Salt Tolerance of Phragmites australis and Effect of Combing It with Topsoil Filters on Biofiltration of CaCl2 Contaminated Soil

De-icing salt used for safe winter driving can have negative impacts on local water quality, vegetation, and soils. This study aimed to evaluate the salt tolerance of reeds (Phragmites australis) against calcium chloride (CaCl2) and the biofiltration effect of combining it with topsoil biofilters for desalination in roadside ditches. Two experiments were conducted in a controlled environmental greenhouse over a period of 150 days. For the first experiment, the salt tolerance of P. australis was examined after treating reeds with five different concentrations of de-icing salt: 0, 1, 2, 5, and 10 g·L−1. In a second experiment, the effect of combining two topsoil filters (expanded clay and activated carbon), each planted with and without reeds, was investigated under a high CaCl2 concentration of 10 g·L−1. As the CaCl2 concentration increased, the electrical conductivity (EC) of soil leachate and the level of salt exchangeable cations (K+, Ca2+, Na+, and Mg2+) significantly increased whereas the acidity (pH) significantly decreased (all p ≤ 0.05). No statistical difference was observed in leaf length or width, while plant height, number of leaves, and both fresh and dry weights were significantly increased with increasing CaCl2 concentrations (p ≤ 0.05). Treatments using topsoil filters, particularly those with activated carbon and reeds, showed the greatest reduction in leachate EC and total exchange cations values. These results suggest that combining P. australis with topsoil filters can assist biofiltration effectively, demonstrating its applicability even in roadside soils subject to extreme levels of de-icing salts.