As the autumn leaves fall outdoors, so do leaves often fall from our houseplants that recently were moved back indoors after enjoying an outdoor respite over the summer. Although this is a bit alarming to a plant enthusiast, the houseplants in question are not necessarily dying. They are simply acclimatizing to life back indoors.
Acclimatization is a term that refers to the adaptation of a living organism to a change in its surroundings. Humans are no exception. The 40 and 50 degree F. nights that seem cold to us in autumn will feel relatively warm, should they occur next February, because we will have acclimatized to cold temperatures by that time.
Houseplants plants are frequently moved outdoors for the summer for a brief reprieve from the poor light and other austere growing conditions typical of an interior setting. No matter how bright an interior room might appear to be, it pales in comparison to the amount of light encountered by plants placed outdoors for the summer. Houseplants often respond by putting on a "flush" of growth under the more ideal conditions. Upon being moved back indoors, lighting is not sufficient to maintain this additional growth.
The most obvious response occurs several weeks after a plant has been moved from an area of fairly bright light to poorer light. The cause for the problem is related to a variable known as the light "compensation point" of the plant. The latter is the point where the amount of sugars and carbohydrates (food) manufactured by the plant through the photosynthetic process equals the amount of the same materials used by the plant during respiration.
At light levels above its compensation point, a plant continues to grow because the food manufactured via photosynthesis exceeds the amount used by respiration. Conversely, when the amount of food used by respiration exceeds the amount manufactured during the photosynthetic process, the plant stops growing and loses leaves. The latter usually ceases when the light compensation point is reached (i.e., the rate of food manufactured from remaining leaves equals the rate of food consumed).
In order to avoid severe leaf drop, acclimatization need to be done gradually. If changes within the plant are fairly slow, new leaves begin to develop that are better suited to utilize lower amounts of light more efficiently. The latter are termed "shade leaves" and morphologically are different from traditional (sun) leaves. Shade leaves usually are larger, thinner and have light-capturing organelles known as chloroplasts more evenly distributed and horizontally oriented between the various layers of the leaf.
Perhaps most importantly, shade leaves have lower light compensation points than do sun leaves. Although they are not as photosynthetically prolific as sun leaves, they are much better equipped to make better use of lower amounts of light than are sun leaves. In short, the development of shade leaves allow full-sun plants to adapted to the low light intensites common to interior settings.
For the reasons described above, houseplants purchased from a retail outlet often shed some leaves after being placed in an interior setting. The plants in question most likely were produced in a nursery where light intensity was relatively high and other growing conditions were good. The shedding of a few leaves simply indicates the plant is acclimatizing to its new environment.
Pre-acclimatized plants are commercially available for those in the interior "plantscape" industry. These plants usually are larger plants that have been acclimatized in the production field or greenhouse by gradually reducing the amount of light they receive. This is accomplished by growing them under shade fabric. As more shade fabric is added, the light intensity experienced by the plants approaches that of a typical interior setting. Plants handled this way usually are a bit more expensive, but perform better when used in an interior setting.
Acclimatization of the root system is another consideration that must be taken into account. While a plant should not be allowed to wilt severely, extending the interval between watering and reducing the amount of fertilizer supplied to the plant can help in the adjustment process. Fertilizer rates should be reduced to about one-half of that supplied under normal production in bright light. This tactic reduces top growth while roots continue to grow. Applying less water also helps harden the plant's foliage so it will be less sensitive to the low humidity typical of indoor settings.
Conversely, take care not to overwater houseplants. It is generally agreed that more houseplants die from overwatering than from any other cause. Most houseplants will recover from slight wilting, upon being watered. At the other extreme, plants that are overwatered often lose their roots due to the stress caused by a lack of oxygen. Houseplants that suddenly wilt most often do so because they are losing/have lost their root system. Plants that wilt because of root loss rarely are able to recover and survive as attractive additions to the interior environment.
REVISED: November 8, 2016