The downtown central library is one of a quartet of new buildings in Minneapolis by “star architects,” but the only one with a significant sustainability goal. This high profile project needed a design that was both innovative in function and aesthetically pleasing.
Designed by The Kestrel Design Group, Inc., the conceptual design for the Minneapolis Central Library’s green roof weaves together local cultural and natural patterns. The concept emphasizes the influence of the Mississippi River on Minneapolis’ street grid and the library building design, as the river bend on which street grids and library building are based is reflected in the green roof pattern of waves. The Mississippi River is emulated by waves created across plant palettes of varying, undulating heights, with 20-30 species in each palette. Wave patterns change seasonally as different waves are more apparent at different times of the year when prominent species are in bloom. Additionally, a seasonal wave movement is also superimposed on 3D wave patterns by a purple/pink burst of color that moves from west to east from spring to fall in accordance with the flow of the Mississippi River. The planting design also complements the frit patterns and geometry of the library windows. The metaphorical connection of the green roof design to the Mississippi River highlights the positive impact of the green roof as it mitigates the effect of storm water runoff from the roof on the Mississippi River.
Further minimizing negative effects of roof runoff on downstream water bodies, the Central Library Green Roof is equipped with a cutting edge irrigation system that utilizes a 7,500 gallon capacity cistern system to collect, store, and distribute rainwater for use by the plants. This irrigation system is intended for use on the green roof only during the plant establishment period and during periods of extreme drought. When the roof does not need watering, roof runoff collected in the cisterns can also be used to irrigate at grade landscape.
Though all green roofs help mitigate storm water runoff from building roofs and are an important aspect of any city’s stormwater management plan, many are being planted as “sedum deserts.” In contrast, the Minneapolis Central Library Green Roof uses a native analog in its plant selection, thereby increasing urban wildlife habitat), emulating a native plant community, and providing a more dynamic, regional character when viewed from library windows or downtown skyscrapers. This project is a model for how green roof design can go beyond the tried-and-true Sedum based green roof to celebrate the local ecology, context, and aesthetic.
An estimated 10,000 years of plant selection have led to a suite of dry plant communities that thrive on cliffs, bedrock, eskers, kames, and scree beds that are adapted to harsh growing conditions similar in many ways to those found on extensive green roofs: hot, dry, windy environments with shallow, free-draining soil profiles. Examples of characteristics that help plants survive such harsh conditions include: thickened cuticles, hirsute stems and leaves, highly reflective surfaces, fine or narrow leaves, sticky surfaces that can hold onto water, leathery rough leaf textures that reduce the speed of wind traveling over leaves, and water storage cells. Green roofs designed using a native analog approach in Switzerland, the U.K. and Chicago, USA provide habitat for large numbers of insect and bird species, some of which are rare, threatened or endangered. While many green roofs today are planted primarily with horticultural varieties of sedums, these roofs have been shown to provide minimal habitat value compared to roofs planted with plant palettes derived from native analogs. Some in the green roof industry have referred to green roofs planted primarily with horticultural varieties of sedums as “sedum deserts.” In addition to maximizing wildlife habitat value, it is hypothesized that using diverse native analogs for green roof design also maximizes other ecological functions. Use of native analogs to design green roofs can also enhance a sense of regional identity, or genus locus.
To maximize the benefits achievable using a native plant analog for green roof design, Minnesota’s Bedrock Bluff Prairie, a subtype of dry prairie that occurs on thin soil over bedrock on bluffs along the nearby Mississippi River, was chosen as a native plant analog for the Minneapolis Central Library green roof design. Bedrock bluff prairie habitats are similar to many extensive green roofs in that they have shallow soil profiles and are exposed to considerable heat, drought, and wind. Using this local plant community as an analog for design, native plant selection was informed by plant species abundance and frequency in Minnesota Bedrock Bluff Prairies, visual/aesthetic design informants of Minnesota Bedrock Bluff Prairies, apparent plant adaptations to the bedrock bluff prairie environment, as well as green roof growing medium depth, commercial availability of plant species, and aesthetic considerations.
While growing conditions on Minnesota’s bedrock bluff prairies at first glance appear similar to conditions on extensive green roofs, plants native to bedrock bluff prairies have not been used in green roof technology long enough to ascertain that these plants can indeed thrive on Minnesota’s extensive green roofs long term. Therefore the Green Roof Designer (The Kestrel Design Group) combined the use of Minnesota’s bedrock bluff prairie plants with proven European green roof plants. However, this may not have been necessary, as to date, the native plant species are thriving on the green roofs.
In summary, the Minneapolis Central Library green roof provides a public demonstration of a wholistic green roof planning approach that:
• helps achieve sustainable stormwater management, protecting the nearby Mississippi River
• uses a native plant community analog approach with plants adapted to local conditions
• maximizes regional identity
• maximizes wildlife habitat
• communicates human care through design to adapt cultural perceptions to an ecologically healthy landscape ecology
• uses artistic design/metaphors to maximize aesthetics and add additional layers of meaning to green roofs
• helps reduce building energy usage
• helps increase roof lifespan
