Earth Repair #2: Phytoremediation & Plant Rock Stars
Though I imagined writing more about microremediation, the plant-lover in me couldn't help skipping ahead to the phytoremediation info in Earth Repair. Please check out Leila Darwish's book for instructions on making effective compost tea, composting for soil contaminated with oil, and more.
As I did in Earth Repair article #1, I must begin by sharing this disclaimer by Scott Kellogg in Toolbox for Sustainable City Living:
"Most bioremediation research has been done in controlled laboratory settings. Community-based, DIY applications of bioremediation have been few. The interactions between molecules, microorganisms, and human bodies are complex and not entirely understood. Much of the [bioremediation information in this book] is still of an experimental nature. At a minimum, these treatments will not cause harm to the soil or to people applying them. There is no guarantee that soils will be safe following remediation. It will take many people participating in this experiment and recording their results to create a more complete picture of what processes work in what concentrations, against what contaminants, and under what conditions."
General Phytoremediation Principles
Phytoremediation is the use of plants to remove and/or transform contaminants in the soil. In general, it's appropriate when there are low to medium levels of contamination present.
The relationship between a plant and a contaminant will vary based on the characteristics of the plant, as well as the chemical or heavy metal. For easy reference, I love the chart below from youarethecity's Brownfields to Greenfields: A Field Guide to Phytoremediation. The entire guide is available for free at the previous link.
In the cases of phytoextraction and rhizofiltration, the plants uptake and store the contaminant, thus contaminating the plant itself. The plant material must then be removed from the site each season and treated as waste. Heavy metals are extracted this way since they cannot be broken down into a simpler chemical form.
Some industrial and agricultural contaminants can be broken down or transformed by plants and microbes, as described above.
Soil Testing Resources
Some plants excel at remediating specific toxins. Before selecting plants for phytoremediation, it's helpful to know which contaminants are present at the site. Patrick and I paid around $200 for a soil contamination soil test through Cornell Cooperative Extension. Check out this helpful Cornell resource for more info about soil testing. I found their “Guide to Soil Testing and Interpreting Results" to be excellent.
In partnership with microremediation, it typically takes years to "complete" a phytoremediation job--perhaps five or more (Darwish). Soil testing at the beginning and perceived "end" of a remediation project can help gauge progress, and offer peace of mind if the land has intended human uses.
If soil testing isn't doable with your budget, consider doing some research about the history of the site. Check out this article about the importance of understanding the history of property where you're gardening or foraging, and some tips for finding that information.
The Plants!
In her phytoremediation section, Darwish includes a beautiful 5-page chart listing 26 types of contaminants, several plants that excel at their remediation, and important notes about each category. Youarethecity's free guide to phytoremediation offers its own chart, offering more information about each plant's phytoremediation style. Below, I'll share a just few of the plants that stuck out as rock stars to me. These are species that appeared numerous times on the chart, and that I have found to be easy to grow in western NY,
As an herbalist, I can't help being drawn to the plants I'm most familiar with--the ones that are edible and medicinal (which includes most of them!) However, I know myself, and cultivating beautiful lamb's quarters and red clover at a site where I can't eat them would kill me. I once witnessed an herb student harvesting nettle at a landfill that had been converted into a park. She knew that nettle was great at pulling minerals--and probably contaminants--out of the soil, but she couldn't help herself. That patch was lush, and contamination is generally invisible; our senses and a forager's lust are powerful forces.
That said, I'd select plants for phytoremediation that I'm not in the habit of harvesting, or that are locally abundant enough to easily find elsewhere.
Sunflowers (Helianthus annuus)
Though sunflowers are edible, their beautiful show each summer makes it easy to sit back and let them do their phytoremediation work. Environmental contamination can reap all sorts of negative impacts for a community-- so why not plant the sunniest, most hope-inducing flower I can think of while we restore the land?
Due to their deep roots, sunflowers excel at extracting and storing contaminants from the soil. They excel at removing lead, mercury, zinc, nickel, cadmium, chromium, copper, cesium, uranium, strontium and hydrocarbons (PAHs, TPHs) (Darwish). For a brief discussion of hydrocarbons, check out the Amherst State Park article.
Sunflower seeds are an excellent food source for birds, but I feel concerned about wildlife eating this part of a remediating plant in particular. Many contaminants are fat-soluble, and I wonder if they might be concentrated in the oily seed. More research is needed on my part. Until then, I would consider removing the heads just before the seeds have fully matured, in the absence of information supporting the seeds' safety.
However, since funding can be a challenge for community phytoremediation, saving some sunflower seeds to replant may make sense. I imagine that the amount of contamination found in seed saved from the site would be vastly outweighed by how much contamination that plant will pull from the site once it's growing. While drying and storing the seeds, do so to ensure that wildlife (including mice and squirrels) will not have access.
Mustards (Brassica spp.)
Anyone who has driven on I-90 between Hamburg and Fredonia, NY in late spring has seen the vast stands of yellow mustard flowers blooming in the vineyards. Though I assumed it was growing wild, I learned that mustard is commonly sown in vineyards to suppress nematodes, add phosphorus to the soil, and more (Sonoma County Tourism, 2019). Check out the previous link for more info and photos of this plant partnership.
Photo: Satdeep gill - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=30436016
Brown mustard (Brassica juncea) is reported to excel at removing lead, zinc, chromium, copper, silver, and cesium from the soil. Mustards, in general, may be selected for removing nickel and cadmium. Because mustards may spread invasively, this is one to keep an eye on. If the plant material is being removed from the site before it can go to seed, this won't be a problem.
Red Mulberry (Morus rubra)
Mulberry possesses tantalizing fruits that should be avoided when doing remediating work. But, it possesses some exciting virtues that warrant its inclusion here. First, Morus rubra is a native plant, found along the east coast and in much of the continental US.
Second, the root zone of mulberries is a special place. Mulberry tends to support diverse communities of soil fungus and bacteria, contributing to its rhizodegradation tendencies. Communities of microbes supported by mulberries roots break down contaminants like PAHs and PCBs-- common industrial contaminants found in WNY and other cities that have hosted industry (youarethecity, 36).
Pumpkins and Gourds (Cucurbita spp.)
In the spirit of Halloween-- the date this article will be published on-- I must praise the pumpkin/squash genus, Cucurbita, for its ability to pull DDT and PCBs out of the soil. These persistent "organic" pollutants (POPs) stick around in soil indefinitely, bioaccumulating in the fatty tissue of animals exposed to them. DDT is a lingering curse across the US after its overzealous use as a pesticide in the 1960s. The chemical can still be found in US homes and in human breastmilk (Jenkins, 2011).
According to youarethecity, pumpkins and gourds use phytoextraction to remove and accumulate these chemicals. I would choose decorative Cucurbita species, such as the gourds pictured above, to adorn contaminated areas while they do their cleanup work. This would reduce the temptation to harvest, which might be present when planting the edible squash of this genus.
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For those interested in supporting the local ecosystem by planting natives, the lists offered by Darwish and youarethecity include such options. Consider hairy goldenrod, species of willow, poplar, and more. (Though yarrow is listed as a native in youarethecity's chart, some sources do not consider it native to North America--though it's considered "naturalized," at least.)
I have no doubt that more plants will be added to this list as more research is performed. According to Darwish, an effective plant for phytoremediation generally possesses these traits:
hardiness
produces a lot of biomass quickly
has a deep or large root system
has a high rate of evapotranspiration
easy to grow (76-77).
Be sure to check out the resources below and throughout this article for more information.
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Sources
Darwish, Leila. Earth Repair: A Grassroots Guide to Healing Toxic and Damaged Landscapes. 2013.
Jenkins, McKay. What's Gotten Into Us? Staying Healthy in a Toxic World. 2011.
Kellogg, Scott. Toolbox for Sustainable City Living. 2008.
Sonoma County. "The Magic of Mustard in the Vineyards." www.sonomacounty.com/articles/magic-mustard-vineyards viewed Oct 2019.
youarethecity. Brownfields to Greenfields: A Field Guide to Phytoremediation. 2011. https://www.youarethecity.com/pdf/fieldguide_youarethecity.pdf
