Plant Lighting Pamela Kock

Growing houseplants successfully requires that we mimic their native growing conditions.  Providing proper light levels is essential.  While most houseplants need the same duration of light – 12 to 16 hours – the intensity of light required is variable.  Further complicating matters, many plants need different light levels depending on the season.  Too little light during the summer, or too much during the plant’s dormant season, and the plant might not bloom. 

Light Levels Explained 

Light meters are sometimes used to determine intensity, and even the spectrum of light present in any particular location.  Fortunately, home gardeners don’t need such detailed information.  When we look at a plant’s care guidelines, its light needs aren’t given in lumens, but as a range from “bright” to “low.” 

Keep in mind that the amount of light that a plant receives in a certain location can be increased or decreased depending on the room’s colors and textures.  A room with white walls will be significantly brighter than a room with dark walls, and reflective textures elevate light levels too. 

Full Sun
Few houseplants besides cacti and succulents require full sun.  This is an area that receives as much light as possible, preferably in a southern or western window, with no curtains or shades to filter the sunlight. 

Bright Light, or Some Direct Sun
Most flowering houseplants and some foliage plants, particularly those with colorful or variegated leaves, fall into this category.  They prefer an area that receives lots of sunlight, such as a southern or western window, but need the sunlight partially shaded or filtered through a sheer curtain during the brightest part of the day.  

Moderate Light
Most foliage houseplants require moderate light, and even shade-loving plants will thrive with this level.  Many flowering houseplants will survive, but probably won’t bloom.  This requires a location within two feet of a northern or eastern window, or between 2-5 five feet away from a brighter window. 

Semi-Shade
Many foliage houseplants that prefer moderate light will also do fine with semi-shade, though variegated leaves may lose their color.  There are also plenty of options for plants that actually prefer this light level.  This would be within 2-5 feet of a northern or eastern window, or 5-8 feet away from the western or southern side. 

Shade
It’s not impossible to grow plants in shade conditions indoors, but the list of varieties that will thrive is fairly limited.  Still, if there’s enough light – without turning on a lamp – to read a book for several hours during the day, the room is bright enough to grow houseplants. 

Symptoms of Light Problems

Even plants that enjoy bright light can suffer if they’ve been kept in a shady environment and suddenly moved to a sunnier spot.  If you plan to move a plant, especially from indoors to outside on a patio or balcony, do it gradually so the plant has time to adapt. 

Too bright:  Leaves appear scorched, developing darkened or brown patches.  Leaves may turn paler than usual, curl up or wilt during the brightest part of the day.  

Too shady:  Plant does not bloom when expected, or grows very little.  New growth may be smaller than usual, stems may get “leggy” and weak with large spaces between leaves.  Lower leaves may turn yellow and drop off, and plants with colored leaves or variegation may turn all green, especially on new growth. 

Light and the Life Cycle 

Mimicking a plant’s native environment sometimes requires artificially altering available light depending on the season.  Without a dormant period in a shady, or even dark, location, it won’t re-bloom the following year.  One example is the poinsettia, which requires total darkness for fourteen hours a day for an eight-week period.  Even a night light, or ambient light from another room, can interrupt this cycle, so the plant may need to be covered with a box or black cloth bag.   Other examples of plants with seasonal light requirements are bulb plants that die back after blooming, such as the amaryllis. 

Supplemental Lighting 

Even those of us with less-than-ideal locations can grow just about anything we want with supplemental lighting.  Most indoor gardeners won’t need specialty grow-lights; a fluorescent bulb will do the job.  Incandescent bulbs shouldn’t be used as grow lights, because they generate too much heat.  For supplemental lighting to be effective, though, there are certain guidelines to follow.  

First, the light must be close enough to the plant.  It’s difficult to grow plants for any length of time in a window-less office with only a fluorescent fixture overhead.  To make a difference, lights must be suspended 12-24 inches above the plants for most foliage varieties, 6-12 inches above for flowering plants.  

Second, the light must be on for at least twelve to sixteen hours a day, unless it’s used to supplement natural daylight.  If the natural sunlight in the plant’s location is only bright enough for ten hours a day, for example, you’d want to turn on the lamps for another two to six hours to make up the difference.

Third, the lights shouldn’t generate so much heat that the plants’ environment is too warm or dry.  Incandescent bulbs shouldn’t be used, because they generate too much heat.  You may need to provide additional humidity for the plants, and check them often to make sure the soil doesn’t get too dry.