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We’ve selected the top 9 plants for green roof gardens from our 5-year study of 216 taxa. Download the results of plant evaluation manager Richard Hawke’s extensive study. 

INFOGRAPHIC: Top 10 plants for green roof gardens.

©2015 Chicago Botanic Garden and my.chicagobotanic.org

The Sky’s the Limit

Results (and Surprises) from the Green Roof

Richard Hawke —  July 20, 2015 — Leave a comment

When the Green Roof Garden was first planted in 2009, everything we knew about long-term rooftop gardening was theoretical. Which plants would live more than one year on the roof? No one knew for sure. Were native plants better to plant than non-natives? Unknown. What about soil depth, extreme weather, pests, diseases? The list of questions was long.

Download An Evaluation Study of Plants for Use on Green Roofs here.

PHOTO: The Roof Garden at the Chicago Botanic Garden Plant Conservation Science Center.

Download the results of this 5-year study. Click here.

Today, after five years of watching, waiting, documenting, and evaluating, we now have actual data to guide us—and others—on the ever-more-popular topic of green roofs! I’ve just published the Plant Evaluation Notes from our research—the first national plant evaluation study of its kind.

Among the data are a few surprises.

The biggest surprise may seem the most obvious—it’s that the green roof survived as well as it did!

I was blown away by the survival rates among plants, and by the fact that so many of them thrived and even excelled in such a challenging landscape. Of the more than 40,000 plants that we installed on both roofs, 30,568 of them were still alive in 2014. Just 14% of the 216 taxa died—that’s a pretty good success rate when you consider rooftop conditions. In fact, adaptability was one of the main criteria that we evaluated each plant on. Here’s the five-point list:

  • Adaptability (to hot/cold, dry, windy conditions, plus shallow soils)
  • Pests/diseases
  • Winter hardiness
  • Non-weediness
  • Ornamental beauty

Other surprises? Definitely the wild white indigo (Baptisia alba var. alba). Although I didn’t expect it to fail, I also didn’t expect it to be as large and vigorous as it has become. By year five, it was nearly three feet tall, with dramatic spires of white flowers. Meadow blazing star (Liatris ligulistylis) was in the same elegant category. But the absolute standout was prairie dropseed (Sporobolus heterolepis). It looked good all year, at all soil depths, and the fragrant flowers made the roof smell like popcorn in August and September.

PHOTO: Antennaria dioica.

Antennaria dioica

PHOTO: Baptisia alba var. alba.

Baptisia alba var. alba

PHOTO: Phlox subulata 'Emerald Blue'.

Phlox subulata ‘Emerald Blue’

PHOTO: View of the Green Roof Garden from above.

The Green Roof Garden today: a tapestry of plant life

It also surprised me that some of the drought-tolerant plants like sulfur flower (Eriogonum umbellatum), tufted fleabane (Erigeron caespitosa), and long-petaled lewisia (Lewisia longipetala ‘Little Plum’) didn’t do better on the green roof. Same goes for sundial lupine (Lupinus perennis). In a broader sense, I’m disappointed that we haven’t had greater success with plants in the shallowest, 4-inch soil depth. It’s the most challenging area on the green roof, so we’ll strive to add more types of plants to this trial area in the coming years.

PHOTO: Richard Hawke, Plant Evaluation Manager.

Monitoring plants in the field

Top 10 starstarstarstarstar Performers
on the Green Roof

  1. Pussytoes (Antennaria dioica)
  2. Dwarf calamint (Calamintha nepeta ssp. nepeta)
  3. Juniper ‘Viridis’ (Juniperus chinensis var. sargentii ‘Viridis’)
  4. Creeping phlox ‘Emerald Blue’ (Phlox subulata ‘Emerald Blue’)
  5. Creeping phlox ‘Apple Blossom’ (Phlox subulata ‘Apple Blossom’)
  6. Creeping phlox ‘Snowflake’ (Phlox subulata ‘Snowflake’)
  7. Aromatic sumac ‘Gro-Low’ (Rhus aromatica ‘Gro-Low’)
  8. Prairie dropseed (Sporobolus heterolepis)
  9. Prairie dropseed ‘Tara’ (Sporobolus heterolepis ‘Tara’)
  10. The 69 other plants that got four-star ratings (good)! 

 

What else is coming to the Green Roof Garden?

We’ll bring in a new set of plants (both native and non-native) to be evaluated and increase the replication of trials in 4-, 6- and 8-inch soil depths. Our goal is to compile a broad list of proven plants so that anyone—businesses, architects, governmental groups, and residential homeowners—has the information they need to grow a green roof. The sky’s the limit!

Visit the Green Roof Garden at the Daniel F. and Ada L. Rice Plant Conservation Center—open ‘til 9 p.m. all summer. The garden has two halves: the Ellis Goodman Family Foundation Green Roof Garden South and the Josephine P. & John J. Louis Foundation Green Roof Garden North.


©2015 Chicago Botanic Garden and my.chicagobotanic.org

Recently I had the pleasure of speaking at a symposium on plant exploration that was held in Des Moines, Iowa. The audience was enthralled following the plant collecting exploits of such luminaries as Dan Hinkley, one of the founders of the renowned (alas, no more) Heronswood Nursery, to far-flung locales such as Vietnam, China, and Bhutan.

Much of my presentation focused on plant collecting a tad closer to home—not as exotic perhaps, but still crucial in support of my research as the Chicago Botanic Garden’s plant breeder. So let’s go seek out the elusive wild phlox.

Phlox is predominantly a North American genus (one species sneaks into Siberia) best known for its gaudily—some say garishly colored—harbinger of spring, the moss phlox (Phlox subulata), and for that summer stalwart, the garden phlox (Phlox paniculata). For an idea of the diversity of the garden phlox, you can see Richard Hawke’s latest evaluation report on Phlox paniculata cultivars. The woodland phlox (Phlox divaricata) and the meadow phlox (Phlox maculata) also have their selections and garden advocates. It’s likely that every midwestern gardener has a phlox or two in their landscape.

Most of the remaining 60-plus phlox species are relatively unknown to horticulture, yet can delight the senses with their almost infinite variation of flower color and fragrance. The underutilized species are admittedly a persnickety group to cultivate, with many of them inhabiting harsh habitats from baking desert valleys to frigid alpine rock outcrops. So phlox breeding efforts in the past have focused (and rightly so) on the more amenable-to-cultivate species mentioned above. 

My breeding work at the Garden has always focused on developing new garden plants from interspecific hybridization, or crossing different species in the same genus. I’ve used this approach to develop new coneflowers (Echinacea) and false indigos (Baptisia), to name a few. In 2006, I started assembling a collection of phlox with the intent of testing my luck in creating novel hybrids between the species here as well. The botanical and horticulture literature wasn’t too encouraging on this front, with perhaps about a dozen authenticated natural and man-made interspecific hybrids known to date. But my perseverance led to two interspecific hybrid phlox, which gardeners may be able to purchase in 2015: Phlox x procumbens ‘Pink Profusion’ and Phlox ‘Violet Pinwheels’.

PHOTO: Pink Profusion phlox.

Phlox × procumbens ‘Pink Profusion’ PPAF

PHOTO: Violet Pinwheels phlox.

Phlox ‘Violet Pinwheels’ PPAF

You may ask,“And where is the plant exploration in this story?” I’m getting there!

Most of the phlox species simply aren’t available in the horticulture trade, yet I desired them for my breeding program. So commencing in 2011, I started my own plant collecting efforts to locate, study, and collect species phlox in the wild. Weeks were spent pouring over old taxonomic literature, maps, herbarium records and the like just to find out where phlox may yet exist in the wild. I say “may,” as the earliest records I located were from the 1940s—never a good harbinger, as urban sprawl, agriculture, and the like all too often swallow up such older stands of native plants. But records from recent years gave me strong hope that some phlox species are still “out there.” Modern collections invariably include GPS coordinates in their notes. Google Earth became my friend at this time, helping to locate potential collecting sites and plan out my trips.

PHOTO: Jim Ault in Russia.

On a trip a few years ago, a bit further afield: an expedition in Russia with colleagues

Finally: boots on the ground! I’ve made local trips around northern Illinois and Indiana, and trips further afield to South Dakota, Idaho, Montana, Wyoming, and Nevada. I’ve settled into a now-familiar routine. Do my research ahead of time, as above. Then go locate the plants in bloom, which translates into days of cruising bumpy, muddy, delightfully scenic and isolated dirt roads out west with one eye on the curves and drop-offs ahead and the other on the disturbed road edges, where so many phlox tend to congregate. Phlox as a rule are resentful of heavy plant competition, and so ironically, often thrive on road edges where the occasional mower or bulldozer damage clears out the competitors. It is that or scramble up steep cliffs and talus slopes, or venture out on to harsh alkaline flats, where yet again the plant competition is light, allowing phlox to thrive.

PHOTO: Haemanthus aliblos in vitro specimen.

Another project in vitro: Haemanthus aliblos specimen
Photo by Jim Ault

As I find populations with plants that appear promising for cultivation, I record field notes and GPS readings, then return in another month or year with collecting permits in hand to collect seed or cuttings. Slowly, I have been building collections of several phlox species, with the hope of ultimately combining through breeding their traits of varied flower shapes, color, and fragrance, plant habits, and adaptability for cold, heat, drought, moisture, high pH, and salinity. Phlox typically take two years from a rooted cutting or a germinated seed to grow into a flowering-sized plant, so the process of growing the species and then using them in breeding is taking time. But this year marked the first I saw a significant number of plants bloom that were hybrids made between garden cultivars and wild-collected plants. As is typical in plant breeding, most of the plants were “dogs” with terrible flowers or habits, or poorly adapted to our local garden conditions. These all got the heave-ho to the compost pile. But a few gems stood out. Stay tuned for future updates!

©2014 Chicago Botanic Garden and my.chicagobotanic.org

Interested in new perennials for your garden? How about ones that have proven to be exceptional—fragrant, colorful, drought tolerant, resistant to disease and pests, and hardy in the Midwest and similar climates? Just turn to our scientists, who have done the legwork for you through the Chicago Botanic Garden’s plant breeding and evaluation programs.

Breeding and selecting new perennials is a long, intense process that begins with cross-pollinating two plants, or moving pollen by hand from the flowers of one plant to the flowers of another plant with different traits. The two related plants—which ideally will produce exceptional offspring—are selected for breeding based on desirable attributes.

PHOTO: Jim Ault poses in a bed of bright pink- and purple-blooming asters he developed at the Garden.

Jim Ault, Ph.D., with Symphyotricum (aster) hybrids developed at the Garden

PHOTO: A closeup of the rich purple buds of Twilite false indigo.

Twilite false indigo (Baptisia × variicolor ‘Twilite’)

PHOTO: Using tweezers, Jim Ault hand-pollinates a Baptisia.

Pollinating Baptisia

“In the best-case scenario, from the first cross to the final plant worthy of introduction, it takes about seven years, maybe eight to ten. I have to think long-term in generation time, from seed to first bloom to maturity,” said Jim Ault, Ph.D., plant introduction manager and Gaylord and Dorothy Donnelley Director of Ornamental Plant Research.

The most promising new plants are propagated by cuttings or tissue culture and then scrutinized by the Garden’s Plant Evaluation Program, managed by Richard Hawke. He compares the plants to cultivars and species already in the trade to ensure that the plants from the breeding program are unique and worthy of introduction. Hawke also recommends plants for use as parents in the breeding program.

PHOTO: Richard Hawke crouches down, examining the progress of a cultivar planted at the Garden.

Richard Hawke at work

“The public can see about 80 percent of the breeding program plants as we are growing them in the ground in the evaluation gardens,” Dr. Ault said. Plants with the highest marks move to licensed commercial nurseries that also conduct field and container trials and then propagate the new plants for sale to home gardeners and the horticultural trade.

In recent years, popular offerings from the breeding program have included the first orange coneflower ever released, Art’s Pride coneflower (Echinacea ‘Art’s Pride’), and Forever Pink phlox (Phlox ‘Forever Pink’). “The interest in ‘Forever Pink’ has exploded,” Ault said. “It has three weeks of peak bloom in late May to early June and then it repeat-blooms on about 10 percent of the plant all summer and fall. It’s compact and, unlike other summer-blooming phlox, has had no powdery mildew whatsoever.”

You can expect to see more noteworthy perennials in coming years. Ault is hybridizing several types, including ground-cover phlox, asters, and other genera. “Something really wonderful should bloom this spring out of the hundreds of new seedlings that we’re growing,” said Ault.

Visit chicagobotanic.org/research/environmental/breeding for a full list of the perennials released commercially through the Garden’s Plant Breeding Program.

PHOTO: A closeup of the unusual bright orange color of Art's Pride coneflower.

Art’s Pride coneflower (Echinacea ‘Art’s Pride’)

PHOTO: A bed of a dozen plantings of Forever Pink phlox in full bloom.

Forever Pink phlox (Phlox ‘Forever Pink’)

PHOTO: Tidal Pool prostrate speedwell.

Tidal Pool prostrate speedwell (Veronica ‘Tidal Pool’)

Support for the plant evaluation program is provided by the Bernice E. Lavin Evaluation Garden Endowment, the Woman’s Board Endowment for Plant Evaluation Research and Publication, and the Sally Meads Hand Foundation.

This post was adapted from an article by Nina Koziol that appeared in the spring 2014 edition of Keep Growing, the member magazine of the Chicago Botanic Garden.

©2014 Chicago Botanic Garden and my.chicagobotanic.org

Trialed and True

Plant Evaluation Program Helps Gardeners Choose the Best Plants for the Area

Renee T. —  April 9, 2014 — Leave a comment

When the glossy gardening catalogs come in the mail, or when you stop by to see what’s new at your local nursery, it’s tempting to dream—wouldn’t those pink-hued purple coneflowers be lovely in the front yard? Or what about that new, show-stopping snowflame hibiscus?

But before you grab your credit card, consider the pertinent question: which plant would work best in your garden? That’s where the Chicago Botanic Garden’s Plant Evaluation Program comes in. For more than three decades, the program has conducted scientific studies to determine which plants offer superior performance in the Upper Midwest and in areas with similar climate and soil conditions.

PHOTO: Overhead view of the bridge and gardens in mid-spring.

A view of the Serpentine Bridge and Plant Evaluation Gardens

 “So many plants have a premium price, and if they don’t perform as expected, people get disenchanted,” said Richard Hawke, plant evaluation manager. “You’ll find what’s hot and new in catalogs and magazines, but I’m all about the tried-and-true. We’re here to tell the average gardener and the green industry how plants performed in our evaluations.”

Few plant evaluation programs are as large or as diverse as this one. There are currently 30 groups of plants growing in the Bernice E. Lavin Plant Evaluation Garden, a 2.5-acre site in full sun, and in the William Pullman Plant Evaluation Garden, which has perennials, vines, shrubs, and small trees growing in partial shade.

Plants are rated based on their ornamental characteristics, how well they adapt to the site, whether they are winter hardy, and how well they resist diseases and pests. “When we look at winter hardiness, it’s not just for cold temperatures but for wet soil, which can be very detrimental,” Hawke said. None of the plants is treated for diseases or insects.

The length of the evaluation varies from four to ten years based on the type of plant. Perennials are studied for four years, while shrubs and vines are a six-year study, and trees may take seven to ten years. “We observe and review them over a long period so we can say with fair certainty how the plant performs for us,” Hawke said.

The results are published in the Garden’s Plant Evaluation Notes, a series of reports made available to home gardeners and the green industry and available on the Garden’s website at www.chicagobotanic.org/plantevaluation.

The latest issue of Plant Evaluation Notes reports on Joe-Pye Weed. Click here to view the full list of plant evaluations.

PHOTO: Closeup of a Joe-Pye weed in bloom.

A Joe-Pye weed cultivar, Eutrochium maculatum ‘Glutball’ in bloom

Hawke is also involved in evaluating the potential for some popular ornamental plants, such as maiden grass (Miscanthus) and smartweed (Persicaria/Polygonum), to be invasive. This is a concern not only for home gardeners, but also for forest preserves and other open spaces where invasive plants compete with native plants.

It’s easy for visitors to check out the plant trials underway—the Trellis Bridge connects Evening Island to the Lavin Evaluation Garden across from the Daniel F. and Ada L. Rice Plant Conservation Science Center, and there’s a new path within the evaluation site. “What’s great about the evaluation gardens is they are densely planted with things that you won’t necessarily see anywhere else in the Garden,” Hawke said. “You can see a group of different filipendulas or lavender growing side-by-side.”


The Plant Evaluation Notes are made possible in part by the Woman’s Board Endowment for Plant Evaluation Research and Publication. This post was adapted from an article by Nina Koziol that appeared in the winter 2013 edition of Keep Growing, the member magazine of the Chicago Botanic Garden.

©2014 Chicago Botanic Garden and my.chicagobotanic.org