Signs of environmental stresses on plants can resemble those of disease.

Signs of Plant Stress

by Janet Beal

Drooping leaves, withering buds, ragged leaves or odd-colored foliage may be symptoms of disease or insect damage to a plant. Equally likely, though, is that your plant is suffering from environmental stresses. Drought, excessive heat or cold, too much water and the wrong soil chemistry can all interfere severely with healthy plant growth. Especially if a plant seems to be struggling chronically, it may be showing symptoms of stress rather than sickness.

1. Common Stress Symptoms

Frequently seen signs of stress in plants are climate-related. Major symptoms include wilting and/or drying of foliage, and drooping or crimping of stems. Every plant has inherent heat and cold tolerances, usually described as hardiness. A plant rated as hardy in U.S. Department of Agriculture plant hardiness zones 3 through 6, for example, can maintain normal growth in climates with zone 3 winter temperatures as low as -40 F. According to the American Horticultural Society's heat-zone map, the same plant could be expected to tolerate the 46 to 60 days of summer heat over 86 F characteristic of USDA zone 6. Within those parameters, the plant is able draw up water and nutrients through roots, absorb sunlight, and absorb or excrete water vapor, carbon dioxide and oxygen through stomate cells in leaf tissue. Any disruption to the balance of temperature and moisture required by the plant manifests in limp tissues, perhaps to a damaging degree.

2. Heat Stress

Excessive heat can produce wilting in even healthy plants. Hydrangeas (Hydrangea spp.), hardy in USDA zones 3 through 9, for example, routinely wilt in the heat, reviving as temperatures drop. When wilt is frequent or severe, the low water pressure in leaf tissues permits them to burn in the sun. Wilting can often, but not always, be minimized by keeping soil consistently moist. Locating wilt-prone plants in partial shade also helps to reduce wilt and burn damage.

3. Cold Stress

Also partially related to soil moisture, winter damage can manifest in evergreen shrubs and trees with brown spots or burned needle tips which may not become apparent until spring or summer. Water in cells swells and bursts tissues when frozen. Damaged tissues absorb water poorly or not at all. Tissues can be further damaged by ice- and snowstorms, leaving dark or black areas of damage on leaves. During cold, dry winters, homeowners may forget to provide extra soil moisture, causing further damage to roots and eventual overall plant health.

4. Drought and Drowning

Dry soil produces what is called matric stress on plants, forcing cells to expend greater energy to draw in water. Prolonged dryness in soil may allow fragile feeder roots to wither, making water-drawing impossible. Wilting progresses to stem droop as plant tissues collapse. Plants tend to die from branches or leaves inward, and in large plants, damage may emerge gradually over several seasons. Symptoms range from burned needle or leaf tips in spring, through branches that fail to flower or fruit in summer, to allover weakness and winter-kill the following year. Maintaining regular watering and trimming back dead sections may enable large plants to survive extended drought. Too much water, however, causes as many problems as too little. Plant roots require both water and the oxygen trapped in tiny soil air spaces to absorb nutrients. Overwatering or chronically wet conditions create oxygen deficiencies, making the processing of nutrients impossible. Like other oxygen-dependent organisms, plants can drown in too much water.

5. Nutrient Deficiencies

A plant placed in soil that does not provide enough nutrients may show symptoms of stress soon after planting. Established plants may also show stress symptoms if necessary nutrients are depleted or soil chemistry is changed. Deficiencies in major essential nutrients, nitrogen, phosphorus, potassium, and minerals needed in lesser quantities, like calcium, iron, manganese and magnesium, are often most apparent in foliage but can affect flowering and fruiting as well as overall growth. Calcium deficiency, for example, produces blossom-end rot in tomatoes (Lycopersicon esculentum spp.), varieties of which are hardy in USDA zones 2 through 10. Plants in iron-deficient soil often have pale or yellowish foliage, a symptom called chlorosis. Testing soil before planting will tell you whether you need to add minerals your plants will need for healthy growth.

6. Soil pH

Soil ph, or the amount of potential hydrogen ions available to make nutrients available to plants, is critical to stress-free plant growth. The pH scale has a range of 14 points, with scores below 7.0 representing acid soil and those above representing alkaline soil; 7.0 is rated as neutral. For example, acid-loving rhododendrons and azaleas (Rhododendron spp.), hardy in USDA zones 4 through 9, need a soil pH of between 4.5 and 6.0 to permit roots to access and process nutrients; in a highly-alkaline soil, they starve and die. Soil salinity, related to pH, also determines how well plants can process available nutrients. Like those of humans and animals, plant metabolisms may have very specific needs. When needs are not met, stress symptoms appear.

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