One of the biggest problems with roadside salt runoff is that it is nearly impossible to extract from the water table once it’s there. Halophytes—plants that tolerate or even thrive in salty conditions—might be a way to mitigate salt near its source.
As part of her master’s thesis, lab and research specialist Andrea Renshaw from the Virginia Polytechnic Institute and State University studied the possibility of utilizing halophytes as a means of capturing salt runoff along roadsides.
Roadside salt, Renshaw says, is incredibly detrimental to plant and soil health. It kills plants—not just aboveground, but also the root networks that promote soil stability underground—and it makes soil more likely to wash away or compact through a process called dispersion. It also makes toxic metals more bioavailable to plants and animals, and it can render drinking water dangerously salty.
“Trying to find a way to stop that salt from hitting the water table in the first place is kind of a priority right now, which is where halophytes come in,” Renshaw says.
Halophytes fall into three main categories: salt excluders, salt excreters, and salt accumulators.
Accumulators are generally the most useful, Renshaw says, because they are capable of absorbing salt through their roots and storing it in their tissues—sometimes as much as 20 percent of the plant’s dry weight.
“Once the salt’s been accumulated up into the biomass, what’s cool about the accumulators is you can then harvest the aboveground biomass and physically pick some of that up and move it somewhere else,” Renshaw says.
Excreters, by contrast, do not store salt—instead, they excrete it onto their leaves, whereupon the wind can disperse the salt crystals. This has its own uses, Renshaw says—excreters can be planted close to the roadside as a way of dispersing salt over a wider area and making it easier for accumulators to uptake the salt without being overwhelmed.
Excluders are the least overtly useful when it comes to remediation, since their salt-tolerance strategy is to not absorb any salt at all. However, they can produce salt-hardy root networks that can stabilize soil and promote incidental beneficial species such as bacteria and fungi.
When selecting halophyte species, Renshaw broadly recommends factors that should be taken into account:
- Low-maintenance plants that are perennial or self-seeding annuals are ideal for reducing labor needs.
- Plants need to be winter- and drought-resistant, and they need to handle a wide range of salt concentrations.
- A biodiverse blend of different species will generally establish faster and be more resilient than a monoculture.
- Invasive species need to be avoided.
Renshaw also warns that halophytes have their limitations: They can only absorb salt as far down as their roots extend (about six inches), and it is difficult to find species that germinate early enough to catch the worst of the spring salt runoff. Halophyte ecosystems might also take two or three years before they become fully established and effective.
“You want to invest the time into it,” Renshaw says. Renshaw presented her findings as part of the 2023 Salt Symposium in August. The event was organized by Bolton & Menk.
—Sophie Koch, contributing writer