Materials

Spartina alterniflora (a.k.a. smooth cordgrass or salt marsh grass) is the main emergent (above-water) growing species in all of our prototypes. This salt tolerant plant can grow in a range of soil conditions. The geomorphological biotope that we call “salt marsh” in North America is only possible because of the salt and saturation tolerance of this species. Over the centuries, marshes have grown by trapping tidal sediment and expanding roots through it, forming wide, flat, living expanses of lush grassy shoots over a bed of thick roots and peat. 

For our engineered mat units, the roots grow to extend throughout the other substrate materials, binding them together solidifying the unit as the original dead plant fibers used in assembly break down. Since 2023, we have harvested Spartina alterniflora seeds from a dozen natural marshes in MA, ME, CT, and RI when the grasses release their seeds in late September. We also buy pre-grown plugs from Pinelands Nursery in New Jersey.

These plants need to stay wet to survive, so our prototype design and assembly processes must take this into account. Many of our prototypes do not have the Spartina added until they are deployed at a coastal location. 

Spartina patens (aka. saltmeadow cordgrass or sea hay) is also salt marsh grass native to the Atlantic coast of North America. Spartina patens resides in the high marsh due to its lower salt tolerance. This grass plays a vital role in protecting shorelines and providing habitat for marsh wildlife, including crabs, birds, and fish. Spartina patens does not spend as much time underwater as Spartina alterniflora so the plant was not generally used in our floating wetland prototypes. However, experiments utilized Spartina alterniflora and patens in our living shoreline installations.

A small amount of marine-grade foam provides sufficient buoyancy to maintain floatation for each individual unit. Biomass in the growing substrate is naturally buoyant at first, but becomes waterlogged after several months in seawater and must be supported to prevent sinking.

The spongy living root mass peat of a thriving marsh is only about 15% heavier than seawater by volume (Boelter, 1968), therefore a small amount of added buoyancy can keep a large peat mass afloat indefinitely.

We have studied many materials for buoyancy in experiments dedicated to documenting comparative material behaviors in ocean water. Plastic foam, in general, has significant drawbacks—high cost, chemical leaching—but is necessary in small quantities to ensure stable buoyancy over time, as the biomass breaks down and the root mass grows. Polystyrene is the most durable marine foam, but is rigid and brittle and therefore difficult to integrate gracefully. Polyethylene is flexible, and much easier to integrate with the other soft materials we use, but degrades faster. We have tested alternatives such as mushroom-based “mycofoam” and wood waste, but have found that any natural material quickly becomes saturated and sinks.