Summer of Weeds: Wild Urban Plants of Boise

The Summer of Weeds is a result of the curiosity and fascination I feel towards weeds. It is also inspired by Peter Del Tredici’s book, Wild Urban Plants of the Northeast, which encouraged me to take a closer look at the weeds that grow in my urban hometown of Boise, Idaho. Del Tredici’s book serves mainly as a field guide for identifying common weeds found in urban areas in the northeast region of the United States. Many of these weeds are found in cities across North America, so the guide is still useful regardless of where you live. Additionally, the book’s 25 page introduction is an excellent overview of how weeds fit in to the ecology of urban areas and an incentive to not only stop and get to know our urban flora but to respect it for its tenacity and durability and its important ecological role.

Excerpts from Wild Urban Plants of the Northeast by Peter Del Tredici:

From the Foreword by Steward T. A. Pickett –

If it is to fulfill its potential, the urban wild flora must be better understood and better used. In other words, its functions, not just its categories – native, exotic, invasive, naturalized – must be appreciated by professionals and citizens alike. Understanding should come before judgement when urban wild plants are concerned.

Defining urban wild plants –

The [plants] that fill the vacant spaces between our roads, our homes, and our businesses; take over neglected landscapes; and line the shores of streams, rivers, lakes, and oceans. Some of the plants are native to the region and were present before humans drastically altered the land; some were brought intentionally or unintentionally by people; and some arrived on their own, dispersed by wind, water, or wild animals. They grow and reproduce in the city without being planted or cared for. They are everywhere and yet they are invisible to most people. Given that cities are human creations and that the original vegetation that once grew there has long since disappeared, one could argue that spontaneous plants have become the de facto native vegetation of the city.

Why weeds are problematic in agricultural and horticultural settings, as well as in natural areas, is fairly intuitive. But why are they seen as a problem in urban areas, outside of our parks, yards, and gardens? –

When it comes to spontaneous urban plants, people’s complaints are usually aesthetic (the plants are perceived as ugly signs of blight and neglect) or security related (they shield illicit human activity or provide habitat for vermin). Indeed, the context in which a plant is growing not only determines the label that we put on it but also the positive or negative value that we assign to it.

Regarding urban ecology – 

[Cities] have their own distinctive ecology, dominated by the needs of people and driven by socioeconomic rather than biological factors. People welcome other organisms into cities to the extent that they contribute to making the environment a more attractive, more livable, or more profitable place to be; and they vilify as weeds those organisms that flourish without their approval or assistance. Regardless of humans’ preferences, an enormous variety of nonhuman life has managed to crowd into cities to form a cosmopolitan collection of organisms that is every bit as diverse as the human population itself.

To illustrate the point that urban weeds are playing a role in the ecology of our cities, Del Tredici lists the ecological functions of each species featured in the field guide portion of the book. These functions include:

  • temperature reduction
  • food and/or habitat for wildlife
  • erosion control on slopes and disturbed ground
  • stream and river bank stabilization
  • nutrient absorption (nitrogen, phosphorous, etc.) in wetlands
  • soil building on degraded land
  • tolerance of pollution or contaminated soil
  • disturbance-adapted colonizer of bare ground

Carbon storage and oxygen production are functions of these plants as well, as they are of all plants; however, as Del Tredici points out, “because [spontaneous urban plants] grow on marginal sites and require no maintenance, [they] are probably providing a greater return in terms of carbon sequestration than many intentionally cultivated species.”

There is much more to say about this “brave new ecology” and the role that urban wild plants play in it. Future posts are in the works. For now, consider this sentiment from Del Tredici’s book: Urban wild plants “are well adapted to the world we have created and, as such, are neither good nor bad – they are us.”

What follows are a few photos of some of the urban wild plants I have encountered in Boise over the last few weeks. These, along with the plants featured in previous Summer of Weeds posts, are a mere fraction of the species that grow wild in my urban hometown. The diversity of weeds alone in urban areas is astounding and should be given more consideration, along with the broader diversity of organisms that exist within our cities.

Creeping wood sorrel (Oxalis corniculata) along the driveway in front of my apartment

Prickly lettuce (Lactuca serriola) in an abandoned lot on Bannock Street

Yet to be identified thistle along 23rd Street

Field horsetail (Equisetum arvense) in front of post office on 13th Street

Pale smartweed (Polygonum lapathifolium) in a ditch at Idaho Botanical Garden

Tree of heaven seedling (Ailanthus altissima) in the backyard of my apartment

Birdsfoot trefoil (Lotus corniculatus) in the lawn at Esther Simplot Park

Weeds taking over a recently abandoned business on 27th Street

Creeping bellflower (Campanula rapunculoides) in the alleyway behind my apartment

Showy milkweed seedlings (Asclepias speciosa) next to horizontal juniper in a median on Parkcenter Boulevard

Purple loosestrife (Lythrum salicaria) on the bank of the Boise River near the Broadway Avenue bridge

Himalayan blackberry (Rubus armeniacus) along the Boise River Greenbelt near MK Nature Center

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Love and Hate – The Story of Purple Loosestrife

In the early 1800’s, seeds of purple loosestrife found their way to North America. They arrived from Europe several times by various means – accidentally embedded in the ballast of ships, inadvertently tucked in sheep’s wool, and purposely carried in the hands of humans. Native to much of Europe and parts of Asia and commonly found growing in wetlands and other riparian areas, purple loosestrife’s appealing spikes of magenta flowers, sturdy, upright growth habit, and ease of propagation made it a prized ornamental; its abundant nectar made it a favorite of beekeepers.

During its first 150 years or so in North America, purple loosestrife became naturalized in ditches, wet meadows, and the banks of streams, rivers, lakes, and ponds while also enjoying a place in our gardens. Concern about its spread was raised in the first half of the twentieth century, but it wasn’t until the 1980’s after an extensive survey was done and a special report was issued by the U.S. Fish and Wildlife Service that attitudes about purple loosestrife shifted dramatically. At that point, it was no longer a benign invader and welcome garden companion. It was, instead, a biological menace that needed to be destroyed.

Lytrhrum salicaria – commonly known as purple loosestrife, spiked willow-herb, long purples, rainbow weed, etc. – is an herbaceous perennial in the family Lythraceae. It reaches up to two meters tall; has square or angular stems with lance-shaped, stalkless leaves up to ten centimeters long; and ends in dense, towering spikes of pink-purple, 5-7 petaled flowers. The flowers attract a wide variety of pollinating insects – mostly bees – and afterwards produce small capsules full of tiny, red-brown seeds. Charles Darwin thoroughly studied the flowers of purple loosestrife; he was intrigued by the plant for many reasons, including its heterostyly (a topic for another post).

Lythrum salicaria (purple loosestrife) – image credit: wikimedia commons

Purple loosestrife seeds remain viable for up to 20 years and are transported by wind, water, and in mud stuck to the feet of birds. Apart from seeds, populations expand clonally as root crowns grow larger each year and produce increasingly more stems. Broken stem pieces also take root in mud, creating new plants. Purple loosestrife’s ability to form expansive populations in a quick manner, pushing other plants aside and forming what appears to be a dense monoculture, is part of the reason it has earned itself a place among the International Union for Conservation of Nature’s list of 100 World’s Worst Invasive Alien Species.

But is this ranking justified? In a paper published in Biological Invasions in 2010, Claude Lavoie compares news reports about purple loosestrife around the turn of the century with data presented in scientific papers and finds that the reports largely exaggerate the evidence. Purple loosestrife was being accused of all manner of crimes against nature and was being condemned before there was sound evidence to justify such actions.

It began with the U.S. Fish and Wildlife Service’s special report published in 1987. According to Lavoie, “a long list of the impacts of the species on wetland flora and fauna [was] presented,” but the claims were not supported by observational or experimental data – “the impacts [were] only suspected.” Regardless, wetland managers began campaigns against purple loosestrife in order to convince the public that it was a Beautiful Killer. News outlets were quick to spread the word about this “killer” plant. When biological control programs began in the 1990’s, news outlets reported on their success. Little empirical evidence had been published on either topic, and debates about purple loosestrife’s impacts remained unsettled in the scientific community.

The flowers of purple loosestrife (Lythrum salicaria) – photo credit: wikimedia commons

Around this time, five reviews were published examining the evidence against purple loosestrife. Lavoie reports that all but one of them “rely on a relatively high number of sources that have not been published in peer-reviewed journals.” After examining the reviews, Lavoie concludes: “although each review provided valuable information on purple loosestrife, most were somewhat biased and relied on a substantial amount of information that was anecdotal or not screened by reviewers during a formal evaluation process. Only one review was impartial, and this one painted an inconclusive picture of the species.”

Research has continued regarding the impacts of purple loosestrife, and so Lavoie examined 34 studies that were published during the 2000’s in search of conclusive evidence that the plant is as destructive to wetlands and wildlife as has been claimed. Upon examination he concludes that “stating that this plant has ‘large negative impacts’ on wetlands is probably exaggerated.” The most common accusation – that purple loosestrife crowds out native plants and forms a monoculture – “is controversial and has not been observed in nature (with maybe one exception).” Lavoie finds that there is “certainly no evidence that purple loosestrife ‘kills wetlands’ or ‘creates biological deserts,'” and “there are no published studies [in peer-reviewed journals] demonstrating that purple loosestrife has an impact on waterfowl or fishes.” All other negative claims against purple loosestrife “have not been the object of a study,” except for its impact on amphibians, which had at that time only been tested on two species, one “reacting negatively.” Certain claims – such as purple loosestrife’s impact on wetland hydrology – should be studied more in depth “considering the apparent public consensus on the detrimental effects of purple loosestrife” on wetland ecosystems.

Lavoie agrees that it is reasonable to control purple loosestrife when the intention is to reduce additional pressures on an ecosystem that is already highly threatened. However, he warns that “focusing on purple loosestrife instead of on other invasive species or on wetland losses to agriculture or urban sprawl could divert the attention of environmental managers from more urgent protection needs.” There is mounting evidence that purple loosestrife invasions are disturbance-dependent and are “an indicator of anthropogenic disturbances.” In order to protect our wetlands, we must first protect them against undue disturbance. Lavoie supports using the Precautionary Principle when dealing with introduced species; however, he finds the approach “much more valuable for newcomers than for invaders coexisting with native species for more than a century.”

A field of purple loosestrife in Massachusetts – photo credit: wikimedia commons

Purple loosestrife has found its way to nearly every state in America and most of the Canadian provinces. Peter Del Tredici writes in Wild Urban Plants of the Northeast, “Conservationists despise purple loosestrife, despite its beauty, and it is listed as an invasive species in most of the states where it grows.” By listing a plant as a noxious weed, landowners are obligated to remove it. Care must be taken though, as removal of purple loosestrife can result in a secondary invasion by noxious weeds with an even worse track record, such as common reed or reed canary grass. “Hardly a gain from the biodiversity point of view,” quips Lavoie.

Claude Lavoie’s paper and the papers he references are definitely worth reading. It is important that we continue to study purple loosestrife and species like it in order to fully understand the impact that introduced species are having on natural areas, especially since it is unlikely that we will ever completely eliminate them. On that note, I’ll leave you with this passage from The Book of Swamp and Bog by John Eastman:

The situation is easy for environmentalists to deplore. This plant, like few others, stirs our alien prejudice. Our native cattails, for example, are almost as rudely aggressive and competitive in many wetland areas as purple loosestrife. Yet, because cattails obvioulsy ‘belong here,’ they seldom evoke the same outraged feelings against their existence. … With the spread of purple loosestrife, we have new opportunities to witness the phases of an ever-recurring ecological process. We can watch it affect, change, adapt, and refit both its own elements and those of invaded communities into new arrangements of energy efficiency. The point is that we might as well study this process rather than simply deplore it; we have few alternatives.

Campaigns Against Invasive Species, part two

Happy American Wetlands Month!

One of the biggest threats to wetland ecosystems is, of course, invasive species. In last week’s post I shared a selection of videos that were produced by a variety of organizations to inform the public about invasive species. Many such videos specifically address invasives in wetlands and waterways. Here are a few of those videos.

Invasive Species of Idaho reminds you to “Clean, Drain, Dry” to avoid transporting aquatic hitchhikers:

Purple loosestrife is a “very wicked plant:”

Commander Ben vs. the Saltcedar Bandits:

Michigan’s Department of Environmental Quality urges hunters not to use Phragmites australis to make duck blinds:

More information about Phragmites by National Geographic:

Texas Parks and Wildlife and the Attack of the Zebra Mussels:

The story of Eurasian milfoil told by students at George Williams College:

Water hyacinth – another “very wicked plant”:

Water hyacinth invasion in Africa:

Attack of the Killer Algae by TED-Ed: