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Salt on roads can harm evergreens

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Wednesday, February 27, 2008
Q. My husband always puts salt on the sidewalk and driveway to melt snow and ice. I believe that the salt he's been using has damaged one of our pines near the driveway. With the recent snowstorm, he once again applied liberal doses of salt on our walkways. How harmful is salt to our pine trees? -- Carol F., Metuchen

A. Many evergreens, including white pine, Norway pine, Norway spruce, white spruce, Canadian hemlock and yew, are susceptible to salt injury. Some evergreens can be very sensitive to chloride ions, which may accumulate in needles and cause plant damage.

De-icing salts dissolve in water to form a solution that lowers the freezing point of water, melting existing ice and preventing further ice formation. The salty water filters down to the soil, where it can affect nutrients, soil microbes and plant roots.

Excessive use of de-icing salts can displace potassium and phosphorus in soils. These salts also may lead to increased compaction and poor aeration. In addition, excess salt can hinder water absorption, causing drought symptoms in extreme cases.

The chlorides in salts can be absorbed by roots and leaves and may accumulate to toxic levels, resulting in tip burn or the death of entire leaves, needles or twigs. Salt can cause direct damage to sensitive plant parts facing roads after repeated applications. Plants that are weakened by excessive salts can be more susceptible to disease and pest problems.

The best way to prevent damage is to reduce the use of all de-icing salts. Mix salt with sand or kitty litter to provide traction while reducing the amount of salt applied.

Alternatives to the traditional sodium chloride include potassium chloride. Potassium chloride is safer for plants but also must be used in moderation. It does not displace the nutrients in soils or worsen compaction to the same degree as sodium chloride. Another de-icing product is calcium magnesium acetate (made from limestone and acetic acid), which is free of harmful chloride ions.

If excessive de-icing salts have been used around plants or plants have been blasted with salt spray, simply wash off excess salts with fresh water or dilute salts in soils around plants with clean water.

Q. I have several rhododendrons on one side of our yard that have been fading slowly over the last several years. We had to remove a large maple tree about three years ago which provided a great deal of shade for the rhododendrons. We planted an oak, but it is still small and does not shade the area. The rhododendrons are very large plants and I don't believe they could be moved. Can I save these plants?
-- Alice B., North Brunswick

A. Rhododendrons and azaleas are found in nature as woodland understory plants. Many cultivars thrive in 15 percent to 30 percent shade or dappled sunlight. In addition, these plants are found in soils with high organic matter and adequate drainage.

I believe you can save these plants if you do several things. First, make certain that both plants have 2 inches to 3 inches of mulch around them that extends out to the drip line or to a point below the outermost tip of branches. Plants must receive adequate moisture throughout the year. Be careful not to overwater plants, and continue to provide adequate drainage for roots. Take a soil test and adjust the soil pH to below 6.

It also may be wise to add a little leaf compost around shrubs before adding the bark mulch. Use a judicious amount of acidic fertilizer around plants. Some of the larger leafed rhododendrons are particularly sensitive to direct afternoon sun. If you can construct a shade cloth or arbor to protect plants from afternoon or direct sunlight, this may relieve plant stress and help plants to recover. However, the best thing you can do to save them is to provide organic mulch and keep plants from drying out. Do not disturb the soil around the shallow roots of rhododendrons. Add the organic materials to the surface and let them break down naturally to improve soil conditions.

Some PJM hybrids (named for Peter J. Mezitt, founder of Weston Nurseries in Massachusetts) and other select rhododendron cultivars can thrive in full sun. These plants still require high organic matter soils with adequate moisture.

Your rhododendrons adapted to the previous shade from the large maple tree, but may still make a comeback. Plants may adjust to the new conditions over time if they receive adequate moisture and a slow, steady release of nutrients.

Resources: Visit njaes.rutgers.edu or ifplantscouldtalk.rutgers.edu for more information on plants and gardening.