be conserved and encouraged. One service that is receiving more attention, as Del Tredici writes, is phytoremediation – “the ability of some plants to clean up contaminated sites by selectively absorbing and storing high concentrations of heavy metals such as cadmium, lead, copper, zinc, chromium, and nickel in their tissues.” Weed species with this ability include prickly lettuce (Lactuca serriola), lambsquarters (Chenopodium album), and mugwort (Artemisia vulgaris). In an article in Places Journal, Del Tredici gives the example of the often despised, introduced plant, common reed (Phragmites australis) cleaning up the New Jersey Meadowlands by “absorbing abundant excess nitrogen and phosphorous throughout this highly contaminated site.”

In the book, Weeds: In Defense of Nature’s Most Unloved Plants, Richard Mabey writes: “As we survey our long love-hate relationship with [weeds], it may be revealing to ponder where weeds belong in the ecological scheme of things. They seem, even from the most cursory of looks, to have evolved to grow in unsettled earth and damaged landscapes, and that may be a less malign role than we give them credit for.” Perhaps, seeing them in this worthy role, will temper our knee-jerk inclination to demonize them at every turn.


See Also: Our Urban Planet and Wild Urban Plants of Boise.