Heavy Metal

Panicum virgatum ‘Heavy Metal’

Generally around here if we are talking about Heavy Metal it is Panicum virgatum ‘Heavy Metal’, we are into more mellow sounds ourselves. You may have noticed as you look at our plant descriptions in the catalog and online, we are talking about different heavy metals now.

In fact, if you use our website’s “Search by Characteristic’ Button and scroll down to the bottom you will find the category ‘Phytoremediation’.

We know that native plants provide many ecosystem services. From stormwater management to sustaining pollinators we know they are invaluable to our ecosystem. But have you considered how the plants affect the soil?

Plants that Do the Cleaning for You

Phytoremediation is the use of plants to clean toxins from soil.  This is not a new technique being used. In fact, phytoremediation was implemented back in 1989 at the site of Chernobyl. It was during this project when Rutgers Professor Dr. Ilya Raskin coined the term phytoremediation. In 1996, Phytotech, Inc a Princeton-based company announced the development of a genetically modified sunflower that could remove 95% of toxic contaminants in 24 hours. These plants were grown in a raft at the end of a pond near Chernobyl and the roots of the sunflowers had concentrations of radioactive metals 8,000 times the concentration of that found in the lake. Hemp was also tested for its potential in soil remediation at Chernobyl in conjunction with Phytotech.

Also in 1996, mustard plants, known as hyperaccumulators – meaning they tend to absorb more contaminants from soil than other plants – were used to clean up an inner-city Trenton, NJ brownfield site. This site, which was the location for the manufacture of lead-acid batteries for more than 30 years. After batteries, the plant manufactured felt-tipped Magic Markers and then the plant was abandoned. The abandoned property with toxic soils and close proximity to a neighborhood school and residential areas. The timing of a pilot brownfield’s grant from the EPA to help offset cleanup costs coincided with Phytotech’s emerging interest in using plants to clean soils. In combination with community education, a phytoremediation experiment began. Indian Mustard (Brassica juncea) seeds were planted in 1996, harvested and replanted another time. The following year a crop of sunflowers (Helianthus annus) was planted. The results showed that plants were capable of removing heavy metals (in this case lead) from the soils. There was a 17% reduction in soil lead contamination where initial contamination rates exceeded 400 mg/kg.

Sunflowers (Helianthus annus) are known accumulators of heavy metals in the soil.

In nearby Fort Dix, lead contaminated soil was washed and then planted with Indian Mustard, Sunflowers and a rye/barley mixture during the 2000 growing season. This process was found to reduce the lead levels in the soil to below 400mg/kg.

In Florida, trees have been used to remove arsenic from sites contaminated with wood treatment chemicals. Plants have been in the news recently as options to help remediate radioactive spills.

Mining Metals

The fact that plants mine metals has been known for quite some time. According to this 2003 article by Niall Kirkwood in Harvard Design Magazine, “early prospectors in Europe scoured the landscape for indicator plants that signaled the presence of metal ores below the surface”.  The miners would look for plants they knew to thrive in metal rich environments and would then know where to dig.  Before phytoremediation came into vogue, plants were used as the miners in the operation. Plants were grown on sites with desirable metals, the plants were harvested and burned and the ash was sold as ore.

The Plants Have Cleaned the Soil… Now What?

This time consuming, weather dependent remediation practice is less expensive than many other options out there. A question that has arisen with phytoremediation is what to do with the plant material once it has accumulated all of those heavy metals and been harvested from the site. It has been common in these operations to burn the plant materials, which may result in secondary pollution. Another option has been to deposit the plant material in a landfill approved for this type of waste. Some plants are mined for the metals they have accumulated, using the funds raised in the sale of the metals to offset the costs of remediation.

You have planted hyper accumulators and they have done their job, now what do you do with the plant materials? Research has shown that hydrophobic contaminants (lead, etc) accumulate in the roots (phytostabilization) keeping the contaminant from moving, and not in the above-ground portion of perennial grasses and so the tops can be safely composted. Other plants used for phytoremediation, typically phytoextraction – where the plants remove the contaminant from the soil and store it in various plant parts, are often incinerated, creating ash that may have the contaminant. Though the contaminant is still present, and most likely needs to be disposed of in a landfill, it occurs in a smaller mass, taking up less space for disposal.

If you encounter the right conditions this may be an option for you. Should you want or need to implement phytoremediation on your site consider some of these plants:

Some Hyperaccumulators:

Solidago canadensis, despite its invasive tendencies abroad, Canada Goldenrod has significant potential for cleaning up soils. From chromium to zinc, lead, mercury and silver this plant is a workhorse in the garden and works magic below the soil too.

Solidago canadensis

Have concerns about copper or lead? Eryngium yuccifolium may be an answer for you.

Eryngium yuccifolium

Even the diminutive violet has soil cleaning capabilities.

Viola walteri Silver Gem

Looking for grasses that will clean the soil? How about the Tufted hairgrass? Plant a fine-textures evergreen grass AND clean up your soils.

Cool-season Deschampsia caespitosa

In addition to beauty the many ecosystem services offered by native plants are often touted as reasons to plant them. Phytoremediation, or their ability to accumulate heavy metals and other contaminants out of the soil, is yet another to add to the list.

Interested in learning more about this topic? You may want to check out the new book Phyto.