Another Reason to Battle Buckthorn

As plant enthusiasts, we often focus on how plants are affected by their environments. Their growth is affected by weather, water, nutrients, etc. But the plant-environment relationship is a two-way street, and plants can have a strong influence on the habitats they live in. We might experience this by walking in a forest and feeling ground beneath our feet that is spongy from the buildup of slowly decaying leaves that accumulated over decades or centuries. The trees in such a forest have “engineered” their environments, changing the very ground beneath them in ways that are beneficial to other plants, to animals, and to ecosystem functioning.

Unfortunately, we can also experience negative engineering effects of plants on their environments when we visit natural areas throughout the Chicago region and beyond that have been heavily invaded by the nonnative common or European buckthorn (Rhamnus cathartica). Buckthorn was brought to the Midwest for ornamental use—it’s great at making hedges—but escaped from human control and is now one of our region’s worst invasive species. As its hedge-producing credentials suggest, buckthorn is good at forming dense, shady thickets. These thickets have been shown to harm native plants and wildlife and to change physical, chemical, and soil conditions where they occur. Where there once may have been an open oak woodland underlain by a thick carpet of grasses, sedges, and wildflowers, we now often see an uninviting tangle of shrubs with little other than bare soil beneath them.

This contrast, between an environment that looks like this

Buckthorn removal has opened this area up and ongoing management restored a robust understory of native vegetation. Photo: Jim Steffen
An open-canopied patch of woodland with robust growth of native vegetation in the understory (Photo: Jim Steffen)


…and one that looks like this

Buckthorn-dominated area; there is almost no vegetation beneath it. Photo: Jim Steffen
Buckthorn-dominated area showing a lack of understory vegetation (Photo: Jim Steffen)

…got me and my colleague Jim Steffen wondering whether the ability of woodlands to perform carbon-storage ecosystem services could be impaired by buckthorn invasion. If so, could restoring impacted habitats back to native woodland vegetation return these services? Some background: Ecosystem services are essentially benefits nature provides to humans (e.g., clean air and drinking water, food, and other resources). Sequestering carbon—removing it from the atmosphere where it contributes to global climate change and instead locking it away harmlessly for potentially hundreds of years—is one such service.

Steffen’s more than two decades of work removing buckthorn from the Garden’s Mary Mix McDonald Woods and restoring native plant species had created the natural, long-term experiment we needed to answer these questions. We had access to areas still dominated by buckthorn for use as control treatments and patches of restored woodland of various ages that had previously been dominated by buckthorn (this is called a “restoration chronosequence”). And we had human capital to put to work: Rachel (Hesselink) Gentile and Chad Zirbel, who participated in the Garden’s Research Experiences for Undergraduates program (funded by the National Science Foundation) in 2009 and 2010, respectively. Gentile and Zirbel, in turn, mentored College First high school students Alan Lane and Kassandra Altantulkhuur.

So why did we think that buckthorn’s engineering of its environment might reduce carbon storage? Why not increase it? Mainly because of all the missed opportunities for carbon sequestration represented by the vegetation that buckthorn displaces: countless individuals of hundreds of species of understory plants, plants that make their living by drawing in carbon dioxide from the atmosphere. During photosynthesis, this captured CO2 is transformed into plant biomass, which occurs largely underground, in a complex tangle of fine, deeply penetrating roots that interact with the soil to produce tough, carbon-based compounds that are very slow to cycle back to the atmosphere. Not incidentally, all of these living understory plants and their deceased brethren (dead plant material is called “litter”) could help stabilize the soil, so that captured carbon would be less likely to wash away in thunderstorms and spring thaws.

What did we find? That buckthorn-dominated areas were indeed bad at storing carbon and that restored areas got better as they got older. Restored patches had lower rates of erosion and higher concentrations of persistent organic carbon in their soil. As restored areas matured, their vegetation continued to rebound, with plant diversity and litter biomass increasing over time. This maturation of the plant community was matched by steady increases in soil-carbon accumulation.

But wait, what about those thickets we see? Surely all that wood must be good for storing carbon? Not so much. We calculated wood biomass by measuring hundreds of trees. Even though restoration involved cutting down a lot of buckthorn, it actually led to a net increase in wood biomass, an increase that was also positively correlated with restoration age. Buckthorn’s thickets may look impressive, but its trunks and branches are puny compared to most trees, there is (almost) “no there there.” We think that taking out buckthorn may have freed native trees that can really get big, like white and red oaks, to better reach their growth potential.

In addition to the well-recognized benefits to biodiversity from active, long-term woodland restoration, our work and that of other scientists shows that there can also be tangible benefits to society. You can learn more about this research in our manuscript recently published in the journal Restoration Ecology. And you can expect to see further interesting work from Gentile (now pursuing a Ph.D. in ecology at Notre Dame), Zirbel (pursuing his Ph.D. in ecology at Michigan State), Lane (an undergraduate at DePaul University), and Altantulkhuur (attending the University of Illinois at Chicago).

©2013 Chicago Botanic Garden and

Workin’ the Berm: Aster Management

Selecting perennials to look good year-round and weather the seasons outside our wall (and next to the freeway) has been a challenge! With its own group of microclimates and an often-harsh growing climate—including high winds and both flooding and drought conditions—cultivating the garden along the Garden Wall and Berm has been a learning experience.

PHOTO: Panoramic shot of the garden visible through and behind our sign on the Edens expressway.
The Garden section in question, located by the big Edens Expressway (northbound lanes) sign.

Originally, the design for the perennial border—which you can see trailing up and down the hill behind the big Chicago Botanic Garden sign—included Tatarian aster (Aster tataricus ‘Jindai’), which stands tall in the fall and produces clouds of small lavender-blue flowers well into late October and even November. The problem with this particular plant choice was its aggressiveness. It’s not exactly invasive, but it’s a bully: its roots spread out and then shoot up a new plant every few inches, which produce a forest of these plants. Unfortunately, they soon encompassed pretty much everything in their path. Even the hearty feather reed grass (Calamagrostis acutiflora ‘Karl Foerster’) began to succumb to this persistent tide of plants, strangling down into mere wisps of their former glory.

PHOTO: Aster tataricus 'Jindai'
A dense grouping of our problematic asters.

Manual removal of the asters was only part of the solution; we needed to find a replacement for these bad boys. In the process, we revisited the vision for this border, and decided to mix drifts of purple coneflower (Echinacea), blazing star (Liatris), and several varieties of ornamental grasses with a replacement for the original Tatarian asters.

The chosen replacement was a smooth aster cultivar, Aster laevis ‘Bluebird’, which is new to the Garden’s plant collection. It bears flowers nearly identical in color to ‘Jindai’ and can grow to the same height as well.

PHOTO: Bluebird smooth aster (Aster laevis 'Bluebird')
Bluebird smooth aster (Aster laevis ‘Bluebird’) is getting along nicely with its neighbor plantings.

Furthermore, this cultivar is more well-behaved and doesn’t spread as aggressively. Bluebird smooth aster simply grows bushier in successive seasons—a win/win situation, to be sure. The replanting process took staging and preparation that began with the removal of any grasses that would need to be relocated or divided, and these were heeled into a well-mulched bed located immediately at the site, and watered generously. Additional EchinaceaLiatris, and grasses were delivered and staged for installation. Once all of the ‘Jindai’ had been removed, it was time to plant the new group. 

The new Bluebird asters came in 2-inch pots and were notably small compared to the 4-foot-tall plants they were replacing; to top it off, these were beginning to bloom in June, so the top half of these small plants needed to be trimmed off, making them even smaller. But we were confident that these plants would be well-sited in full sun, so their potential growth was a slam-dunk.

Once the grasses were in place—Panicum virgatum ‘Rotstrahlbush’, Calamagrostis acutiflora ‘Karl Foerster’, and Sorghastrum nutans ‘Sioux Blue’—the rest of the perennials were planted, with the idea of pulling the taller Liatris more toward the middle or back of the border and allowing the Echinacea to fill toward the front. The new asters would eventually stand toward the middle and back of the bed, as tall as most of the grasses. It was important to maintain the colors placed well among the grasses, as they would be the last to bloom. Planting and mulching happened simultaneously to avoid damage to the new asters.

PHOTO: Panoramic shot of the new plantings in full bloom.
This thriving section of the replanted Garden Wall and Berm has a prairie theme: blazing star (Liatris), coneflower (Echinacea), Bluebird asters, and a variety of grasses.

That was a year ago, and the border looked fine in the fall, but the question remained—how will it look this year? We’re happy to report that the Bluebird asters are rocking it: they were already as tall in July as their neighboring grasses, and they’re filling out and ready for a spectacular fall display. Of the 3,200 plants that were either moved, divided, or planted anew, the survival rate is exceptionally high: fewer than 1 percent of the plants were lost! It’s mid-August at this writing, and the colors are popping. So, the next time you drive by, carefully check it out, and enjoy this part of a lengthy border of native beauty.

©2013 Chicago Botanic Garden and

King Cracker

The king cracker (Hamadryas amphinome) is our featured butterfly this week. 

The king cracker is part of a larger group of butterflies called crackers because of the sound males make with their wings when they fly. You guessed it, they make a cracking sound! Only males can make the sound, but both males and females can detect it. We still don’t know why they make a cracking noise; perhaps it has to do with mating, or maybe to deter a potential predator. Regardless, it is fun to hear!

PHOTO: King Cracker butterfly
King cracker male (Hamadryas amphinome)
Photo by Bill Bishoff

The king cracker is native to Mexico and Peru, but has been spotted in the southern United States as well. It has a very unique color pattern that is truly remarkable to see. On the top of its wings is a stunning blue and white calico pattern. Underneath is a brilliant brick-colored patch that gives the butterfly its other common name, red cracker.  

The king cracker is a master of disguise. The mottled blue and white tones allow it to blend easily into the bark on trees. Tree trunks are a favorite resting spot for king crackers and they, unlike most butterflies, rest with their wings open; ready to take flight at any moment. Even the the pupae are disguised to look like withered leaves.

Another interesting fact about the king cracker is that it feeds mainly on decomposing fruit and not nectar. It is easy to spot a king cracker grabbing a quick bite on any of our four feeding dishes. So come on out to the Butterflies & Blooms exhibition and see if you can see or hear one today!

©2013 Chicago Botanic Garden and

Veggies Go Vertical

Quite often, it’s the vining vegetables that cause the unruliness: never-ending indeterminate tomatoes, crazy-prolific cucumbers, and winding-everywhere pole beans can make a mockery of a gardener’s carefully measured beds and neat rows unless they get the support they need.

Horticulturist Lisa Hilgenberg searched out several practical, good-looking, decently-priced solutions for bringing veggie vininess under control in the Regenstein Fruit & Vegetable Garden. Forget the rebar, scrap lumber, and snarls of string—your garden deserves these so-much-classier, smart-yet-modern takes on trellises and tuteurs.

©2013 Chicago Botanic Garden and

The Goldenseal Dilemma

In late October 2012 when I was driving down a country road in rural northwest Illinois, I spotted some bright sky-blue asters blooming near the corner of a woodlot. I was traveling between nature preserves in this area of the state collecting seeds for the Dixon National Tallgrass Prairie Seed Bank, so my mind was already tuned in for interesting native flora that might produce a collection for the seed bank. It was impossible to make a positive I.D. traveling at 60 m.p.h., but the color of that patch of blue was intriguing enough to warrant turning around to go back for a closer look.

As I had hoped, the attractive blue flowers belonged to a fine native species called Short’s aster (Symphiotrichum shortii), which inhabits high quality woodlands. This being the case, I thought it prudent to take a peek into the adjoining woodland to see what else might be growing there. To my surprise, within 30 feet of my entry point I stumbled across a large patch of goldenseal (Hydrastis canadensis)—a once common but now rare plant of Illinois woodlands. Its rarity is attributed to its past popularity as a medicinal plant, which led to its overharvest. Along with goldenseal, other quality woodland plants such as bellwort (Uvularia grandiflora) and baneberry (Actaea sp.) were also present.

PHOTO: golden seal (Hydrastis canadensis) in bloom.
This native woodland perennial produces two large, broad, palmately divided leaves atop a 1-foot-tall hairy stem. A small cluster of greenish-white flowers are produced at the base of one of the leaves in spring when the leaves are expanding, and they mature into a cluster of bright red fruits by midsummer. The knotty, bright yellow root of goldenseal has been harvested by humans for centuries for a variety of medicinal uses. Its popularity has led to extreme harvesting pressure culminating in drastically reduced natural populations.

I did not have time to explore the woodland further that day, so I made a note of where it was located and planned on returning to the site soon to explore it further. However, I had one question that I needed to answer before this was to happen: Who owned that woodland? I was confident that it wasn’t a nature preserve, because I had lists and maps of all of the protected preserves in the area. My guess was that it was privately owned. When I left the site, I recalled that there was a home situated just off the road near the middle of the woodland, so I thought that would be a good starting point.

PHOTO: Hydrastis canadensis leaves (with one ripe fruit).
Hydrastis canadensis leaves (with one ripe fruit)

As luck would have it, the owners of the home also owned the entire woodland. I spoke to the owner about the goldenseal I found in the corner of his property and the possibility of making a collection for the seed bank. It turns out that he already knew about the goldenseal population from a conversation that he had had with a local forester years ago. The forester thought that there was a good chance it had been planted there to be harvested for its medicinal value at a later date. This was a likely scenario, considering the size of the population. The owners agreed to allow me to collect its seeds as well as seeds from any other species that I sought for the seed bank. The only problem was that for the seed bank we are primarily interested in preserving the seeds of natural populations, not introduced ones. Seeds from natural populations represent individuals that are ideally suited to that environment by natural selection across generations, and are therefore of more value to those seeking genetically adapted seeds from a particular area.

PHOTO: Hydrastis canadensis fruit.
The raspberry-like fruit of goldenseal is considered inedible, but the roots have many medicinal properties.

So here lies the dilemma: large populations of goldenseal are rare, because of overharvesting. I rarely see this plant in woodlands, and when I do, it is always in small numbers. If this population was cultivated for the medicinal value of its roots, there is a good chance that it does not represent a natural population. I did not collect seeds of goldenseal that fall—the seeds had ripened and dropped much earlier in the summer. Any seed collection for the seed bank could not occur until the following year. This gave me plenty of time to contemplate whether or not to make the collection.

Since my first visit to the woodland, I have made several seed collections of many quality native woodland plants for the seed bank, including a collection of the Short’s aster that led me to the goldenseal discovery. During those collections I have become more familiar with the woods and have discovered additional colonies of goldenseal—some quite distant from the original population. Could these additional colonies represent multiple plantings? Maybe, but the sizes of the additional colonies are quite a bit smaller than the original. Perhaps they represent offspring from the original colony. If that is the case, this may be an indication that this particular woodland is an ideal habitat for goldenseal—even if it is not the original habitat. Or, there is a chance that this is a remnant population that for some reason survived overharvesting forays years ago.

PHOTO: A field of goldenseal in fruit.
A population of goldenseal on the property; one of several colonies where seeds were harvested

I completed a collection of the goldenseal population (estimated at more than 500 plants) for the seed bank on July 24, 2013. A notation in our database notes will read: “Population may not be natural.”

©2013 Chicago Botanic Garden and