City Flora #2 - Sidewalk Cracks

Green Carptetweed (Mollugo verticillata), also known as devils-grip, originated in Central and South America, but has now become a common resident of parking lots, sidewalks, and driveways throughout temperate North America.

What would you say that the opposite of a meadow full of wildflowers is? How about a shady forest full of big, old trees? Answers will probably vary, but I suspect many people reading this will say that it is some variety of paved surface. On the spectrum of land cover types, paved areas such as parking lots, sidewalks, freeways, or city centers seem to offer the most striking contrast to the green spaces and wilderness areas that most people picture when they hear the world “nature.” Whereas lawns, parks, and gardens represent spaces in which humans attempt to shape communities of plants and animals in ways that aligns with our own wills, paved areas seal off the major sources of nature’s vigor — the sun, the rain, the soil — from each other and in doing so, appear to more or less remove nonhuman beings from the equation all together. Of all the walls that humans have erected between culture and nature, this one certainly seems to be the most formidable.

Impervious surfaces like pavement and concrete are indeed a major part of why urban areas are so challenging for many organisms to survive in, but this does not mean that they are completely devoid of all life. As with anything we build, cracks form in the façade and from these cracks emerge dusty, scraggily plants rich in symbolic currency. Serving as the background to a motivational poster, a flower rising from the parched, uniform gray of a parking lot punctuates sayings about the benefits of perseverance or the presence of beauty even in the bleakest of circumstances. More radically, a sidewalk plant might be seen as the zombie fingers of Mother Nature herself, engaged in an always just beginning counter-attack which sees pavement broken into soil and old roads or parking lots turning into meadows of disreputable wildflowers. For every floral regiment which succeeds in taking over a space like this, millions more appear as fleeting, tragic heroes, growing to a modest height before being pulled-up, cut down, or succumbing to the limits and excesses of life in the pavement. 

I think that there is a lot of value in all of this symbolism and in the way it guides our appreciation for urban plants, but I also think that we have to be careful about the assumptions that underlie our interpretations of these images, assumptions which often amount to an acceptance of a fundamental boundary between humans and nature. There is nothing wrong, of course, with thinking about sidewalk plants as models of persistence, sources of beauty, or calls for humility, but I think that we sometimes go farther than this and come to see sidewalk cracks implicitly as a kind of metaphysical battlefield. The plant growing out of the sidewalk crack is not just persistent, but so persistent that it is able to break through, however briefly, from the earth into the human realm — that’s why it’s so impressive. This story acknowledges that the wall between humanity and nature has cracks, but it still accepts that the wall is there in the first place. 

In this essay, I want to propose an alternative view, one in which the cracks that sidewalk plants grow out of do not merely suggest the permeability of the wall between nature and culture, city and wilderness, but that it was never there in this first place. In doing so, we will be talking about the fascinating and underappreciated community of plants that not only survive in these harsh habitats, but thrive, as well as how they might help us to think about our ethical responsibilities as the primary shapers of the urban environment. 

So, get ready for some weird stares — we’re gonna’ have to get nose to pavement for this one. 

Welcome to the (Concrete) Jungle:

One of the most important questions in urban ecology is why some organisms seem to do well in city habitats while others do not. Answering this question has both theoretical implications for our understanding of species distribution in general and practical ones related to how we can better manage our cities for environmental conservation. There are a few different approaches to answering, often emphasizing different aspects of how organisms relate to their environments, but they are all generally compatible with each other and can be combined to produce a multidimensional picture of what tends to work and why. 

One approach which is especially useful when it comes to sidewalk crack plants centers on the concept of exaptation. In the grand scale of things, urban habitats haven’t been around for very long, and so there aren’t many organisms with evolutionary histories that are deeply intertwined with them. Most just haven’t had the time to develop such a relationship. And yet, there are a fair number of organisms that not only do very well in urban habitats, but also have particular features which contribute to this success. Evolution does not anticipate the future, but only shapes the characteristics of a population to fit the environment that it currently resides in, so any traits which make an organism well adapted to colonizing novel urban habitats probably evolved originally in response to the demands of a different environment. Then, when the organism was introduced to an urban environment, the traits’ usefulness carried over and continued to provide it with a fitness benefit. Such organisms are considered exapted to urban life (Del Tredici; Winchell et al. 2023).

The plant community found in any given place is always informed at least in part by local eccentricities of the physical environment and a little bit of randomness. These mint plants (family: Lamiaceae) are not commonly found in pavement habitats, but have in this case spread from my parent’s garden bed to a crack in their driveway, which is wide enough to support their root systems and deep enough to shelter them a bit from trampling. The shadow of the house also provides them with protection from overheating.

Exaptation is a useful concept because it helps to explain how both individual organisms do so well in novel urban habitats and how distinct and consistent communities form within those habitats. The formation of the biological community present at any given location is complex and always involves a degree randomness, but in general, it ends up being composed of a subset of species with access to the location and which are able survive within the environmental limitations of the available habitat. The pool of species in the geographical area of any given city is composed of both native species that have been evolving there for a long time and non-native species recently introduced by humans. Across any geographic area with a similar pool of available species, novel urban habitats with similar conditions will host similar (though rarely identical) communities of organisms exapted to those conditions. 

You can easily observe how all of this theory plays out in real life by taking some time to get to know the community of plants that grow in the pavement cracks of your city. As I already mentioned, there is generally a degree of randomness (and therefore variability) to the composition of any biological community, but the more you look, the more you’ll start seeing familiar faces in crack after crack. If you do a little research into the plants that you are seeing, you’ll find that many of them originally evolved in exposed, sunny environments with sandy soils or rocky outcroppings and cliffs (Lundholm and Marlin 2006; Lundholm 2011; Del Tredici 2020). These habitats share many important features with sidewalk cracks, including compacted soils and extreme temperatures, as wells as sources of disturbance such as wind and trampling. You may also notice that many these plants look rather similar — a good sign that these similarities reflect adaptations whose benefits have carried over into life on the pavement.

Low and Slow:

Easily the most obvious of these shared features is a prostrate growth habit, or a tendency to grow flat across the ground. We’ve talked a little bit about prostrate growth habitats before in the context of lawns, where plants such as the common dandelion (Taraxacum officinale) and the dwarf cinquefoil (Potentilla canadensis) use it as a strategy for avoiding disturbance from mowing. Plants growing in sidewalk cracks also use prostrate growth habits to avoid disturbance, but in their case, it is mostly for avoiding damage from trampling (Warwick 1980; Del Tredici 2020). Plants that grow flat on the ground tend to do better in situations where they are at a high risk of being trampled because, unlike vertically growing plants, they do not have to deal as much with the force of gravity pulling down on them while experiencing the addition force of a trampling boot or rolling tire. Trample-resistance plants also tend to have extra tissue built up around the nodes delineating segments of their stems and to have shorter internode segments, which provides better support and improves resistance to breakage at these especially vulnerable points. These exaptations seem to work quite well for pavement plants — while they tend to prefer less disturbed road and parking lot edges, they are still able to form sizable mats around cracks that are run over with some regularity by large vehicles.

The prostrate knotweed (Polygonum aviculare) adopts a prostrate growth form in heavily trampled habitats, such as pavement cracks, and is more upright is less disturbed areas, such as lawn edges. Unlike many other ground cover plants (but like most sidewalk plants), it does not root at the nodes of its shoots as they grow out over the ground. The shoots are attached to the ground only by the central taproot. 

Features that allow plants to resist trampling are often phenotypically plastic, meaning that their expression varies depending on the environment that an individual plant finds itself growing in. A study of two populations of prostrate knotweed (Polygonum aviculare) growing in an unpaved parking lot at the University of Brussels, for example, found that individuals from the population growing in a more heavily trafficked part of the lot tended to be more prostrate and to have smaller leaves, smaller shoots, and smaller internode shoot segments than individuals from a less-trafficked area (Meerts and Vekemans 1991) Again, these are all useful adaptations for resisting trampling. When researchers collected seeds from both populations and grew them in a greenhouse, however, these differences were not present in their offspring, indicating that they were the result of individual plants responding to their environment, rather than evidence of underlying genetic differences between the two populations.

Another interesting response to trampling that the researchers in this study found was that heavily trampled knotweeds appeared to dedicate more resources to reproduction and to produce many more seeds than those in less trampled areas. They suggest that this is because heavy trampling tends to create areas of compressed soil where seeds have a harder time sprouting and juvenile plants struggle to become established. Thus, producing more seeds increases the likelihood that at least one will manage to survive to adulthood. This tactic is perhaps even more useful in rocky or paved habitat, where seeds have to find their way into the narrow space of a crack in order to access any soil at all. 

The Streets Have Roots:

Broadleaf Plantain (Plantago major) is also known as “white man’s footprint,” a name given to it by Native Americans due to it appearing to follow settlers wherever they went. Despite its non-native status and its commonality, plantains are not generally considered invasive because they don’t outcompete other plants. 

Once a knotweed seed does find its way into a sidewalk crack, it still needs to contend with soils compressed by the weight of the pavement and by people or vehicles traveling over it. This is especially problematic for the establishment of a root system, since compressed soils are harder to push through than looser ones. To survive in these kinds of soils, most sidewalk plants have evolved a taproot — a relatively large, central root from which other, smaller roots extend outwards, similar to those found in root vegetables such as carrots, beets, and turnips (Del Tredici 2020). This is as opposed to a fibrous-root system in which a plant puts out a system of many, smaller roots of around the same size. 

Taproots grow vertically down through the soil, whereas fibrous-root systems tend to spread out more horizontally, making taproots an ideal root system for plants that grow in small spaces, such as in gaps between rocks or in sidewalk cracks. They also tend to be very strong and so are able to push through and loosen up highly compressed soils before sending out smaller rootlets to spread out horizontally. Once a taproot is established, it is often very hard to remove — much to the frustration of would-be sidewalk weeders, pulling up the leafy portion of most sidewalk plants is not enough to kill them, since the surviving taproot generally has enough resources stored inside of it to send up new shoots.

While most sidewalk plants do have a taproot, it is not a completely universal feature. Grasses like the smooth crabgrass (Digitaria ischaemum), for example, have very strong, fibrous root systems that can penetrate compressed soils, though individuals growing under these conditions tend to be a bit smaller than those in better drained soils. The broadleaf plantain (Plantago major), meanwhile, has a mixed root system that includes both a small, thick taproot and a fibrous-root system that extends out around it, presumably giving it some of the benefits of both vertical and horizontal root growth. 

Some Like It Hot:

Once a sidewalk plant has found its way into a suitable crack, germinated in spite of the compressed soils, and adopted a prostrate growth habit to avoid trampling, there is still one more thing it has to deal with — extreme temperatures. Most of the materials that we use to construct paved surfaces have a very low albedo — that is, they tend to absorb rather than reflect sunlight. This is why the blacktop of a parking lot or a school yard feels hot to the touch on a warm sunny day while a patch of grass (which has a high albedo) does not. Over the course of a day, heat is absorbed and builds up in the paved surfaces of a city before being slowly released during the night. This also why cities tend to be much warmer on average than surrounding rural areas, a phenomenon known as the urban heat island effect (Douglas and James 2015).

All plants and animals that live in cities (including humans) have to deal with the urban heat island, but its impacts are especially acute for those like the sidewalk plants, which live in heavily paved areas. Aside from the obvious impacts on water retention, extreme heat can reduce the efficiency of photosynthesis through its impact on RuBisCO, a crucial enzyme which is responsible for “fixing” carbon collected from carbon dioxide (CO2) to what will eventually become the sugar molecules that plants use to store energy. Sometimes, RuBisCO will mistakenly fix oxygen instead of carbon to a growing sugar molecule, which wastes energy in a process that is supposed to be creating it. As temperatures climb, this mistake becomes more likely, both for reasons specific to the biochemistry of RuBisCO and because plants tend to close the small pores or stomate in their leaves during warmer periods, a response which reduces water loss while also preventing gas exchange, including the discharge of excess oxygen. To get around this limitation, certain plants that live in especially hot environments have evolved a special photosynthetic pathway in which simple organic molecules shuttle extra carbon dioxide to the region where RuBisCO is most active, increasing the ratio of carbon dioxide to oxygen and improving the odds of RuBisCO fixing the right atom. This is called the C4 photosynthetic pathway (Forseth 2010).

Common Purslane (Portulaca oleracea) originated in Eurasia and is well-adapted to growing in hot weather through its unique, hybrid C4-CAM photosynthetic pathway and its succulent leaves, which help to hold water. It is commonly eaten throughout its native range as a part of salads, stir-fries, soups and stews and has a somewhat sour flavor due to the malic acid produced during CAM photosynthesis.

Only about 3% of the world’s plants use the C4 pathway because the extra energy investment required to create CO2 shuttling molecules is really only worth it if a plant is growing under consistently, very high temperature conditions — such as, for example, those found on an exposed rockface or chunk of burning hot asphalt. Indeed, of the common sidewalk plants pictured in this article, every single one (with the exception of the mints and the ever-adaptive broadleaf plantain) is a C4 plant. Some, like the common purslane pictured above, even take things one step further by adding a third photosynthetic pathway into the mix. This pathway, known as Crassulacean Acid Metabolism or CAM photosynthesis is an adaptation which allows plants growing in very dry conditions to get around the tradeoffs associated with closing up their stomate to prevent water loss. During the day, CAM plants keep their stomate closed and photosynthesize just like any other plant. Then, during the night when things have cooled down a bit, they open up their stomate and start to absorb carbon dioxide from the atmosphere. Because there is no sunlight that can be used to produce sugars at night, this carbon cannot be used in photosynthesis and so it is stored as malic acid in special vacuoles (basically cellular storage areas) until the morning. When the sun rises and temperatures start to climb, the plant closes its stomate, converts the malic acid back into CO2, and shuttles it to the chloroplasts, where RuBisCO is waiting to fix it to a sugar molecule (Forseth 2010).

Most plants use either the typical C3 photosynthetic pathway, the C4 pathway, or the CAM pathway, but the common purslane is one of a few plants that can use both the C4 pathway and the CAM pathway interchangeably (Ferrari et al. 2020). For a long time, it was thought that it was impossible for any plant to use both the C4 pathway and the CAM pathway simultaneously in the same cell, but researchers have discovered only in the past few years that the common purslane is actually able to do this (Moreno-Villena et al 2022)! Whichever of the two that they use, C4 and CAM sidewalk plants are not only well-equipped to photosynthesis in extreme heat, but also to avoid competition with C3 plants by doing much of their growth in mid-summer, during the hottest time of the year. 

Keystone Ethics:

I could go on about sidewalk plants for a long time, but I think you get the point. You can do the kind of analysis that we’ve been doing here on just about any kind of urban plant community by investigating what habitats their constituent plants evolved in and thinking about how adaptations to that habitat might continue to be useful in their new life in the city. By coming to better understand how plants and animals relate to and interact with the urban and suburban ecosystems that they live in, we can start to change our pictures of these places, bringing the more-than-human world that was always there more into the foreground. This is especially profound (I think) with sidewalk crack plants because, as we talked about in the beginning of this essay, the paved habitats in which they have found a suitable niche are usually seen not as habitats, but as the negation of habitat. Concepts like exaptation help us to see our relationship to nature with more nuance, allowing us to recognize that we have not so much paved over paradise for a parking lot as gone from paradise for one group of organisms to paradise for another (in this case, drought tolerant, trample resistant plants and car dealership owners).

This is a bit of an extreme characterization of an important idea — that there is no metaphysical wall separating humans from nature — and because it is extreme, there is a heightened risk of misinterpretation. One way that I have seen some people misinterpret this idea is through the assumption that, if humans and nature are not separate, then anything that we do is natural and therefore, morally okay. This view appears to be rooted in the common idea that the natural/artificial binary is fundamentally related to the good/evil binary and that if the former is broken down, then the only alternative is an amoral, state of pure nature. This seems to be the underlying assumption, for example, of a controversial article by biologist R. Alexander Pyron, published in 2017 by the Washington Post. Trying to save endangered species from extinction, Pyron argues, is misguided because extinction is an important part of evolution and therefore perfectly natural.

The Spotted Spurge (Euphorbia maculata) is native to eastern North America. In younger plants, the leaves have small, rusty red spots on them.

The problem with this argument is that it rejects one picture of our ethical responsibility towards endangered species — that in driving them to extinction, we are messing with the natural order of things — and then jumps wildly to the conclusion that we must have no ethical responsibility towards endangered species at all. The connection between nature and the good is an old one that has been drawn by many cultures throughout history, but it is not the only option for justifying our moral obligations to the more-than-human world. An alternative that I like is based on two, very simple facts: that we humans participate is chains of cause and effect and that we have a uniquely advanced ability to step back, analyze, and make value judgments about our role in these chains. The first of these may apply to other sources of extinction, but the second does not — an asteroid hurtling towards the earth or a saber-tooth cat crossing the isthmus of Panama aren’t kept up at night wondering if what they're doing is right or wrong. 

On their own, the existence of cause and effect and our ability to analyze it in ethical terms are not enough to get us any specific moral rules or guidelines, but they do make it more difficult to hand wave our obligations to the more-than-human world, even in cities. Much like beavers building damns or elephants trampling forests, we humans are a keystone species, a species which has a disproportionate impact on the character and composition of the ecosystems that we live in. The difference between us and other keystone species is not that their activities are natural and ours are not, but that we have modified 70% of the Earth’s surface, pushing out any species that does not share our habitat preferences, all while having the ability to recognize this, make a value judgment about the changes we have made, and come up with alternative courses of action (Masson-Delmotte et al 2019). We have moral responsibility for our actions simply by virtue of being able to think about them morally and decide to act differently. I believe that for cities and other habitats that we have a heavy hand in creating, modifying, and inhabiting, this means that we have to always be asking ourselves: are we doing the best we can to create an environment in which as many people and other organisms can thrive as possible? 

While it is probably best known as a lawn weed, the Smooth Crabgrass (Digitaria ischaemum) is also very well-suited for growing in sidewalk cracks, where its strong root system prostate growth habit, and drought tolerance make it a formidable member of the pavement plant community. 

In the case of pavement, I would argue that the answer is a resounding “no!” While having paved surfaces as a part of our urban habitats does have an undeniable benefit in terms of accessibility and broken axles, it also has lots of problems. In addition to homogenizing diverse landscapes of species-rich habitats into expanses of species-poor, rockface analogues, impervious surfaces such as asphalt and cement block rainwater from being absorbed by the soil, instead forcing it to flow in rapid pulses through otherwise drought-stricken urban stream beds. This results in more pollutants being washed into urban streams and cancels out the free flood control services offered by streams and the soil at a time when global warming is making the risk of flooding much more prevalent (Du et al 2015; Chithra et al 2015). Meanwhile, the urban heat island effect, driven in large part by paved surfaces, makes ever more intense heat waves even more dangerous in cities, contributing to heat-related deaths and the worsening of chronic illnesses such as asthma via its link to air pollution (Piracha and Chaudhary 2022). Because of the explicitly and implicitly racist laws and practices that informed urban planning in the twentieth century, the brunt of the risks associated with these problems fall on poor people of color (Hoffman et al. 2020).

All of these problems exist where what we might call “the natural world” interacts with “the artificial.” They can be addressed in large part by technological means and environmental management, but they also came to be problems in the first place because we failed to see how our activates might fit into larger chains of cause and effect — or we did and just didn’t care because of racism. If we do not want to repeat these mistakes, we need to start looking at the world and our obligations to others more holistically. We need to stop dividing things into culture and nature, economics and ecology, our neighborhood and their neighborhood, and instead see our cities as great big habitats that we all share, and which are in turn part of a larger sea of cause and effect, a moral community in which we all must live out our lives as best we can. That sea of cause and effect, which we humans view through a moral lens, is nature.

This is the lesson of the sidewalk crack plant and, whatever changes we may make for the benefit of others in response to it, I hope that we will always leave at least a little room for them. 

Sources:

Chithra, S. V., Nair, M. H., Amarnath, A., & Anjana, N. S. (2015). Impacts of impervious surfaces on the environment. International Journal of Engineering Science Invention, 4(5), 27–31.

Del Tredici, P. (2020). Wild urban plants of the northeast: a field guide. Cornnell University Press. 

Douglas, I., & James, P. (2015). Urban ecology: an introduction. Taylor and Francis.

Du, S., Shi, P., Van Rompaey, A., & Wen, J. (2015). Quantifying the impact of impervious surface location on flood peak discharge in urban areas. Natural Hazards, 76, 1457–1471.

Ferrari, R. C., Cruz, B. C., Gastaldi, V. D., Storl, T., Ferrari, E. C., Boxall, S. F., … & Freschi, L. (2020). Exploring C4–CAM plasticity within the Portulaca oleracea complex. Scientific Reports, 10(1), 14237.

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Masson-Delmotte, V., Pörtner, H. O., Skea, J., Buendía, E. C., Zhai, P., & Roberts, D. (2019). Climate change and land. IPCC Report.

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Moreno-Villena, J. J., Zhou, H., Gilman, I. S., Tausta, S. L., Cheung, C. M., & Edwards, E. J. (2022). Spatial resolution of an integrated C4+ CAM photosynthetic metabolism. Science Advances, 8(31), eabn2349. 

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