University of Maryland Extension

Leaf Yellowing-Environmental Stress - Annuals, Bulbs, Groundcovers, Perennials, and Vines

                                                  stunted impatiens

Leaf yellowing is often one of the first symptoms of plant stress. If the stress causing condition is not alleviated, yellow leaves may turn brown. A prolonged period of environmental stress causes overall stunting and poor growth. Noticing the pattern and progression of symptoms will help to diagnose the cause of the problem.

Many times plants exhibit symptoms that are attributed to insects or disease when the real problem is in the plants' environment. A plant problem that is not caused by a living organism is called "abiotic" injury. Abiotic injury may be caused by drought or excess water, exposure to low light or extremely sunny conditions, or may be associated with poor soil. Environmental factors that contribute to abiotic problems, in many cases, cannot be changed. Selecting plants that are suited to a location becomes extremely important. Sometimes gardening practices, such as planting procedures, time of planting or fertilizer application, can lead to abiotic injury. 

Drought

Plants wilt and leaves curl when roots are unable to supply sufficient moisture to the stems and leaves. Wilting for short periods of time does not harm plants. Sometimes a plant wilts on a hot day because moisture is evaporating from the leaves faster than the roots can supply it. If there is ample soil moisture, the plant will absorb water in the evening to firm up the stems and leaves. Over a prolonged period, however, drought will cause serious damage, such as yellowing, leaf scorch, browning, or leaf drop and stunted growth. Extended periods of drought also inhibit flower formation. Severe heat and water stress when a plant is in bloom may cause scorching or browning of flower buds and blossoms. Plants vary in their ability to tolerate drought and some may die suddenly after extended periods of drought.

drought stressed salvia
Drought damaged annual salvia

Excess Water

Problems with excess water can result from poorly drained soil or overwatering. Excess water reduces oxygen in the soil, which damages fine roots and renders the plant unable to take up water. Plants exposed to excess moisture show the same symptoms as plants under drought stress. The primary symptom of excess moisture is wilting or yellowing of lower and inner leaves. If excess water continues, plants may show other drought symptoms, such as scorch, leaf drop, and/ or plant death. To avoid problems, select plants tolerant of moist soils or adjust watering practices to allow soil to dry between watering.

daylilies with yellowing leaves
Daylilies growing on a wet site

Insufficient Sunlight

Many plants require full sun for optimum growth. Sun loving plants growing in partial sun or shade are unable to produce as much growth as a plant growing in full sun, due to reduced photosynthesis. Plants growing in shade will exhibit elongated spindly growth, with fewer side shoots, and thin pale colored or yellow leaves. The leaves are usually larger and thinner (providing more surface area for photosynthesis) than leaves of the same species growing in full sun. Flowers are reduced in size and quantity. Often plants are grown in the shade of nearby trees. Tree roots compete for available water and usually win, causing further stress on plants growing in the shade.

Excess Sunlight

Some plants grow best in partial sun or shade. When shade-loving plants, such as hosta or impatiens, are planted in full sun, leaf scorch or sunburn can occur. Strong sun and heat causes the breakdown of chlorophyll in the leaf. Damage appears as pale, bleached or faded areas. These areas eventually become brown and brittle. Symptoms are more severe when strong sun is combined with dry soil conditions. To avoid problems caused by excess sunlight, select plants suited to the growing conditions in your yard. Most shade loving plants will tolerate morning sun, as long as adequate moisture levels are maintained. Avoid planting shade tolerant plants in locations exposed to strong mid-day sun.

Sunburn may also occur on greenhouse grown plants that are adapted to low light conditions and planted directly outdoors without a "hardening off" period. To avoid sun scorch on greenhouse grown plants, place plants in full sun for 2 hours per day initially and gradually increase the length of sun exposure.

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