February, 2005

Avoiding salt damage to trees along roadsides

by MARY M. WOODSEN

Heavy snows in the Northeast and beyond mean highway departments have laid on the salt a bit thicker than usual this winter. On well-traveled highways, that can amount to 40 tons to 80 tons of salt per lane each year. Yet we won't see the effects until we've long since forgotten the snow and ice.

"Road salt damage can mimic the symptoms of plant disease," said Jana Lamboy, ornamentals specialist with the New York State Integrated Pest Manage­ment Program. "Knowing how to distin­guish abiotic problems like this from insect and disease damage is critical in choosing the right treatment - and avoiding the wrong one:"

According to Lamboy, trees that are stressed due to road salt damage are also more susceptible to insect and disease problems, as healthy plants can create physical barriers that prevent attacks. "The epidermal cells at the plant surface add lay­ers, such as waxes and tannins, to cell walls even as they repair wounds," Lamboy said. "But stressed plants aren't able to muster their normal defense mechanisms:"

The worst damage is along heavily trav­eled roads where cars and trucks speeding through slushy snow spew a fine salty mist into the air. Brian Eshenaur, horticulturist at Cornell Cooperative Extension, Mon­roe County , NY , said this fine aerosol can coat needles and twigs on trees up to 150 feet from a highway. "At high concentra­tions, the salt burns the needle tips of conifers," he explained. "It's not just on new growth but on older needles, too. When the tips are burnt back, it's likely that the needles will fall off within a year."

Eshenaur noted deciduous trees gener­ally suffer less because bare stems aren't as sensitive to salt. But salt can enter twigs through bud scales and leaf scars and kill dormant buds. Flower buds are especially vulnerable - look for reduced flower and seed set on black walnuts, lindens and sus­ceptible maples this year. Sometimes salt damage even causes witches'-brooms to erupt out of branch ends.

Salt that washes into the soil near roads and walkways can slow growth and reduce vigor by interfering with nutrient avail­ability. The sodium can restrict uptake of magnesium and potassium, two nutrients essential in making chlorophyll.

In fact, if the brine in the soil is more concentrated than the sap in the roots, you've got a chemical drought; the roots can't even take up water. "Many plants synthesize defense com­pounds that taste bad to insects or are toxic to microbes that cause diseases;" said Lamboy. "Manufacturing them takes extra energy and resources that a plant doesn't have if it's struggling just to take up water and nutrients:"Young trees are more sus­ceptible than mature ones. Indeed, research suggests that salt promotes soil compaction by chemically disrupting the natural aggregation of soil particles, thus causing the soil structure to fall apart. A tree in a poorly drained site or downhill of a heavily salted road is at greatest risk.

What mental "search image" should green industry professionals use for diag­nosing salt damage? "Look during early summer for marginal leaf scorch on deciduous trees - yellow, brown or fallen needles on evergreens, especially on the side toward the road," suggested Eshenaur.

Because salt damage can mimic dis­eases such as bacterial leaf scorch or injury to the inner bark, Eshenaur stressed pro­fessionals evaluate the entire site. "You need to look for other sorts of damage, too," he explained. "Anything that affects the trunk or roots, like putting in a curb or hitting the tree with a lawn mower, could produce similar symptoms:"

Mary M Woodsen is a science writer for the New York State Integrated Pest Manage­ment Program, Cornell Cooperative Exten­sion Service. Cornell Universitv. Tthara.

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AMERICAN NURSERYMAN MARCH 15, 2003