Ragweed in the City: Part 1

This is the first of three blog posts I will be writing about my current research on the causes and consequences of disparities ragweed pollen in urban areas. Read on!

Why ragweed!?

As an urban ecologist, I am interested in how urban environments shape the distribution and traits of plants and animals. When I began my PhD in 2013, I spent a few weeks walking around Minneapolis identifying different species and observing urban habitats. I quickly noticed that there were a few plants that thrived. One of these plants was ragweed (Ambrosia artemisiifolia), the bane of many allergy sufferers. Ragweed produces a prolific amount of pollen, causing allergies in approximately 15-20% of Americans, and triggering asthma in some individuals. If you have allergies in late summer or fall, you are probably allergic to ragweed!

Ragweed living that city life in an alley in Minneapolis

Curious observations

I found ragweed just about everywhere: along bike paths, railroads, and riverbanks and in sidewalk cracks, lawns, parks and vacant lots. I also noticed that ragweed wasn’t evenly distributed across Minneapolis. Some residential neighbourhoods had lawns and boulevards with few ragweed plants and lots of trees. Others had more vacant lots and recently disturbed areas, fewer trees, and more ragweed plants. These two observations (ragweed growing in lots of different habitats, ragweed distributed unevenly across the city), led me to focus my dissertation on studying how selection in response environmental variation at different spatial scales (within a city, between urban and rural areas, and across latitude) can shape the evolution and adaptation of plants.

As my research took me to other U.S. cities (St.Louis, New Orleans, and others), I continued to observe differences among neighbourhoods in the abundance of ragweed plants. Was this a real pattern or simply an observation? Why might some neighbourhoods have more ragweed than others? And does this lead to differences in the amount of allergenic pollen across neighbourhoods? My dissertation research went in one direction, and these questions were left percolating in the back of my mind.

Is ragweed pollen in urban areas an environmental justice issue?

Fast forward to 2018: I was writing a research proposal for the Grand Challenges Postdoctoral Fellowship. The goal of this fellowship is to encourage interdisciplinary research that contributes to addressing major societal challenges identified by the University of Minnesota. My observations about the disparities in the abundance of ragweed in cities seemed like a good fit because I was increasingly realizing that it might be an environmental equity issue.

Tree canopy coverage and greenspace in the city

By now I had learned that most cities only have a single pollen count, but in New York and Detroit, the amount of pollen produced by trees and weeds can vary at scales of 1-10 km. This indicates that wind-dispersed pollen like ragweed may not move as far in urban areas as expected. Single pollen counts may not be representative of the actual pollen levels in different neighbourhoods. Ragweed is also more abundant in vacant lots in Detroit, and vacant lots are more common in lower income neighbourhoods Furthermore, wealthier neighbourhoods tend to have more trees or canopy coverage, which likely reduces the available habitat for weedy species like ragweed that tend to thrive in recently disturbed areas.

I decided to step out of my comfort zone and proposed to study the environmental and societal causes of inequality in urban plant allergens by diving into the fields of mathematical biology, public health, and environmental policy. I was fortunate to find three mentors (Dr. Allison Shaw, Dr. Bonnie Keeler and Dr. Jesse Berman), who were equally enthusiastic about these questions. In 2019 I was awarded the Grand Challenges Postdoctoral Fellowship for 2019-2021 to pursue this research.

Up next: pollen traps, field surveys and lots of ragweed

In the coming weeks I will be writing updates for two components of this research I’ve started working on in 2020:

1) Measuring ragweed pollen at small spatial scales in Minneapolis using pollen traps

2) Assessing the abundance of ragweed plants and green amenities using field surveys.

Stay tuned! 

Ragweed flower anatomy and updates from the field!

Today we got rained out from our regular day of field work, so I thought I would post an update!

CBS undergraduates Eric Holton and Liz Sampson collecting data at the River Terrace field site

CBS undergraduates Eric Holton and Liz Sampson collecting flowering data at the River Terrace field site (rural location)

All the ragweed plants seem to have finally passed peak male and female flowering – data collection was pretty crazy for awhile. They are producing a huge amount of pollen, which is not surprising given that it’s a wind-pollinated plant and a major source of hay fever! Luckily, neither Eric nor I are allergic (yet).

A. artemisiifolia involucres. In the lower right is a single male flower, with two marks along a ruler indicating a length of 1 mm. Image taken from www.backyardnature.net

A. artemisiifolia involucres or “hats” as I call them. In the lower right is a single male flower, with two marks along a ruler indicating a length of 1 mm. Image taken from http://www.backyardnature.net

Common ragweed is a member of the Asteraceae, which is a large family of flowering plants that includes sunflowers and daisies (among many others!). Like other Asteraceae, common ragweed has composite flowers. The male flowers of common ragweed are clustered into little green upside-down bowl-like structures known as involucres, each of which is a single inflorescence. Multiple inflorescences are found along elongated spikes or stalks, and these stalks are found all over the plant! This means a single plant can have thousand of male flowers, each of which is releasing pollen into the air. Hello allergies!

A. artemisiifolia female flowers at the junction of a leaf and the main stem. Image by Graham Calow, taken from www.naturespot.org.uk

A. artemisiifolia female flowers at the junction of a leaf and the main stem. Image by Graham Calow, taken from www.naturespot.org.uk

The female flowers are a little bit harder to spot – they are generally located on stalks below the male inflorescences or at the junction of a leaf and the stem of the plant. Initially all you can see are tiny protruding stigmas (see picture to the left!), which then develop into a single seed.

In my experiment this summer, there are also some really neat all female plants that don’t have a single male flower on them! They are already starting to develop mature fruit as well. I’ll try to include a picture of one of these plants in a future blog post.

Sad, wilted ragweed plants at Scooterville on August 4th, 2015.

Sad, wilted ragweed plants at Scooterville on August 4th, 2015.

The past two weeks were pretty hot in Minneapolis – there were a few 35 C days and it didn’t rain for over a week. I was a bit of a scared that a significant number of plants at the Scooterville site (urban location) were going to die, but I resisted the temptation to water them. I am looking for differences in adaptation between populations after all!

Nevertheless, I am pretty happy that it rained today.

Until next time, fellow ragweed enthusiasts!

Adaptation to urban and rural environments – Germination + Transplanting!

It’s a little late, but finally a post about the experiment I’m doing this summer!

In Fall 2014, I collected seeds from multiple common ragweed (Ambrosia artemisiifolia) populations within urban Minneapolis and in the surrounding rural area. This summer, I am conducting an urban-rural reciprocal transplant experiment using those seeds to determine whether ragweed populations have adapted in response to the selection pressures present in urban and rural environments.

I have four field sites – two located in urban Minneapolis (called St.Paul and Scooterville), and two located in rural locations outside of the Twin Cities (called River Terrace and Rosemount). The Scooterville site is located on the University of Minnesota East Bank Campus and is part of the UMN Living Labs program.

Tilling Scooterville

Eric Holton (UMN undergraduate in EEB), tilling the Scooterville field site

In mid-May, I germinated the seeds in the greenhouse at the UMN and I prepared the sites for planting. This included lots of tilling, which is basically like using a giant lawnmower, but scarier.  In early June, I transplanted the ragweed seedlings out into the four field sites (with the help of others, of course). Overall, the transplanting was a success and most of the plants survived! Hurray!

I’ll be collecting data on these plants over the course of the summer, and I’ll try to post an update now and then with some pictures too! Stay tuned for more on everyone’s favourite plant, ragweed.


Ragweed germinants successfully transplanted


The Rosemount field site after transplanting