University of Maryland Extension

Chlorine Toxicity - Trees and Shrubs

chlorine damage on dogwood

Back to Common Problems - Trees and Shrubs

Scorched leaves may be the result of chlorine toxicity. Chlorine is a micronutrient, essential to plant growth. However, too much chlorine can accumulate in leaf tissue, resulting in leaves with a scorched or burned appearance. Trees with scorched leaves have brown or dead tissue on the tips, margins, or between the veins of the leaf. Leaf tissue may appear bleached, instead of scorched. Leaves may be smaller than usual. They may yellow and drop early. Chlorine toxicity can result from air pollution, in the form of chlorine gas, or from excess chloride in the soil.

Excess chloride can build up in the soil from swimming pool runoff, irrigation water, or excess soil salts (sodium chloride). Chlorine (Cl) converts to chloride (Cl-) in the soil and is absorbed by plants in this form. Chloride toxicity is most common in irrigated, dry regions, seacoast areas, and near roads frequently treated with salt in the wintertime. Chloride levels can be reduced with the use of gypsum. Incorporate gypsum into the soil at a rate of 58 lbs. per 1000 square feet, in loam soils. Less gypsum is needed in sandy soils, more in heavy clay soils. Water thoroughly to leach toxic levels of chlorine from the soil.

Damage to plants from chlorine gas is less common than damage from other air pollutants, such as sulfur dioxide, fluoride, and ozone. Chlorine gas is a by-product in the manufacture or incineration of glass, plastics, paints, and stains. It is released from refineries or as a result of chemical spills. Reducing air pollution at its source is the best solution to reduce damage to plants and people. Careful watering practices can reduce air pollution damage to plants. Soil should be dry during periods of exposure to air pollutants, followed by thorough watering after exposure. Wetting the leaves of sensitive plants may help to reduce damage during periods of poor air quality. Trees sensitive to chlorine are ash, boxelder, Siberian crabapple, dogwood, horse-chestnut, silver maple, sugar maple, pin oak, sweet gum, and yellow-wood.

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