HORIZONTAL – Goeppertia plants have recently become popular indoors because of their vibrant colors and unique patterns. From left are Goeppertia zebrina, Goeppertia makoyana and Goeppertia insignis. (Photo by MSU Extension/Eddie Smith) Alt text -- Three potted plants have green leaves with stripes or spots.
by Bronwynne Bailey
It’s so cold, I can’t even think about my garden, but I am focusing on my houseplants. One of the most important elements for plants is light.
General guidelines indicate that east-facing windows receive morning sun and remain cool during the day, while north-facing window don’t get any direct light at all. North-facing windows are good locations for Christmas cactuses, pothoses and cyclamens. South and west windows receive the most light and heat during the winter. Put your poinsettias and herb containers in these windows.
It’s not hard to find illumination guidance for specific types of houseplants—usually categorized as bright, medium, or low light. The problem is what I may think as bright light is actually not enough for herbs I want to place in that window. In other words, what I perceive as brightness may only be low light when measured quantifiably, and this can make a huge difference to a houseplant!
Another potential problem is that some directions for houseplant specify situating plants next to or a certain number of feet from north-, south-, east-, or west-facing windows. However, windows come in many different sizes, and this can affect the amount of sunshine passing through those windows significantly. So do any outside obstructions near those windows, such as nearby buildings and trees. This might be the correct amount of light, but who knows? Here’s where our handy meters will come to the rescue, allowing us to replace our subjective assessments with objective ones.
Meters may be analog, meaning they don’t require batteries or electricity or digital, powered batteries. There aren’t many of the basic analog meters available. Analog light meters measure only in foot candles or lux. A foot candle is a unit of measure used to denote the amount of light emitted by a candle falling on a surface exactly one foot away from the light source at all points, and this is equal to one lumen per square foot. This is an archaic unit of measurement and the problem with these meters is that they favor green light waves rather than blue and red light waves. Generally, plants are more sensitive to blue and red light waves. Most plant foliage reflects green light waves back rather than absorbing them, which is why foliage looks green. Red-leaf lettuce reflects red waves so would be more sensitive to green light. And many blue-tinted plants are found in desert habitats reflect blue waves rather than absorbing them, a boon to the plants in hot climates because those waves are hotter.
Digital meters offer more features. Some digital devices provide measurements in foot candles and lux, while the others measure PAR (photosynthetically active radiation). Be sure to read the manual that is provided with your device so you can utilize all of the options available. Power up your meter, remove the lens cap, and decide if you are measuring lux or PAR. Next, choose the range if necessary. On the website Gardener’sPath.com, “changing the range moves the decimal point, measuring in increasing or decreasing orders of magnitude by a power of 10. If you are testing very bright conditions—near windows, for instance—you want the decimal point to move towards the right, effectively testing within a bigger range.
On the other hand, if you are testing dim conditions, you can select the range to be much smaller, moving the decimal point towards the left. If the value on the display seems too low for what you are measuring, check to see which range you have selected – there may be a “x10” or a “x100” displayed below the value, meaning you need to multiply the number you see on the display by 10 or 100 to get your result. For instance, you may be seeing “5.00” on the display, but if “x100” appears below this number, you’ll know the value is actually 500.”
Now that you have set up your meter, hold the sensor close to the plant’s foliage, directed towards the window or lamp. The value may fluctuate but will provide a range of the light your plant is receiving. The leaves closest to the light source will receive more light than the leaves on the opposite side of the plant. This is why turning your plants, provides consistent color and better plant health. Ideally place the meter in the center of the plant for the reading. Cloudy and sunny days will provide different amounts of light as will different times of year. Record the variations in your garden journal to find the average light available. If your meter has a peak button this provides you with the max amount of light during the reading.
Per Gardener’sPath.com, “Values between 75 and 200 fc are considered good levels for low light conditions. For these areas, reach for low-light tolerant houseplants like snake plants, grape ivy, or ZZ plants.
Values between 200 and 500 fc are considered good luminosity levels for plants requiring medium brightness. For values in this range, opt for houseplants adapted to moderate levels of light like begonias, prayer plants, or moth orchids (phalaenopsis).
With values of 500 to 1,000 fc, you are safely within the bright light houseplant category and can choose from a wide variety of houseplants such as birds of paradise, fiddle-leaf figs, and some succulents, such as jade plants.
Over 1,000 foot candles is considered very high, and in this range plants will be getting some direct sunlight, where you can grow gardenias or herbs such as basil indoors.”
Plants can generally survive in a level lower than ideal but will produce more foliage and flowers at the highest level they tolerate. You can also expect that some cultivars prefer a level lower or higher. If you find that your plants do not have enough light, consider grow lamps. We will learn more about them in another article.
