Indoor Plant Basics: Light, by Jim Hole

Plants have several basic needs: light, comfortable temperature, humidity, soil, water, fertilizer and physical space. Placed together on one list, the basics look a little daunting, but understanding their significance requires a very small (but thoughtful) investment of your time. And when it comes to plants, a little knowledge really does go a long way.



Plants, like people, need energy to grow. But whereas people seem to obsess over avoiding carbohydrates, plants obsess over making them. I am, of course, referring to photosynthesis—the process by which plants take energy from the sun and convert it into sugars that can be used to grow. This process has been called the most important chemical reaction in the world.

Measuring Light

Light is the single most important factor in determining whether your houseplants will thrive or die. Unfortunately, it is also the most poorly understood factor. It comes down to understanding that the amount of light your plants receive will determine whether they are rapidly dying plants, slow-dying plants, status quo plants (neither gaining nor losing growth), slow-growing plants or rapidly growing plants.

Because light can’t be held in one’s hands or poured into a glass and measured, you have to think of it in terms of intensity, quality and duration.

  1. Intensity of light: the strength of light available
  2. Quality of light: the wavelengths or colours of light
  3. Duration of light: the amount of time plants are exposed to light in a 24-hour period

The relationship between these three factors is important. For example, if the quality of light is high, but there isn’t much of it (intensity), or it doesn’t last very long (duration), will your plant do well? Not on your life! Ideally, you want to give your plants the perfect intensity of the highest quality spectrum light for the optimum amount of time. But that will never happen. So the situation comes down to compromise and manipulation. The fact is, as wonderful as a short burst of perfect light is, 12 hours of lower quality light is better.

Light Factors

By far, the greatest challenge you’ll have is providing your houseplants with enough light. Providing the ideal quantity of light seems like a fairly easy task initially, but consider the complicating factors that reduce the amount of natural light that gets to a plant’s leaves.

  • Not as much sunlight enters your home in the winter as it does in the summer. In fact, winter light may be 20% of summer light. This, of course, is because the days are shorter and the sun is lower in the sky.
  • Moving plants even a few extra feet away from a window will cause a dramatic reduction in sunlight. A few feet doesn’t sound like much, but it is not uncommon to see a 100-fold drop in light when a plant is moved from a windowsill to a table a few feet away.
  • Windows are not a source of sunlight—they merely allow light to pass through with, at best, 93% sunlight transmission. The sunlight transmission may drop to less than 50% if the glass is tinted.
  • South-facing windows usually provide the greatest amount of sun exposure due to the sun’s apogee (or track across the sky). For us in the northern hemisphere, the sun tracks across the southern sky with a high angle in the summer and a low angle in the winter. Because of this angle, south-facing windows tend to get the greatest amount of year-round light exposure, with west-facing and east-facing windows coming in second and third respectively. North-facing windows receive little, if any direct light but can capture enough indirect sunlight to grow a few low-light plants.

Other factors that contribute to inconsistent natural light:

Many other factors may contribute to inconsistent natural light throughout the year, including fog, cloud cover, elevation, drapes and window treatments, the presence of ultraviolet-blocking coatings, dirt or dust on the window, reflections from light-coloured interior paint and the presence of awnings, overhangs or shade trees near windows.

Artificial Lights

Fortunately for plant lovers, artificial lights can be used to supplement the natural light that windows provide. What’s unfortunate, however, is that there tends to be a lot of confusion about what artificial lights can and cannot do.

The majority of the lights available in garden centres are referred to as grow lights, implying that they have some magical ability to grow plants above and beyond that of standard lights. The reality is that virtually every common household light is a grow light. In other words, they all emit at least some light in the visible light spectrum—otherwise, why would we use them? Some of these lights are better than others when it comes to light spectrum, but all of them (with the exception of those used by professional growers) are quite wimpy. What I mean by that is typical grow lights from hardware stores kick out very little total light energy and, therefore, may starve your plants. Many so-called grow lights do have plant-friendly light spectrums, but they are equivalent to one wonderfully flavoured, but Lilliputian-sized appetizer at a dinner party—something that will satisfy your tastebuds but starve you if it’s the whole meal.

To give you an idea of what grow lights can and can’t do, let’s compare some of the common ‘grow lights’ you might find in a store.

Fluorescent Lights:

  • Cool white-light fluorescent tubes: Cool white-light fluorescents emit a high percentage of blue light and green light (which is why people’s complexions look a little dull under them). These lights are convenient, economical, energy efficient and—best of all—don’t produce excessive heat. Although these lights stimulate some of the processes needed for growth, they don’t provide enough of the entire spectrum of light that plants need to grow to their full potential.
  • Warm white-light fluorescent tubes: Warm white-light fluorescents are slightly better than cool white-light flourescents in that they provide a greater portion of the light spectrum (specifically the red band), but these lights don’t entirely meet a plant’s needs either.

Incandescent Lights:

  • An incandescent light is any light that has a filament. Although incandescent light is the light people tend to prefer, plants don’t really value it. Most of the electrical energy from an incandescent light is converted to heat, and only 6–12% is converted to usable light. Because incandescent lights produce a lot of red light, they need to be used in combination with fluorescent tubes in a ratio of about 1:3 to prevent plants from becoming stretched and lanky. So for 100 watts of fluorescent light, provide about 30 watts of incandescent light. This ratio ensures a better red-to-blue light balance.

Note: the intensity of light drops dramatically as distance increases between light bulb and plant, so height-adjustable fluorescent tubes are ideal for tweaking that optimum distance.

Did you know?

Chloroplasts are structures that contain chlorophyll and give plants their green colour. The primary duty of chlorophyll is to take electromagnetic energy (sunlight) and change it into chemical energy. Without this conversion, electromagnetic energy can’t be turned into the usable energy we call food.

Managing Light Levels

It’s important to realize that a plant that requires low light can be placed in a room with bright southern exposure. The key, however, to keeping that plant from becoming stressed is moving it an appropriate distance away from the window. The closer a plant is to a window or skylight, the more light it receives. Just remember that light intensity and duration change with the seasons.

Outdoors, the intensity of sunlight cast in the middle of a summer day may average between 10,000 and 12,000 foot-candles. Indoors, light intensity depends on a combination of artificial and natural light. The best way to measure light is with a light -meter.

A light meter can show how light levels change as you move away from a window. At one time, light intensity for plants was measured in foot-candles, but that is a term more suited to measuring the human eye’s sensitivity to light than it is to measuring the light energy that plants use. The correct term for plant light intensity is a bit of a tongue twister—photosynthetic photon flux. The actual units are measured as micromolecs per square metre per second. If this sounds complicated, try thinking of measuring light as we do raindrops. The volume of water that hits each square metre every second can be measured and quantified.