Light is the single most important factor to consider when growing plants.  This is because plants literally eat the light – via photosynthesis.  Photosynthesis is a process whereby plants use the solar energy of the sun to power chemical reactions that trap the sun’s energy in the form of sugars.  One can think of plants as living solar panels.  In order to be successful with lights either indoors or in a greenhouse, you need to know a little bit about what you are growing, and a little bit about the properties of light.

Properties of light

Light has many properties, but when growing plants under lights, you only need to be aware of three – light quality, light quantity, and the Inverse Square Law.

Quality of light – the color or form of the light, often expressed as a color or a wavelength (each wavelength corresponds to a particular color).  For many orchids, you don’t have to have the ideal quality of light, as long as the quantity of light is strong.  However, for optimal results in blooming, you should strive for a spectrum that mimics sunlight’s color.  Color can be measured in PAR (Photosynthetically-Active Radiation), CRI (Color-Rendering Index), or PPFD (Photosynthetic Photon Flux Density).

PAR is a metric that came about from an old study in the 1970s that sought to determine which colors of light affect photosynthesis the most.  While PAR is good, we later found that it’s not a complete measure of the actual light that plants use, but because it was the first metric available for growers, it’s still used today.  The issue with PAR is, is that it does not measure far red (>700nm wavelength) or far blue/UV light (<400nm) that plants need and use.

CRI is a measure of how closely-matching a beam of light’s color is to the sun.  CRI goes from 1-100, and a measurement of 95+ is closest to the color of the sun.  This includes the far red and far blue that plants need.  As a grower, you can get away with any light bulb that has a CRI rating of 90+.

PPFD is a measure of PAR at a specific distance, so you can determine how much light is hitting the plants’ leaves.  PPFD is a more accurate measure of light color at the plant’s leaves, as opposed to at the center of the bulb, even though it only accounts for the PAR.

Quantity of light – the amount of photons (pockets of energy) that are hitting the plant.  Quantity is measured in brightness or lumens, but can also be measured with PPFD, as that metric combines quality and quantity.  Indoors, we measure light in terms of hours of ambient light vs hours of direct sun that a plant is getting.  With light bulbs, we can know the number of lumens.

Light Indoors - Windows

Indoors, we can take advantage of windows to get free light for our plants.  When thinking about how much light a plant needs indoors, one must consider the direction that the window faces, whether or not it’s obstructed, your latitude on the earth, and the number of hours of direct sun that come through that window.

In the northern hemisphere, the sun rises in the east, swings to the south, and sets in the west.  That means that south-facing windows will have the highest quantity of light.  East and west windows will have a bit less, and north-facing windows will have the lowest quantity of light, assuming that the windows are unobstructed.  There are some folks who claim that one can burn a plant in a window, and that’s only true for south-facing windows for folks below the 37th parallel (the northern border of Oklahoma).  That means that plant placement is important!  In general, follow these rules for window placement:

Additionally, placement of the plant in relation to the window is important – always place a plant IN a window, not next to it.  Always ensure that the plant can “see the sun” from its location and get hit with as many rays as possible.  A beginner’s mistake when growing plants is to either place plants too far from a window, or to place them off to the side of the window, where the plant is facing the wall, and isn’t in the window.  No plant should be more than 3 feet from a window.

Figure 1- A window. Plants should only be placed in the area where the direct sun hits, as indicated by the measurements.  The closer the plant is to the glass, the better.

Be also aware that the more light and heat a plant gets, the more frequently they will need to be watered.  This may change with the seasons, and may change if there is a source of heat or air conditioning nearby.  Plants can be placed near an air conditioning unit, as long as the air is not blowing on them.  Plants should be 3 feet or more away from heaters.  In older homes with radiator heat, the radiators may be in the windows.  In that case, one should get a radiator cover and some kind of shelf that’s a few feet above the radiator, so that the plants can get the light without being cooked to death.

Light Indoors – Artificial Plant Lights / Grow Lights

When thinking about setting up grow lights for your plants, the most important factor is the bulb.  You can be as creative as you like about which kinds of lights or the setup that you choose to setup.  Be sure that you follow the basic rules of lightbulbs below to ensure that your plants are getting enough light.

Also, some basic information about bulbs:

Light In Greenhouses – Artificial Plant Lights / Grow Lights

The cheapest and easiest way to light up a greenhouse if you are lucky enough to have one is by utilizing the power of the sun.  However, in northern climates above the 37th parallel, orchids may need a little lighting help in the winter.  If your greenhouse is smaller and created by DIY means, then follow the instructions for indoor lighting.  If your greenhouse is installed by a greenhouse company, consult with them or another reputable greenhouse supply store about plant lighting options.  Photosynthesis is universal, so you can use any grow lights for any plants, such as cannabis lights for orchids.