Notes on Landscape Design: Morningstar’s Green Roof Terrace
If you’ve ever shopped in 108 N. State at the corner of Washington and State, chances are you haven’t been aware that a green roof hovers above you about 7 floors up. Chicago boasts more than 500,000 square feet of green roofs throughout the city – more than any other city in the U.S. A number of Chicago programs promote installing green roofs to improve stormwater capture, building insulation, local heat island reduction and new green space for people and wildlife. We show a number of them on the rooftop section of our site, but I wanted to illustrate the design thinking behind one we recently had photographed: Morningstar’s roof terrace.
Views from this roof allow great observation of historic Chicago architecture, especially the Reliance Building and the former Marshall Fields building. The surrounding buildings reach higher than the roof, and so the architecture creates a sense of enclosure. Because of this, and the relatively small size of the terrace, one of our design intents was to create a feeling of spaciousness.
We did two things help make the space feel more expansive:
1. Kept everything on one plane.
Trees and plants are not in raised planters but are flush with the paving. We felt that separating the space with anything other than vegetation, trees, and seating would minimize the scale of the terrace too greatly. We were able to do this in part because we collaborated early on with the architect, Gensler, and specified what structural loads we would need for the more significant soil depths. The photo below shows how deep the planting areas are:
2. We made sure views beyond the roof were unobstructed by a wall.
Because the guard rail is glass, they are nearly invisible and aren’t perceived as a visual enclosure. This gives you uninhibited views while sitting on the terrace and reinforces the feeling created by the cantilevered terrace, that you are floating mid-level in the city. These images show the cantilever and how the railing disappears visually when you are on the roof:
Trees on the Roof
A significant consideration when designing rooftops is how plant material will make it to the roof and how it will survive once it is there. Trees of this size don’t ride an elevator. These were craned in and then were placed individually in the tree pits designed for them.
This fact influenced our strategy for the placement of trees so that they appear a little less formal. Unlike everything else around them – the city streets, the windows of the architecture, the trees are not planted ‘on grid’ but appear naturally (or randomly) placed. A dead tree in a formal layout would be much more clearly visible – like a missing tooth – and can’t be filled without the redeployment of a crane. In a more random arrangement a void won’t feel like a hole. It can be filled with a smaller specimen to reflect a natural growth process, where younger trees succeed older ones. With any roof condition, it’s worth having an arborist on board to maintain tree health – replacements are tricky.
Green Roof Planting Considerations
Rooftop conditions are among the harshest environments for plant material. Reflective surfaces from surrounding buildings can literally burn up new trees or plantings in a single season and rooftop wind can dry them out just as quickly. As a result, we design with ‘bullet proof’ plant varieties for these conditions (like the honey locust trees). At the same time, we carefully consider at where the plantings will be placed within the roof. At Morningstar, influenced both by growing conditions and by the desire to create a park-like grove, we grouped everything very closely together in a concentrated mass instead of spreading the planting out over the entire square footage of the terrace. The effect is the opposite of what you expect: though there is more paving than vegetation on the roof, it feels the other way around.This grouping also responded to the client’s desire to have a space flexible enough for multiple uses – employee meetings, private coffee-break conversations, and corporate parties. The benches within the vegetated area allow for intimate conversation, while the movable seating on the paved area is completely dictated by an individual or group’s preference.
A relatively simple palette of plantings beneath the trees is concentrated together to achieve the appropriate scale for the space. The ground plane is planted with euonymus coloratus, a hardy goundcover that turns red in the fall and is able to withstand any reflective light or hotspots that come off the building. Bulbs, like the aliums in the photo above, and rotations of annuals mark changes in the seasons.