Dealing with salt
When we think about storm damage, we do not always think of how to mitigate the damage that salt inflicts on plants. But for the director of horticulture at Brooklyn Bridge Park this was a major concern in the aftermath of Hurricane Sandy.
Salt damage to cherry laurel above and below the water line. - Photo ©: Julienne Schaer
In October 2012 Hurricane Sandy devastated coastal regions of New York and New Jersey. Much of Brooklyn Bridge Park (BBP) was underwater for four hours. Sandy was the second ‘100 year storm’ in two years, and researchers at MIT say we should now expect such extreme weather events every three to 20 years.
In light of our changing climate, robust ecological landscapes must now comprise more than native plants, sustainable construction, and organic management practices. Landscapes should be created with rising waters and extreme weather events in mind, and management practices for storms and floods should be researched, established, tested, and shared.
Below is a case study for horticulturally-focused storm management, using BBP’s experiences and experiments after Hurricane Sandy. The techniques utilised might be helpful in other coastal or storm-prone landscapes. Management strategies and (very) preliminary experiment results are laid out below with the hope that other landscapes might benefit from and join the conversation about how to manage horticultural storm damage in the future.
Assessing the storm damage Brooklyn Bridge Park is an 85-acre (34 ha) public park built on reclaimed shipping piers along the East River. The park contains many distinct ecosystems – from meadows to forests to wetlands – and is managed with an emphasis on ecology. While our many piers were hit hard by Sandy, the park weathered the storm relatively unscathed.
Large areas of the piers, uplands, and even surrounding streets of the park were submerged in brackish and salt water for up to four hours during the storm. The park lost electricity that BBP is still working to restore (editor’s note – this article was first published in January 2013), and two playground surfaces were badly damaged. Three young trees toppled over, and anything that wasn’t secured, from 200-gallon (900 litre) planters to shipping containers, floated incredible distances around the park. However, that was the extent of the damage. While it is too soon to tell if submerged plant material will return with full vigour in the spring, initial surveys indicate minimal plant loss and we are cautiously optimistic. We owe this adaptability to forward-thinking park design and proper management in the
days and weeks after the storm.
The designers of the park, Michael Van Valkenburgh Associates (MVVA), carefully considered waterfront location, shoreline conditions, climate change, and rising sea levels to create a park capable of withstanding storms and major floods.
Topographical changes blocked incoming flood waters, soft edge treatments of rip-rap and salt marshes held up against violent water forces, and the park itself soaked up waters that might have damaged the surrounding neighborhoods further. The sandy soil profiles used in the park (between 70 and 90%) helped the initially salty soils drain quickly. Plants were selected for salt tolerance and placed with rising water levels in mind, using many salt tolerant natives like pitch pine, beach plumb, and Baccharis in flood zones.
It is this kind of design sophistication that can help create the adaptable and climate-change-appropriate landscapes of the future. However, proper post-storm landscape management can be just as critical for ensuring long term plant survival.
Flushing out the salt
Following the storm and after four hours of salt water inundation, our biggest horticultural concern was flushing soils of plant-toxic salts. Remember Carthage? Elevated salt levels can kill plants through reverse
Rendering of the completed Brooklyn Bridge Park. - Image ©: MVVA Inc.
osmosis and make soils uninhabitable for plants in the future. There was a concern regarding adding water to potentially drowned plants, but salt was determined to be a more critical factor.
Without electricity, the extent of our irrigation system was limited to a single zone at a time. With 100 zones per pier, this was not enough. Water trucks were brought in to spray foliage and flush soils in low-lying lawns and beds. We estimate that soils received two to three inches of water through these means in the week following the storm. While the Nor’easter that swept New York just a week after Sandy was devastating to many neighborhoods, compounding prior damage, it was incredibly beneficial to BBP, which soaked up the additional inch of water happily.
When flushing soils, salt does not simply dissipate; one has to chase the salt down the soil profile. We were aiming for a safe salinity level of less than two parts per thousand
to a depth of 18 inches (46 cm), out of the reach of the majority of the plants’ roots. It is estimated that 12 inches of water (30cm) will reduce salinity by 80%, and while this is by no means always accurate, we used it to set a goal of applying 12 inches of water to the park’s more affected areas. Between irrigation, rainfall, and water trucks, we estimate that the park received about 14 inches of water (36 cm) in the month after the storm.
Testing salinity-reducing soil additives
In addition to water, we received many recommendations regarding soil additives that might help bind or drain salts. It has been my experience that the data on these methods are limited and some are feared potentially harmful to soil food webs, so we divided the park into quadrants to test the efficacy of our various options, as well as potential damage to soil biology.
Humic acids were applied to the park. The acids have been shown to aid plants with nutrient uptake and reduce stress in saline soils, as well as potentially lowering soil salinity levels. Originally, lignite-based humic acids were integrated into a compost tea mix and sprayed in with water, and later applied again to all areas in a granular form. As this is a proven technique, and carries no potential ecological drawbacks, we applied humic acids to all flooded areas and did not keep a control.
Gypsum (calcium sulphate) has been shown to reduce salinity and increase drainage in sodic and clay soils. While our soils were neither clay nor sodic, we wanted to test the hypothesis that gypsum would reduce salinity in severely compacted areas as well. We manage horticulture at BBP with an emphasis on ecology: a healthy soil food web is the basis for the plant material, habitat, and wildlife. We monitor and cultivate microbial biodiversity. As was expected, inundation in salt water decimated soil ecology directly after the storm (see Table 1). While there were undocumented concerns that gypsum might harm the soil food-web, it was decided that elevated salinity was more detrimental and we proceeded with experiments. We took soil samples for food-web analysis before application of gypsum to selected locations. We will sample again next year to determine both short – and long-term effects on soil biology.
Native plantings, post-storm, apparently unscathed by the flood waters. - Photo ©: Jeanne Rostaing for Gardenista
Preliminary results (Table 2) indicate that the majority of our soils drained quickly back to safe salinity levels regardless of treatment. Only the most compacted areas remain potentially problematic. Soil compaction is a major factor in salinity drainage because water has a hard time flowing through compacted soils to flush the salts.
Comparing results and planning for 2013
Due to limited data, it is too soon to say how our various amendments affected soil salinity. In the days after Hurricane Sandy, we were focused more on helping the park rather than on science. It is our hope that patterns will appear with increased testing in the coming year. I invite anyone in similar situations or with experience in the matter to contact me regarding post-storm landscape management. Through comparing notes and techniques with many coastal landscapes in the region, it is my goal to construct and share best management practices for flooded coastal areas. I would welcome collaboration in this process.
While the true extent of the plant damage from salt water flooding will not be known until well into next season, preliminary tests indicate that trees, shrubs, forbs and turf sustained only minimal damage. We were lucky that the storm hit after most of the plants had begun going dormant for the winter. With good design, good management, and good luck, we hope that most of the
Immediately post-storm, several areas appeared to be holding up well. - Photo ©: Jeanne Rostaing for Gardenista
plants at BBP will thrive in the coming year. Visible plant deaths so far have only occurred in recently transplanted trees, ironically pitch pines. While we had hoped to take much
of the established planting beds off of irrigation this year, we will continue to water for 2013, especially in the case of drought. We will continue monitoring all plants closely and keep track of survivor and vulnerable species for future use.
The evolution of ecological landscapes
must take our changing climate into account and seek to create and manage landscapes that perform the ecological processes and services capable of surviving and even mitigating extreme weather events. We have to plan for 100-year storms and expect them every decade. Using the unfortunate event of a super-storm to test remediation strategies for future hurricanes can turn a potentially devastating event into an opportunity for science.
AFTERWORD – June 2013
Park plantings have been affected far more than expected. Many of the survivors/victims fly in the face of printed ‘salt tolerance’, (most of our London planes are dead) so we’re cataloguing and sharing the information with as many parks as possible. Because we work with many native plants, there isn’t thorough research available on resilience.
The amazing thing is how briefly the plants were inundated, and how quickly the salinity levels returned to normal, and still how much damage we see.
Replacing dead material is a slow and careful process. No trees will be removed until we are absolutely certain they are at the very least 80% dead, which, with trees, can be difficult to ascertain. Many of the shrubs we feared dead are re-sprouting from their crowns, and will simply need to be cut back.
Michael Van Valkenburgh, of MVVA, relayed a lovely sentiment last week, that in our replanting, we should focus on making the park more resilient for the future, but we don’t want to hide the history of the landscape. We don’t want to create an immediately ‘perfect’ park that never went through a storm. The irregularities in plant growth that will result from the flooding can be truly beautiful.
This article was originally published under the title ‘Weathering the Storm: Horticulture Management in Brooklyn Bridge Park in the Aftermath of Hurricane Sandy,’ in the January 2013 edition of the ELA newsletter (newsletter of the Ecological Landscaping Association). You can find this and other articles at www.ecolandscaping.org.
Rebecca McMackin is the director of horticulture
for Brooklyn Bridge Park.