Houseplants make it obvious when they lack enough light by growing longer, weaker stems and lighter smaller, greenish-yellow leaves. Some plants, however, can grow happily in a dark corner of a city apartment. You know these favorites--Chinese evergreen, rubber plant, sanseverias, peace lilies and their cousins. These plants share some common characteristics that make them able to stay healthy indoors through the dark days of winter.
The level, or illuminance, in nature varies widely from 10,000 foot candles (fc) in full sunlight to 100 fc on an overcast day to 10 fc on a cloudy day or in shade. Light and heat are both energy and, air masses and seasonal norms being equal, sunny days have warmer temperatures than cloudy days. Indoor spaces without additional artificial light have little more illuminance than an overcast day. Indoor light also varies with time of day, artificial light, reflective surfaces--even the number, area and cleanliness of windows. Latitude affects illuminance, too, as any Northern city dweller who has escaped a dreary winter's day to emerge from the airport into a blinding tropical sun can attest.
Low-light houseplants are primarily tropical plants because tropicals are acclimated to warmer temperatures than hardier plants grown in temperate climates. Centrally heated houses and apartments often exceed the daytime comfort range of most temperate-climate plants and almost always exceed it at night.
Tropical plants have adapted to the light levels of the floor of the rainforest or jungle where sunlight is blocked by mid-level and canopy plants. Researchers in Florida found that plants developed sun leaves to process high levels of sunlight and shade leaves that process available light more efficiently. Low light tropical plants, acclimatized to dappled and deep shade, require only 25 to 75 fcs to stay healthy, well within the range of most interior spaces.
Light energizes chloroplasts, the little green factories of photosynthesis that produce chlorophyll, the chemical that helps plants turn carbon dioxide, water and minerals into food. Chloroplasts are where low-light houseplants acclimatize; their chloroplasts are highly efficient, requiring less illuminance to provide adequate chlorophyll for the plant.
Flowering requires extra energy and nutrients that low light plants just cannot provide. Many, like the lovely peace lily will flower as light levels rise in spring and summer. Their flowering is not regular, however. Most have plain green or gently variegated leaves for the same reason; the bright colors of the croton requires bright sunlight and the ability to waste red light. Ferns, a primitive, non-flowering plant are understory plants and are often successful low-light plants.