Across most of Canada, there always seems to be a dry, clear day in late winter when a telltale, greyish white film coats every available outdoor surface. The residue is immediately recognizable as the accumulation of road salt, and its pervasiveness is astonishing.
Spring rains may wash away the visible evidence, but the salt doesn’t disappear. Much of it goes into the water table or finds its way into gardens that lie beside roads and walkways, where the salt can lurk in heavier soils for up to three years.
According to Environment Canada, five million tonnes of road salt is applied each year to sidewalks, streets and highways across the country (the highest volumes are used in Quebec and Ontario, the lowest in the western provinces). In 2001, following a five-year study, the federal government concluded that road salt is harmful to aquatic ecosystems, plants and wildlife, and recommended control measures (such as pre-wetting the salt or spraying it as a brine).
Of course, it’s not just gardeners with patches near roadsides who have to worry about too much salt in their plants’ diets; those by the sea must also contend with its ravages. However, though both conditions dictate the need for salt-tolerant plants, seaside green thumbs may actually have an easier time licking their salt problems thanks to soil composition (see Gardening by the seashore for more information on seaside gardening).
Researching salt-tolerant plants
But finding a comprehensive list of salt-tolerant herbaceous plants, shrubs and trees can be difficult. That’s why Laura Deeter, a professor of horticulture at Ohio State University Agricultural Technical Institute, decided to do graduate work on salt-tolerant perennials.
Deeter chose 38 perennials, four or five of which she thought might withstand salt (the others were just common plants) and applied varying amounts of salt water to them in a greenhouse. In the final stages of her research, she spread sodium chloride around several of these specimens on an outdoor field.
Deeter found that the negative effects of plant contact with salt included black or brown leaf edges and spots, leaf drop, plant death, delayed flowering and stunting, as well as plants that simply refused to surface come spring—such as ‘Blue Stocking’ bee balm (Monarda 'Blaustrumpf').
"Some of [the results were] as expected, that certain plants just don’t tolerate any kind of salt in the soil," says Deeter. She also noted that plants often thought of as tough—members of the Asteracae family, black-eyed Susans (Rudbeckia hirta) and purple coneflowers—were not at all amenable to salty soil.