During the latter part of February and early
March, the harmful effect of excessive use of de-icing salts used along
sidewalks and roadways becomes crystal clear.
Salts deposited on beds and branches can cause damage ranging from
complete branch dieback, browning of the needles or foliage, to lack of
flowering, and tufting, which is also known as witches broom.
Grass along sidewalks can be completely dead due to overuse of salt.
"Witches Broom" on Tilia (Linden).
Years ago, I can vividly remember the lawsuit
conducted by Schenck Farms and Greenhouses of St. Catharines with the Province
of Ontario. Because of the
proximity of the Queen Elizabeth Highway and the resultant drift of salt spray
onto their nearby peach orchards, this condition resulted in a significant loss
of fruit production. It took years
for Schencks to successfully win their case.
Salt damage on turfgrass.
Road salt can also cause damage to the plants
when it is absorbed by the roots and accumulated in the plant to toxic levels.
Root physiology is drastically affected.
Photosynthesis is dramatically reduced as is the production of
chlorophyll. Root dehydration is
increased due to the fact that the solute concentration is higher outside of the
cell than it is inside the cell. In
a nutshell, when salt goes into solution it interferes with water uptake by the
plant. Water leaves the cell faster
than it may be absorbed due to the fact that the sodium ions in the salt tend to
replace both the phosphorus and potassium ions in the soil, making them
unavailable to the plant. With a
high accumulation of salt around the roots, plant growth is adversely affected
and the roots die of drought.
Pinus strobus (White Pine) showing damage on windward
On a small scale, roadside plants can be limitedly
protected by wrapping plants with burlap, or constructing burlap or durable
plastic screens along fence lines to shield them from traffic splash.
At the homeowner level, avoid sodium and calcium chloride salts near salt
sensitive plants. Use other
alternative products that are not harmful to plants.
When salt is used, avoid throwing salt laden snow onto adjacent shrubs or
Oakes Garden Theatre-Niagara Falls, Ontario.
Protection of boxwood hedge from salt damage.
In 1974, G.P. Lumis, Department of
Horticulture Science, and G. Hofstra and R. Hall, Department of Environmental
Biology, University of Guelph, published results of their research on Salt
Damage to Roadside Plants. Their
description of injury symptoms for trees and shrubs is as follows:
specific to evergreens:
needle browning moderate to extreme, beginning at the tip;
needle browning and twig dieback on the side facing the road
but none or very little on the back side;
no needle browning or dieback in branches near the ground
under continuous snow cover;
needle browning and twig dieback less severe further from
browning usually first evident in late February or early
March and becoming more extensive through spring and summer.
specific to deciduous plants:
leaf buds on the terminal part of branches facing the road
very slow to open or do not open;
new growth arises from the basal section of branches facing
the road, resulting in a tufted appearance;
flower buds on the side facing the road do not open but
flowering normal on back side.
injury more severe on side facing the road, plants one-sided
due to branch dieback;
damage more pronounced on downwind side of highway;
plants further from road injured less;
branches covered by snow not injured;
injury to evergreens apparent in late winter, injury to
deciduous plants not evident until spring;
branches above the spray-drift zone not injured or injured
damage increased with the volume and speed of traffic and
the amount of salt applied to highway;
plants damaged over several years lack vigor and soon begin
less winter-hardy plants injured more severely;
salt spray penetrates only a short distance into dense
plants in sheltered locations lack injury symptoms.
salt is used, it should be used as limitedly as possible.
Better than this, alternative products should be considered for use to
avoid ground water contamination as well as damage to landscape and roadside
plants. Some species of trees and
shrubs are much more tolerant of salt than others.
A selective list of salt tolerant plants is as follows:
||Acer campestre (Hedge
|Aronia spp. (Chokeberry)
(Siberian Pea Shrub)
|Cytisus spp. (Broom)
|Hamamelis spp. (Witch
angustifolia (Russian Olive)
|Hibiscus syriacus and cvs.
(Rose of Sharon)
|Hippophae rhamnoides (Sea
||Fraxinus americana (White
Ash and cvs.)
|Hydrangea spp.and cvs.
|Hypericum (most) (St.
||Ginkgo biloba (Maidenhair
|Lonicera tatarica 'Zabelli'
inermis and cvs. (Honey Locust)
xylosteum (European Fly Honeysuckle)
(Kentucky Coffee Tree)
||Nyssa sylvatica (Blackgum)
|Philadelphus spp. and cvs.
||Populus spp. (Poplar)
||Prunus x cistena (Sand
|Rhamnus frangula (Glossy
|Rhodotypos scandens (Black
||Quercus rubra (Red Oak)
|Salix and cvs. (Willow)
types) (Shrubby Roses, e.g.. Rosa rugosa)
||Sophora japonica (Japanese
|Sorbus (Mountain Ash)
(Silver Buffalo Berry)
|Tamarix ramosissima (Fivestamen
||Larix decidua (European
|Viburnum dentatum (Arrowwood)
||Larix kaempferi (Japanese
||Picea glauca (White
|Picea pungens glauca
(Colorado Blue Spruce)
||Pinus banksiana (Jack
|Yucca filamentosa (Adam's
||Pinus mugho mughus
||Pinus nigra (Austrian
|Pinus parviflora (Japanese
|Arctostaphylos uva-ursi (Bearberry)
||Pinus ponderosa (Ponderosa
|Erica spp. (Spring Heath)
||Pinus thunbergii (Japanese
||Taxodium distichum (Bald
|Vaccinum and cvs.
It should be clearly understood that salt
tolerance for plants depends upon a great number of factors as previously
discussed. The above list is
representative of plants that will grow in an environment that is periodically
subjected to air-borne road salt spray.
Salt damage on Taxus (Japanese Ewe)
Acer platanoides, the Norway Maple, has been
purposely left off this list. While
this tree is incredibly hardy and very resistant to salt spray damage, there
are other good reasons why I feel that it should not be used as a roadside
tree or used nearly as extensively as it is in the landscape.
This will be another article appearing in Hort-Pro at some meaningful
time in the future.
Lumis, G.P, Hofstra, G., and Hall, R.
Salt Damage to Roadside Plants in Ontario Shade Tree Newsletter,
Vol.5, No.1. Toronto, 1973.
Lumis, G.P., Hofstra, G., and Hall, R.
Plants Resist Salt in Landmark 2(4), 1990.
John A. Morley
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