On the other hand I am amused with their choice of plants, non-native species.
In defense of non-native landscape plants, many such as Sedum are extremely well behaved and proven to survive in harsh conditions. Sedum is beautiful and Sedum is economically important to many green roof plant growers.
I am not bashing Sedum, lets get that out of the way up front. Once this discussion gets started, Sedum proponents immediately change the conversation from "Sedum is not a native" to "Sedum is not invasive and has been used for years".
The underlying issue here lies not even in the fact that the U.S. National Park Department of Interior's main headquarters building in our Capital possesses a green roof with a non-U.S. native (though there are a few native Sedum species in the U.S. they are not used on the roof) plant selection.
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| National Park Service, Dept. of Interior Headquarters Green Roof |
The real point here is the photo of of the DOI's green roof is used on a publication entitled "Illustrated Guidelines on Sustainability for Rehabilitating Historic Structures". These guidelines are what every local Preservation Appropriateness Review Committee use as a standard for renovation projects.
The document states very plainly under the Recommended approach column, "Selecting sustainable native plants that are drought resistant and will not require excessive watering of a green roof".
The document then goes on to post several photographs portraying the Sedum as U.S. native plant species.
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| Illustrated Guidelines on Sustainability for Rehabilitating Historic Structures |
I am working on a project now proposing to incorporate a green roof onto a non-original additional to a structure in a Preservation District and every reviewer is looking to the Illustrated Guidelines on Sustainability for Rehabilitating Historic Structures for guidance.
We have been extremely careful during plant design phase to use only low growing native plant species, those found growing not just in the U.S. but those plants native to the specific area where the green roof is being proposed.
How then does our government publish guidelines and not abide by them themselves? I suspect the U.S. Park Service and Department of Interior really do want to encourage the use of native plants and their efforts are visible with all the references to wildflowers and native plant research they support, making this issue all the more confusing.
DOI should then start by being the example everyone looks too.
I would suggest either replacing the non-native rooftop plants with drought tolerant natives on the DOI green roof or change the language in the Illustrated Guidelines on Sustainability for Rehabilitating Historic Structures to allow for use of exotic, foreign plants.
Two levels of standards, one for the government and one for us regulated is not equitable or fair.
The question is really about native plants and exotic plants.
The question is not really about exotic invasive plants.
Yet there is another even more bizarre twist to the situation.
Exotic invasive plants are typically those plants non-native to an area AND exhibiting tendencies to displace native wildflowers and plants.
Certainly there are levels of invasivity. Sedum and kudzu cannot be fairly compared as to damage caused.
Interestingly though, the Sedum shown on the DOI's green roof in the above photographs appear to be S. acre, and other Sedum species now listed on many U.S. and worldwide invasive species lists.
I did confirm the plants shown in the photograph on the DOI building are in fact Sedum, through a project description located on the manufacturer's website.
Included below is a brief excerpt from a previous post concerning invasive species issues on green roofs:
Here in the US there are many red flags and concerns now being raised concerning Sedum use and the potential for invasive activity.
For example;
- The City of Indianapolis cautions against the use of both S. sarmentosum and S. hispanico in the Indianapolis Stormwater Green Roof Technical Design Manual
- The Invasive Plant Association of Wisconsin lists S. acre and S. purpureum as having invasive qualities
- Missouri's Exotic Pest Plant list includes S. sarmentosum as a category B exotic present in Missouri
- Virginia's Department of Conservation and Recreation ranks S. sarmentosum as an invasive species
- Massachusetts suggests it is necessary to gather additional data on the S. telephium as part of the Managing Invasive Plants program.
- The Great Lakes Indian Fish and Wildlife Commission list Sedum spp. as a terrestrial invasive
- Wyoming's Natural Diversity Database report lists S. acre as a noxious weed plant located across F.E. Warren Air Force Base
- New York's state Early Detection Invasive Plant Program lists both S. sarmentosum and S. telephium species
- U.S. Fish and Wildlife Service list S. telephium on their invasive plant list
- S. acre, S. albo-roseum, S. purpureum, S. ruprestre and S. sarmentosum are listed as Alien Plants by Illinois' State Museum site
- USDA Forest Services, Non-Native Invasive Plant List lists S. telephinum/ S. purpureum on their Invasive Plant list
- US Natural Resource Conservation Service maps naturalized and escaped non-natives, including Sedum. Their database of non-native Sedum species occurrences is informative. They suggest "These plants are introduced to some part of the PLANTS Floristic Area, though they may be native in other parts. While many are harmless or beneficial, others that are not already invasive or noxious have a high potential to become so in all or part of their range. In general, introduced plants are likely to invade or become noxious since they lack co-evolved competitors and natural enemies to control their populations." Sedum species listed include;
- S. acre
- S. aizoon
- S. album
- S. dasyphyllum
- S. dendroideum
- S. hispanicum
- S. hybridum
- S. kamtschaticum
- S. linare
- S. mexicanum
- S. ochroleucum
- S. praealtum
- S. refluxeum
- S. sarmentosum
- S. sexangulare
- S. spurium
- S. stoloniferum, and
- S. ternatum
Other Countries too, view Sedum species as an invasive plant, including;
Many Not-For-Profit organizations and preserves also consider Sedum to be potentially invasive. A few include;
- Nature Conservancy of Canada - Garry Oaks Ecosystems lists S. acre as an established invasive species
- Wisconsin Botanical Information System tracks county by county where the many different Sedum species have escaped and become naturalized
- Washington State University Extension Blog has a comment " My plant peeve right now is sedum. It's used and promoted extensively for green roofs and rooftop garden use, because it grows so well in extreme conditions -- which makes it also quite willing to grow in not-so-extreme conditions. I planted a handful that a neighbor had given me, and it now has spread through one garden bed, along the base of a rock outcrop on the other side of the yard (I took a snippet and parked it from the bed onto the outcrop, not knowing how it would spread and spread), and now is moving into the lawn. It's easy to pull out, but a tiny piece will root, so composting is out, and vigilant disposal is necessary. I have yet to see sedum on an invasive species list, but predict that it will show up on a bunch of them in the next few years, as pieces make their way off of rooftops and into the on-the-ground landscape. "
- Mid-Atlantic Gardener's Guide says 'Many smaller sedums are invasive..."
- Grassroots websites suggest Illinois' Draft Invasive Plant documents list both S. sarmentosum and S. telephium are invasive
- The New York Botanical Garden's notes S. sexangulare has been newly noted in the Torrey Range as an exotic species
Truly there may be places for Sedum use.
Yet to confuse potential project applicants with photos and guidance language speaking to the use of native plants yet portraying and using a plant native not to America when referring to historic structures does not seem logical.
In fact, it is lack of consistency in guidance and procedure that can lead to expensive and time consuming litigation.
Thee is an easy solution. Either recommend native plants and use native plants or don't recommend native plants if DOI wishes to continue to use exotic landscape plants on their green roofs.
All the Department of Interior had to do was consult with their own plant experts. A monoculture of only a few genera never is optimum for biodiversity support.
On the other hand, a green roof filled with many different native wildflower families, genera and species could make a powerful statement about America's heritage at the headquarters of the Department of Interior.
