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Superplants clean up toxins from contaminated soil

“It is this ability to bear what is unbearable and to go on living, to go on doing what one is used to doing — it is this uncanny ability that the existence of the human race is based on.”

— Christa Wolf, author of “Accident: A Day’s News,” 1989

In this quote, German literary critic and author Christa Wolf is referring to the aftermath of the Chernobyl disaster in the mid-1980s, when a reactor at a nuclear power plant exploded, releasing more than 100 radioactive materials into the atmosphere. Wolf’s novel was one of the first of an entire literary genre that emerged after the incident.

Last week, I shared tips on handling smoke ash in the garden that may be contaminated with toxins from fires. Small amounts of petrochemical and metal particles within the ash, such as asbestos, arsenic, mercury, lead, copper, zinc, cadmium and chromium, can leech into the soil wherever the ashes fall.

Besides being poisonous to people and pets, accumulated petrochemicals and metals in the soil can reduce soil fertility by disrupting microbial populations and activities. If your garden is located close to a house fire and accumulates a thick layer of ash, the OSU Extension Service recommends testing the soil for contaminants and adding fresh soil or compost to the garden.

However, it turns out that the best way to clean contaminated soil is to grow plants that have evolved mechanisms for decomposing and removing toxic residue from soils. These plants are called hyperaccumulators because they are able to take up 100 times more metals and petrochemicals than other plants.

Hyperaccumulator plants include certain species of trees, grasses, ferns and flowering plants, some of which may already be growing in our garden or landscape. Since the 1980s, these “superplants” have been increasingly used in a process called phytoremediation, a term that comes from the Greek root “phyto” (plant) and the Latin root “remedium” (to correct or remove an evil).

Two of the most well-known examples of phytoremediation made use of sunflowers and other hyperaccumulators to clean up radioactive sites after disastrous explosions at nuclear power plants in Chernobyl, Ukraine (1986) and Fukashima, Japan (2011).

In fact, phytoremediation has been utilized in landfills, mining sites and other places where toxins have been released into the soil and groundwater from industrial waste, agricultural chemicals and sewage. Using plants to remove contaminants in large areas has shown to be more cost effective than soil excavation.

I couldn’t find a definitive list of hyperaccumulator plants, but here are several that have been used for phytoremediation efforts:

Trees: willows, poplars, date palms, red maples, pines

Flowering plants: sunflowers, hydrangeas, carnations, alyssum, poinsettias, snapdragons, alpine pennycress, hemp

Edible plants (don’t eat if used as hyperaccumulators): Brassicas such as mustards, kale, collards and broccoli, and corn

Grasses: Indian grass, alfalfa, buffalograss, wheatgrass, bluestem, fescue

Water/wetland plants: cattails, bracken ferns, water hyacinths, duckweed

In most cases, phytoremediation involves using a combination of plants because many hyperaccumulators absorb only specific toxins. For example, hydrangeas draw out aluminum from the soil, and ferns are hyperaccumulators of arsenic.

On the other hand, some hyperaccumulators are generalists; they clean up a variety of toxins in the soil. Sunflowers are good at absorbing metals such as lead, arsenic, zinc, chromium, copper and manganese. Indian mustard removes lead, selenium, zinc, mercury and copper. Both sunflowers and Indian mustard were used to help clean up the Chernobyl site.

How do these superplants work their magic? Hyperaccumulators trap and store contaminants in their cells (roots and plant tissue), and then they metabolize the toxic elements into less harmful molecules. These molecules either stay within the plant or they are released as gases through transpiration. Once the hyperaccumulated plants are harvested, they’re burned and the ash is safely discarded; sometimes the metals are extracted from the ash for repurposing.

Phytoremediation has limitations. Hyperaccumulator plants can only remove soil contaminants as deep as their roots, and it may take several growing seasons before positive effects are observed. The success of phytoremediation can also be constrained by other factors: soil chemistry, growing time, climate, and the age/size of the plant (younger plants draw up toxins more effectively than older plants; larger plants draw up more toxins than small plants).

Of course, the extent of soil contamination also effects the time it takes to remove the toxins. Scientists predict the 18-mile zone around Chernobyl’s nuclear power plant will not be habitable for as long as 20,000 years.

People may not be around anymore by then, but hopefully hyperaccumulator plants will be here to clean up the soil and water contamination that humans have left in their wake.

Rhonda Nowak is a Rogue Valley gardener, teacher and writer. Email her at Rnowak39@gmail.com. For more about gardening, check out her podcasts at https://mailtribune.com/podcasts/the-literary-gardener and her website at www.literarygardener.com.