Problem type: Environmental
Name of problem: Improper lighting
Plant name(s): Any house plant
Symptoms / Characteristics:
Plants that do not receive enough light exhibit spindly growth and often "bend" towards the light. Stems have elongated internodes (spaces between leaves) and lower leaves become pale and thin. New growth is small and pale. Flowering is inhibited and the buds that do form often drop from the plant without opening. Plants with variegated leaves lose their characteristic leaf colourations and often turn completely green.
Plants that receive too much light exhibit a compact growth habit and often wilt during the day due to rapid water loss. Leaves often curl downward and appear pale or bleached due to the breakdown of chlorophyll. Yellowish-brown dry patches develop on leaves and flowers. Flower buds are often sacrificed in order to redirect energy into maintaining the leaves. When the buds do open, the flowers are small and short-lived.
Sudden changes in lighting can also have a negative effect on plant health. For example, plants that have been produced under high-light greenhouse conditions often drop leaves when relocated to low-light home conditions and severe leaf drop can result in plant death. This is quite common in Ficus species. Conversely, plants that are moved from a fairly shady location to a sunny window often exhibit signs of scorching and wilting. Even plants that prefer bright lighting will react this way if they are not properly acclimatized
Control / Preventions:
All plants require a certain amount of light in order to produce food via photosynthesis. Optimum light levels vary from plant to plant; certain plants perform best in bright sunshine and others prefer shade. At the time of purchase, select plants that will adapt well to their future location in the home. Plants that tolerate low light conditions include Chinese evergreen, pothos, philodendron, Norfolk Island pine, arrowhead plant and some ferns. Variegated plants and flowering plants often (but not always) prefer bright light. In areas of the home that receive high-light, high-heat conditions, try growing cacti and succulents that are better equipped to tolerate moisture stress. Most house plants fall somewhere in between the two extremes, preferring moderately bright, indirect light.
Plants that exhibit symptoms of light deficiency should either be moved to a brighter location or supplemented with artificial light. There are many types of artificial lamps available and different combinations may yield different results depending on the plants in question. When supplementing with artificial lighting it is important to ensure proper spacing between the lights and the plants. Lights that are placed too close to the plants may burn the tissue and lights that are placed too far away may result in spindly growth. Once the light deficiency has been corrected, pinch spindly plants back in order to encourage compact growth. Plants should be turned on a regular basis in order to avoid "bending" and promote a more uniform growth habit.
Plants that exhibit symptoms of excess light are usually suffering from the heat stress that accompanies that light. Essentially, direct sunlight causes rapid heating of the plant tissue and the plant loses water at a faster rate than the roots can compensate. The wilted plants often recover overnight. To correct the situation, the plants should be moved to an area that receives only a small amount of direct sunlight. Another option is to filter or shield the light, especially during the hottest part of the day, in order to reduce the amount of exposure to direct light. This can be achieved by using sheer curtains or partially opened blinds in south facing windows.
Plants must be properly acclimatized when moving them from one location to another. The plants should gradually be moved closer to the intended location over a period of at least a week. This will help to avoid sunburn and leaf drop symptoms that often result from abrupt changes in lighting. This principle applies to plants that are introduced into the home, plants that are moved within the home, and plants that are placed outdoors for the summer. When moving plants, it is also important to adjust water and fertilizer applications accordingly. Plants in high light conditions are more active and tend to use soil nutrients more rapidly than low-light plants. Supplying a low-light plant with the same fertilizer concentration as a high-light plant may actually result in nutrient toxicity. Plants in high light conditions also require more water than low-light plants due to higher evaporation and transpiration rates associated with increased temperatures. Avoid overwatering low-light plants.
Relevant web sites:
Blashfield, J.F. 1980. The Healthy House Plant. Little, Brown and Company Limited, Toronto. 328 pages.
Chase, A.R. 1987. Compendium of Ornamental Foliage Plant Diseases. The American Phytopathological Society, Minnesota. 92 pages.
1976. Garden Fax: House Plants – Lighting. Alberta Agriculture, Alberta. 2 pages.