The Chicago Botanic Garden’s Plant Evaluation Program seeks to determine, through scientific evaluation, which plants are superior for gardens in the Upper Midwest. Plants are rated on ornamental qualities, cultural adaptability, winter hardiness, and disease and pest resistance.
Early to mid-June is the greenest and lushest time in our experimental and ever-changing Green Roof Garden, one of the few rooftop landscapes in the area that invites the public in for a visit.
Native plants create a prairie-like effect atop the Plant Science Center.
Now in its fourth growing season, the 16,000-square-foot garden creates an oasis atop the Daniel F. and Ada L. Rice Plant Conservation Science Center. The green expanse helps reduce rainwater runoff and insulate the building from heat and cold. The garden also serves as living laboratory under the stewardship of Richard Hawke, plant evaluation manager.
“This is so new to us. We didn’t know what to expect. It’s been a complete learning process,” said Hawke, who has evaluated nearly 230 types of plants each year since the fall of 2009. “We want to keep testing and trying and increasing the palette each year.”
Hawke has watched nature take a hand in creating a pleasing meadow effect across the less formal Ellis Goodman Family Foundation Green Roof Garden South. “More people are drawn to this than ever,” Hawke said. “It’s just become a true landscape.”
The skyblue aster (Symphyotrichum oolentangiense) is “finally established and starting to do its own thing,” he said, and flowering for the first time are the dwarf false indigo (Amorpha nana) and largeleaf wild indigo (Baptisia alba var. alba). The latter takes its time in a landscape, first putting down a taproot—hard to do in eight inches of growing medium! Once established, it sends up a tall spike of showy, white flowers that bloom for several weeks. Like other members of the bean or legume family, wild indigo improves the soil by increasing its nitrogen levels.
Wild columbine (Aquilegia canadensis)
The deep coral native columbine (Aquilegia canadensis) is beginning to spread, blending into the lavender and white blooms of the hairy beardtongue (Penstemon hirsutus) and foxglove beardtongue (Penstemon digitalis) growing nearby. Bright yellow lanceleaf coreopsis (Coreopsis lanceolata) and the silvery, feathery foliage of field sagewort (Artemisia campestris ssp. Caudate) add to the prairie aesthetic with their layered colors and textures.
Foxglove beardtongue (Penstemon digitalis)
The more formally planted Josephine P. & John J. Louis Foundation Green Roof Garden North features wavy patches of sedums, a popular rooftop variety, alternating with familiar cultivars of plants. Flowering bulbs bring the garden into color in early spring. Mourning doves, robins, swallows, mallards, Baltimore orioles, and purple finches count among its bird visitors.
Evaluations of perennial plants typically take four years, but Hawke is sensing that it may take more time than that to tease out the “best that can be grown on a rooftop.” Prairie dropseed (Sporobolus heterolepis) has proved to be the top-performing grass, providing an elegant fine texture and remaining dark green even in the worst of last summer’s drought. Some plants have thrived, others have merely survived, and others have all but disappeared from the landscape over the evaluation period. Hawke has learned, for example, that the pale purple coneflower (Echinacea pallida) is doing better than the more commonly known purple coneflower (Echinacea purpurea). Yarrow is not doing as well as he expected, but prostrate junipers show promise. “Our goal was to learn everything we possibly could about growing plants on a green roof,” he said. “I still think there’s a lot more to learn.”
It’s hard to believe that the Green Roof Garden has been in place for three full growing seasons already. Our horticultural and research staff is pleased to see how the green roof is growing and evolving as plants settle in and move around by rhizomes and reseeding. Despite the challenging weather of 2011 and 2012, the vast majority of the 240 taxa currently on the green roof have thrived. While our goal is to minimize the care and resources put into maintaining the roof, we had to give the plants supplemental water once in July 2011 and twice in June 2012 during periods of extreme heat and drought. Not surprisingly, the greatest stress was on plants in the shallow 4-inch-deep plots. But the great news is that plants rebounded quickly once they received the additional water.
The Plant Conservation Science Center Green Roof Garden
The Green Roof Garden contains a mixture of plants that are commonly grown on green roofs and other taxa that are uncommon or untested for this use. The plant evaluation component is a particularly exciting aspect of our green roof—fully half of the 16,000 square feet is dedicated to testing a broad variety of new plants for green roof culture. In a way, the sky’s the limit on what we can try. In fact, we initiated a new trial of prickly pear cactus (Opuntia spp.) in 2012 to see how these succulents from the western U.S. perform next to our local species, Opuntia humifusa.
Common mountainmint (Pycnanthemum virginianum)
There have been many success stories on the roof, but here are just a few native plants that I’ve found particularly strong performers in 2012:
Common mountainmint (Pycnanthemum virginianum) is native to eastern North America and found in dry and moist prairies and calcareous fens in the Chicago region. Plants remained ornamental and healthy regardless of droughty conditions; it is growing in the semi-intensive 8-inch substrate depth. While stress caused lower leaves to drop, plants flowered well and maintained strongly upright habits at all times. Plants were 22 inches tall and 10 inches wide in 2012 and the small white flowers were in bloom from early July to late September. Bees, moths, wasps, and flies are frequent visitors on common mountainmint.
Although pitcher sage (Salvia azurea var. grandiflora) is an introduced plant in the Chicago region, it is native to dry prairies in Illinois. Pitcher sage features exceptional sky blue flowers from late summer to frost. A serendipitous planting of pitcher sage, stiff goldenrod (Solidago rigida), and big bluestem (Andropogon gerardii) made a spectacular show last year. We’re growing pitcher sage in extensive to semi-intensive plots so flowering stems ranged from 30 inches tall in the 4-inch plots to 56 inches tall in the 8-inch plots.
Pitcher’s sage (Salvia azurea var. grandiflora)
Stiff goldenrod (Solidago rigida)
Big bluestem (Andropogon gerardii)
Prairie dropseed (Sporobolus heterolepis) and the cultivar ‘Tara’ are being grown in the extensive to semi-intensive plots. Variable plant sizes, from 14 to 40 inches tall and 26 to 40 inches wide, were observed in the different plots. Leaves remained green throughout the growing season, even in droughty conditions, and turned a beautiful orange in autumn. The pungently fragrant flowers opened in August and remained effective for many weeks. Prairie dropseed is native to mesic and hill prairies in Illinois. ‘Tara’ is generally shorter than the species by about a foot and has darker green leaves and a vase-shaped habit.
Arrowleaf violet (Viola sagittata)
Arrowleaf violet (Viola sagittata) naturally occurs in sandy old fields and dry prairies in the Chicago region, and is known to occur in sterile abandoned clay fields where the topsoil has been eroded off. Given its adaptability to poor soil conditions, arrowleaf violet was recommended for testing by Jim Steffen, one of the Garden’s ecologists. Arrowleaf violet bloomed from early April to early May and then again later in the summer. It is a generous re-seeder and has begun to pop up around the Green Roof Garden. Plants are grown in extensive to semi-intensive plots, but greater vigor has been noted in 6 inches of growing media than in 4 inches.
Plant Evaluation Manager Richard Hawke updates us on the growth of the Green Roof Garden
September marked the second full year of the Green Roof Garden, so we want to update you on how it’s growing.
Overall, it’s been a good year, and most plants are thriving. We started the year with a good amount of precipitation, but an especially hot and dry period with sustained temperatures nearing 100 degrees in early July made us worry that a significant number of plants wouldn’t survive. Though we agonized over the decision, we watered for a half hour on two days in the first and second weeks of July (about a half an inch of water each time), which stopped the decline. Thankfully, about a week after irrigating, we received enough rain to saturate the growing medium; subsequent rains throughout the remainder of July reinvigorated the plants, and we did not need to water again. Most of the plants that were struggling recovered well, and ended the season in good health. Although we irrigated twice, we still think the roof is sustainable.
Josephine P. & John J. Louis Foundation Green Roof Garden North in August 2009
Josephine P. & John J. Louis Foundation Green Roof Garden North in October 2011
On the Josephine P. & John J. Louis Foundation Green Roof Garden North, we added plants where there were losses in 2010 including several new evaluation plants such as Aster oharae. We collected this plant on the seashore in the far east of Russia and didn’t expect it to do as well as it has in the heavy soil of the Lavin Plant Evaluation Garden. It seemed like a natural to try it on the Green Roof Garden in the coarse growing medium, which is much more like its native growing conditions; its blue flowers appeared intermittently throughout the growing season. Scabiosa olgae, collected in the Caucasus Mountains, is also doing well; its lavender-blue flowers rival any of the cultivated Scabiosa, and it has unusual fuzzy leaves.
Overall, we’ve lost about 10 percent of the 40,000 original plants, with two significant losses over the past two years: Butterfly weed (Asclepias tuberosa) and perennial lupine (Lupinus perennis). Both were replanted this year to retest them, because we like to give every plant at least two tries at evaluation.
So far, the depth of planting medium seems to influence survival. We have 4-inch, 6-inch and 8-inch planting depths. During the drought, we saw the greatest suffering in the 4-inch depth. On the Ellis Goodman Family Foundation Green Roof Garden South, we’ve been using 4-inch-deep square black plastic planting trays to demonstrate a common commercially available green roof product. We observed that the plants in the trays, especially those growing close to the edges, suffered the worst during the drought. The black plastic holds heat more than the other planting areas, which can cause additional stress on the plants.
Ellis Goodman Family Foundation Green Roof Garden South in August 2009
Ellis Goodman Family Foundation Green Roof Garden South in July 2011
Overall, the ornamental grasses are doing very well and none of them have sustained losses, including feather grass (Calamagrostis brachytricha), little bluestem (Schizachyrium scoparium), and prairie dropseed (Sporobolus heterolepis) — both the species, and the cultivar ‘Tara’. Woody plants such as the junipers and Gro-low sumac are doing well in 8-inch depths, as are the lavenders and Rosa carolina.
As part of the evaluation we are not only monitoring which plants survive, but also observing the ornamental characteristics, size, and how the plants can be used in the landscape. The Green Roof Garden was designed by Oehme, Van Sweden and Associates to be an ever-changing landscape of perennials, with interesting color, blooms, and texture. Some of the plants are performing similarly to how they grow in a typical garden and some are not, so we’ll be assessing both their ornamental qualities as well as their suitability to green roof culture. One such example is New England aster (Symphyotrichum novae-angliae), which blooms well, though its lower leaves turn brown and are unattractive during the flowering period. The fact that the plants are thriving on the roof but have some ornamental issues could be solved by interplanting with ornamental grasses that would hide the aster’s lower stems.
A few plants have spread a bit too aggressively by seed and were selectively removed this year: black-eyed Susan (Rudbeckia hirta), seeded widely on the south roof and was removed wherever it overgrew other plants; hawkweed (Hieracium ‘Leopard’), which has a pretty spotted leaf and yellow flowers, spread from the north roof into many areas on the south roof; and partridge pea (Chamaecrista fasciculata var. fasciculata) reseeded densely rather than widely, smothering many neighboring plants. Another factor we are evaluating is where the plants move on the roof; for example, are they moving into different soil depths or into more protected or shaded locations on the roof? In general, we’ll allow them to move in the evaluation areas, as long as they are not acting like thugs or weeds.
This year, the public responded more positively to the native side (South Green Roof Garden) than last year, because the plantings had filled in more. We added a native blue salvia (Salvia azurea) that blooms late in the season, and large beardtongue (Penstemon grandiflorus), which has big, beautiful red or pink flowers in spring. Both plants were especially well received by visitors. This year we noted that sideoats grama (Bouteloua curtipendula) looks better planted close together en masse in the 8-inch area, as opposed to more sparsely planted in the 4-inch area. We are seeing plants like goldenrod (Solidago sp.) and tickseed (Coreopsis tripteris) appear on the roof this year; these must have seeded onto the roof from the ground below, as they were not planted here. An important question we’re considering in the evaluation plots is whether this a garden or a habitat, and depending on the answer, should we let plants move around and seed-in from other landscapes?
Fall color began in late September and early October, and a number of plants put on exceptional autumnal displays. Big bluestem (Andropogon gerardii) is showing nice deep-purple fall color; the foxglove beardtongue (Penstemon digitalis) has turned a beautiful wine red. The various asters are blooming really well; some that are particularly nice this fall include azure aster (Symphyotrichum oolentangiense), silky aster (Symphyotrichum sericeum) and smooth aster (Symphyotrichum laeve), the latter of which has nice big clusters of lavender-blue flowers. Aromatic aster (Symphyotrichum oblongifolium) is similar to the New England aster in flower, but has a bushier habit on the green roof. We had a heavy infestation of lace bugs, which caused many asters to look sickly earlier in the summer. Fortunately, the asters recovered their health and flowering was unaffected.
We saw a variety of birds and insects on the Green Roof Garden this year. Swallows were very active all summer, and there were several killdeer pairs nesting early in the season. Other common visitors were robins and mourning doves, and we occasionally saw a pair of mallard ducks. A variety of butterflies, bees and grasshoppers were also observed. A visiting scientist was monitoring the green roof for ants in early summer, but he didn’t find any. He will continue to monitor the roof for the next couple of years. Thankfully, no geese, chipmunks or squirrels have appeared on the green roof.
Plans are underway to publish a full evaluation of the Green Roof Garden and hopefully in the next few years we’ll be able to recommend a good list of plants for green roofs. Though it is warmer, windier and drier on green roofs, clearly it’s not completely inhospitable to plants, as evidenced by the many thriving plants in the Green Roof Garden.
The Green Roof Garden is located on the roof of the Daniel F. and Ada L. Rice Plant Conservation Science Center. The 16,000-square-foot green roof is accessible to the public via a grand staircase, and an overlook with interpretive panels educates visitors about aspects of rooftop gardens. Two distinct areas serve specific functions: the Ellis Goodman Family Foundation Green Roof Garden South features regional and national native plants, many of which are not currently used as rooftop plants; the Josephine P. & John J. Louis Foundation Green Roof Garden North features a mix of plants known as good green roof plants, plus native and exotic plants that have potential for green roof use. Generally, the plants are sun-loving, drought tolerant, and can withstand windy conditions.
Richard Hawke, plant evaluation manager, gives us an update on the Green Roof Garden one year after the opening of the Daniel F. and Ada L. Rice Plant Conservation Science Center. The Ellis Goodman Family Foundation Green Roof Garden South features regional and national native plants, many of which are not currently used as rooftop plants; the Josephine P. & John J. Louis Foundation Green Roof Garden North features a mix of plants known as good green roof plants, plus native and exotic plants that have potential for green roof use. Visit our website for more information.