The cast iron tree grate is a near-ubiquitous feature of urban (and suburban) streetscapes. It has design intentionality and an ambiguous environmental purpose.
On the design side, we have an artful casting process and beautiful variations in composition that provide a complement to a tree’s flare, integrating it materially into the surrounding human-dominated landscape.
As an inspiration, is the ornamental metalwork of American architect Louis Sullivan as well as the elemental process of working with iron. Christopher Sancomb, RISD sculpture faculty poetically describes ironwork:
“Iron is something that runs through our blood and is one of the earth’s most elemental and common metals. [We] artists respond to the transformation of iron from elemental earth to a liquid state fueled by fire; emerging as a new solid form, with an organic life that changes over time as it begins the slow return to its origin.” Understood this way, cast iron grates are a tree’s natural guardian-companion.
Many architects and landscape architects select or commission tree grates with the same attention they give to selecting tree shape, fragrance, timing of leaf set or schedule of flowering.
Finished cast iron tree grates waiting to be shipped from foundry packing houses retain almost magical patterns that seem both ancient and modern. Below are ten examples of these designs. Simple grids. Spiraling orbs. Sunbeams like sharp shards. Poems in scaled fonts. Nisga’a cultural symbols.
On the environmental side, we have the initial and seemingly charitable purpose of tree grates protecting the tree’s roots. After all, urban trees are disadvantaged from the start—plunked down into compacted soils, amidst over- and underground transportation, sewage, water and utility infrastructure, chemical contamination and excessive heat.
But do cast iron tree grates protect street trees’ roots? In a sense, yes. Iron grates prevent bare roots from being driven or walked on and the slits in the iron (necessarily narrow to meet ADA specifications) allow water to flow down to roots. And yet, there’s a lot going on both above and below ground that makes cast iron tree grates problematic.
Gary Johnson, professor of Urban and Community forestry at the University of Minnesota Extension, writes that many trees die prematurely because of dysfunctional, distorted roots created by stem girdling — something partly attributable to cast iron tree grates, at least those that don’t have easily movable collars.
Legendary landscape engineer, Laurie Davidson Cox in his 1916 A Street Tree System for New York City, wrote on the trouble with iron grates,
Where gratings are used, they should be of a design which permits of easy removal for frequent cultivation. The chief value of the grating lies in its improvement of aeration conditions by protecting the ground surface from becoming compacted by traffic. Hence an immovable grating which prevents frequent cultivation destroys its main reason for existence. There are a number of gratings in Manhattan which are thus largely worthless.
Nina Bassuk, head of the Urban Horticulture Institute acknowledges the continuation of the tree grate problem, noting the expense of cast iron tree grates — they can cost more than the tree — which encourages buyers to purchase smaller grates which then girdle the tree if not widened as the tree grows. “Since they are made of cast iron,” she says, “that makes it difficult to modify without breaking the grate entirely.”
Adjusting to the climate changes of the anthropocene, the US Forest Service is very interested in large, landscape-scale urban forestry. Street trees assume additional value now for their ability to absorb storm water surges and mitigate an intensified urban heat island phenomenon, among other benefits. The architecture of urban tree roots has become an urgent area of inquiry.
Recent research on tree root ecology in the urban environment by arboriculturalists like Susan Day at Virginia Tech, has revealed that “tree roots are primary contributors to the development of soil structure and, in the longer term, soil formation.”
There are many factors in the urban environment that contribute to degradation of soils and in particular, soil structure. Thus, the potential of tree roots to influence soil structure is of considerable interest. This new appreciation of the influence of roots on soil is redefining and enlarging our concept of rhizosphere: the area where soil interacts directly with living roots. Tree root contributions to soil structure not only affect plant growth, but [they also affect] a host of other soil functions that provide ecosystem services such as stormwater runoff mitigation through enhanced soil permeability. via
Again Nina Bassuk, “Urban trees have a foreshortened life compared to rural trees. The dilemma we face is that tree roots need a penetrable soil so that they can grow for long distances to get the water, nutrients and oxygen they need. On the other hand, when we have sidewalks surrounding trees, engineers require that the soil be compacted so that there is a load bearing surface to lay their concrete on.” As a contemporary design and materials solution, Nina and a team of applied researchers designed CU Structural Soil, to meet engineers’ specifications while allowing tree roots to penetrate freely through (structured) soil to get the water, nutrients, and oxygen that they need.
Of tree grates and new ways to protect roots, Nina prefers a simple open soil bed, covered in shredded bark mulch to manage root surfacing / pavement heaving. If that isn’t possible, she points to unmortared granite blocks or concrete pavers—both of which have collars that can be unfussily opened as the tree flare grows.
Beyond the mechanics of our domestic tree grates, I think of the densest cities of the world in India, Nigeria, Mexico, China. Their local geologists and environmental chemists, like Sayo Fakayode, look at heavy metal contamination of roadside topsoil as informing the design of current and future urban landscapes, including street trees.
I also look to the abundance of traditional art that expresses a deep knowing about trees, which have been around upwards of four million years. I have a 39-page book, La Vie Nocturne des Arbres / The Nightlife of Trees. Six thousand copies of the book (I have #92) were screen-printed by Gond illustrators from Madhya Pradesh in the center of India. The ink sits heavily and brightly on an ebony background, each page thick and fibrous. The book presents the distinctive trees of the region that provide shade, shelter, food and an extremely rich fantasy world of stories and of beliefs that look at the characters trees become against night skies and underneath our feet and homes.
It’s a bit exaggerated to call cast iron street tree grates iron lungs, as I’ve titled this feature. But when I think back on my grandmother’s reminisces of the polio era, I see the ambiguous techno-medical miracle of iron lungs. The device helped polio victims breathe, but with extreme discomfort—their changing bodies encased in a gargantuan, static, metal clam shell. That compromised material-design solution, for right or wrong, reminds me of living street trees and the widely-used, but dubiously protective, iron street tree grates.
The old Gond wisdom of tree roots lingers as something both ornamental metalworkers and urban horticulturalists are striving to maintain….
Les racines des arbres s’enroulent autour de la Terre, pour la maintenir en place. (Tree roots wrap around the Earth, to keep it in place.)
