University of Vermont Extension
Department of Plant and Soil Science
Fall News
Article
SALT DAMAGE TO
PLANTS
Dr. Leonard Perry, Extension Professor
University of Vermont
Most people are only too aware of the damage and
corrosive effects of salt on automobiles, but may not be aware of
damage that
winter salt on roads and walks can cause on plants. On heavily
traveled
highways from 40 to 80 tons of salt per lane mile per year may be
applied.
Landowners along these roads also are aware of the damage to plants
that such
salt can cause.
Deicing salt is usually refined rock salt consisting
of about 98.5 percent sodium chloride, 1.2 percent calcium sulfate,
0.1 percent
magnesium chloride, and 0.2 percent rock. Calcium chloride is
reported to be
less toxic to plants but is seldom used on roads because it is much
more
expensive than rock salt and more difficult to handle in bulk.
When sprayed onto plants from passing cars and plows,
salt may enter plant cells or the spaces between the cells directly.
One result
of this "salt application" is that buds and small twigs of some
plant
species lose cold hardiness and are more likely to be killed by
freezing.
Salt accumulation in the soil also may cause plant
injury. This frequently occurs when salt-laden snow is plowed off
streets and
sidewalks onto adjacent lawns.
Anyone who has tried to get table salt out of a wet
shaker knows how readily salt absorbs water. Rock salt exhibits the
same
property in the soil and absorbs much of the water that would
normally be
available to roots. Thus, even though soil moisture is plentiful,
high amounts
of salt can result in a drought-like environment for plants.
When salt dissolves in water, sodium and chloride ions
separate and may then harm the plants. Chloride ions are readily
absorbed by
the roots, transported to the leaves, and accumulate there to toxic
levels. It
is these toxic levels that cause the characteristic marginal leaf
scorch.
The symptoms of
excessive salt resemble those caused by drought or root injury. They
include
stunted, yellow foliage; premature autumn leaf coloration; death of
leaf
margins (scorch); and twig dieback. When
conifers are injured by salt spray, the affected foliage turns
yellow or brown
in early spring. If spray is the primary cause of the salt deposit,
discolored
needles are soon masked by the new season's growth. However, if salt
is
excessive in the soil, the new needles may die as chloride ions
accumulate in
them. This could be lethal to the entire plant if it occurs for
several
consecutive years.
One characteristic
of salt injury that aids in diagnosis is that it is often confined
to branches
facing the road. Trees closer to the road suffer more damage than
those set
farther back. You can often observe this on pines and other
evergreens near
interstates where they’ve been affected by the salt spray thrown
from passing
plows at high speeds and with wind.
Here are 10 measures you can use to prevent or lessen
plant injury from salt.
-- Rather than sodium chloride, use calcium or
magnesium or potassium chlorides. These
are more expensive but you need to use much less. Or use sand,
cinders, or one of the
relatively new and safer deicing products (these are often liquids
and consist
of chemicals more akin to fertilizers). Liquid products wont track
about
outside or indoors as do granular products.
--In the coldest areas, use a calcium chloride product
as it works down to about 5 degrees (F).
In contrast, rock salt only is effective down to about 15 degrees
(F). Other products fall in
between. If below 5 degrees (F), don’t
use chemicals but rather sand for some traction if really icy.
-- Pre-apply any chemicals prior to icing or snow, as
they are more effective then and you can use less. Don’t apply on
top of snow, rather shovel it
away and then apply.
-- Generally use less salt and deicing chemicals, as
many landowners tend to apply too much—small applications more often
are more
effective.
-- Late season applications (after March 1) are most detrimental
and should be avoided if possible since this is the time plants are
coming out
of dormancy and are most susceptible to injury.
-- Screens of fencing or burlap may be erected to ward
off salt spray from roads.
-- Salt and snow should not be piled around plants or in
places where the resulting salt water will drain into plants when
the snow
melts.
-- Construct a shallow trench between walks or drives
and plants or lawns. This will help
channel salt runoff away from desirable garden areas.
-- If weather permits, it's a good idea to flush the
area around roots exposed to salt with fresh water as soon as the
snow melts.
-- Where new trees and shrubs are to be planted and
where exposure to salt is likely, select species or cultivars
resistant to salt
spray injury. Examples of salt-tolerant evergreens include white
spruce and
Austrian pine.
Salt-tolerant deciduous trees include ash, birch, honey locust,
poplar, tamarack (larch), white oak, red oak, mountain ash, and
weeping willow.
Salt-tolerant shrubs include serviceberry, broom, pea shrub,
hydrangea,
winterberry, northern bayberry, mockorange, potentilla, most shrub
roses,
tamarisk, and spreading cotoneaster.
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