Preliminary Survey of the Plant Species on Carleton’s Green Roof
(Fall 2005)
By Sonja Mae Langton-Yanowitz ’06
with help from Mark McKone, Phil Camil, Miranda Fix, Jennifer See, Jason Lord & Richard Strong

Introduction

During our design process, we thought that the unique conditions of our green roof, which the prairie plants would have to adapt to, would be a shallow soil depth, and a dry and hot environment. Prairie plants typically have deep root systems. However, the soil depth of our green roof ranges from 6” to 2”. We predicted that the green roof conditions would be dry because the plants would not have an underlying water table to draw water from in times of low precipitation. Also, because of the exposed position on top of a roof, we predicted that the green roof environment would be hot as compared to a typical prairie system. With these conditions in mind, we were curious to see which plants would grow during the first season. The following survey explains what we found.

During the spring of 2005, root stock was hand planted in sections on the roof and a seed mixture was spread all over the roof. During fall, 2005, we noticed that species not present in the root stock or seed mixture were also growing on the roof; these plants will be referred to as colonists from this point forward.

Survey Method

The Daubenmeyer vegetation plot survey method was used to estimate the percent cover of each plant species on the roof. The roof was subdivided into 10 plots (Figure 1), and within each plot, each species was estimated as covering: 0, 0-1, 2-5, 6-25, 26-50, 51-75, 76-90, or 91-100 percent of the plot. For example, a person could look at a plot and estimate that dandelions are covering 6-25% of the plot, 6-25% of the plot is barren (no plants are growing), and clover is covering 51-75% of the plot. The data that we collected is presented in the tables following Figure 1.

I was not able to identify all plants growing on the roof because not all plants were in flower, and it can be difficult to identify a plant without the flower. For this same reason, some plants were only identified to family. It is likely that the identifiable species composition will evolve as the years pass because some prairie species require a period of dormancy (sometimes ranging up to a few years) before they are able to send up a flower stalk and can be identified.

Results

Overall, 75 species were planted on the roof; of these species 9 were found growing on the roof. Twenty-eight colonist species, which were not planted on the roof, were also found growing on the roof.

Suggestions for the Future

I would recommend surveying the roof during the summer, because more plants will be in bloom and identification will be easier. If anything, this survey underestimates the species richness on the roof, but at least it is a good preliminary survey. During this project, it was suggested by Myles Bakke (Carleton Arboretum Manager) that since many prairie species thrive under periodic burns, it may be worthwhile to simulate the effects of a burn. He suggested mowing the roof in the spring, burning the mulch, and then spreading the burnt mulch over the roof. This would give a competitive advantage to the prairie species by eliminating their competitors and would reintroduce nutrients back into the soil.*

Also, Mark McKone (Biology Professor) suggested the possibility of growing vines on top of the roof. He said that vines are often a hardy species that can grow in little soil and are not very bothered by dry or wet conditions. Also, he said that the leaves of the vines would probably be good at absorbing heat, just like the other roof-top plants...

*There is evidence that increasing the surface light intensity by removal of standing dead has the same effect as burning tallgrass prairie plants; in this case we can simply mow the roof without burning the mulch, as the ashes have no effect on productivity (see Hulbert, Lloyd C. Causes of Fire Effects in Tallgrass Prairie. Ecology 69 (1): 46-58 Feb. 1988).

The green roof as viewed from Plot I (see Figure 1 in PDF). The red flags pictured help mark the plot boundaries.

Click here to download the 27-page PDF

For more information about our Carleton College building and design process, see the Greenroofs.com July 2005 Student Guest Article "The Carleton College Green Roof Project" by David Holman, Jason Lord, Jake Gold, and Andrew Kaplan of Carleton College.

Sonja Mae Langton-Yanowitz is a senior Biology major at Carleton College. Her interest in plant identification began when she worked as a Naturalist for the Superior National Forest Service during the summer of 2004. She will be attending Dental School at the University of Minnesota in the fall of 2006. Contact Sonja Mae at: Sonja.Mae.Langton-Yanowitz@alumni.carleton.edu.