Indoor plants and Light
- GardenMaestro
- Feb 23, 2023
- 7 min read
Updated: Oct 15, 2023
Very often I’m brought or shown pieces of a plant or a few leaves by people in order to tell them what is wrong with the plant. Much can be told from a plant just by looking at a sample of it. The size, spacing, and texture of the leaves can tell you the location of the plant. This in conjunction with the presence of pests on the plant, or evidence of their damage, and even the variation in coloration of the leaf can build a pretty accurate picture of where the plant is situated, and what the problem or problems with the plant may be.
Plants in low light intensity
Problems associated with low light intensity may be caused by incorrect placement of the plant in the home, removal of the primary light source, overcrowding by other plants, or sheltering by the dense foliage of the same plant.
The leaf size of plants varies according to their environment. A container plant growing in conditions that are close to its natural environment will have a leaf size roughly the same as its wild cousin.
If the light intensity is too weak or very low, the plant tries to compensate by increasing the size of the leaf area, this is a slow process and takes a number of weeks before a noticeable difference.
The plant will increase the length of its stem in order to search for a better quality and quantity of light, the inter-nodal distance (space between leaf attachments), becomes exaggerated and the color of the leaf deepens as more chlorophyll is produced in an effort to produce more food.
Plants will always grow towards the strongest light source; this is a physiological mechanism called phototropism; some even angle their leaves as the sun tracks across the sky in an effort to maximize the light exposure. Indoor taller plants may develop or lean towards the strongest light source.
The plant may even shed some of its older leaves (normally near the base of the plant) so that its growth efforts are directed into producing height as it looks for light.
The leaves may lose some of their hard, waxy covering layers the “cuticle” in low light, and become softer and more pliant. Cuticle thickness is normally related to exposure to Ultraviolet light.
Plants in this condition are easy targets for sap-sucking insects and fungi and can be easily sunburnt if moved into direct light (or outdoors) due to the reduced thickness of the cuticle.
In variegated plants, the natural variations in the leaves will be reduced in low light as the plant tries to compensate by producing more chlorophyll.
Leaf variegations
Plants that have either mottled or striated green and white markings on the leaves are called “variegated”.
Those that have a combination of white, green, yellow (or cream) markings are called “picturated”.
Common examples of these types of plants are Ficus benjamina “Variegata”, Cordyline australis “Variegata”, Epipremnum aureum (also known as Scindapsus, Pothos, Snake plant, Money plant, and Devil’s Ivy)
Variegations and markings in the leaf can be caused by: natural inheritance in the wild, or the manipulated crossing of breeding stock plants for aesthetic purposes. One method used to introduce variegations into a plant is to infect it with a plant virus; the virus may live in the plant and prevent it from producing chlorophyll in certain portions of the leaves. In this type of symbiotic plant/ virus relationship, the virus does not kill the plant as it requires a live host. Plant viruses are normally very specific to a particular genus of plants.
Mostly, these markings are caused by the lack of chloroplasts, the green food-producing organs in the leaf cells. In low light the plant will start to produce more chloroplasts in an attempt to maintain an adequate food production level; as this happens the variegation in the leaf will start to revert back to plain green.
This effect can be seen if you look into the center of a densely leaved variegated plant; such as a Ficus or Epipremnum. The leaves at the darkened center of the plant will be plain green and those exposed to light will be variegated.
The reverse is also true in exceptionally strong light; the variegated portion of the leaf is much more significant than that of the green part, as the plant has no problem producing sufficient food with its limited leaf capacity.
Containers should be rotated slightly to ensure the plant retains a straight growth habit. Ficus plants are notorious for shedding leaves if the container is moved, this is normal, however, up to 30% of the leaves can be dropped!
When taking a plant home, you can normally judge the position a plant needs to be placed in by the color and size of the leaves. Plants with large dark green leaves will normally tolerate lower light as their leaves have adapted in size and color to compensate for this. Plants with small or variegated or even colored leaves require brighter light to survive and need to be closer to a stronger light source.
Light Quality/ Frequency
Light quality is just as important as the amount of light that plants receive. All plants grow naturally outdoors in natural sunlight, and the amount of light they receive in nature is related to their environment, this is why many tropical/ jungle plants make very good indoor plants as they are adapted to low or filtered light as they live under a canopy. Conversely, succulents are best suited to high-light (and hot) environments.
Plants are green as they reflect green light wavelengths and absorb (visible) light in the blue and red frequencies (as well as a certain amount of Infra Red and Ultra Violet).
Visible light (to humans) falls between 400 nanometers (the violet end of the spectrum) and 700 nanometers (the red end of the spectrum) in frequency.
Green light falls somewhere in the middle of the spectrum, between about 495 and 570 nanometers, and is reflected by the chlorophyll in the plant
Blue light (between 400 and 500 nanometers) promotes vegetative growth
Red light (between 600 - 700 nanometers) promotes flowering and fruiting in plants
Sources of light
The ultimate source of light is natural sunlight, this is perfect for outdoor plants, but indoor, lighting becomes more complicated.
Fluorescent light, incandescent light, LED lights as well as indirect sunlight (through glass windows or doors) are all sources of light for indoor plants; however, not all are of equal benefit due to the frequencies of light that they emit as the image below illustrates.
As you can see from the above illustrations, LED light (besides being more cost-effective and cooler than incandescent and brighter than fluorescent), has a far better light spectrum than incandescent as well. Of course, this is a generalization as there are different brands, qualities, and grades of LEDs; however, most modern hydroponic systems now use LED lighting, normally a mixture of Blue and Red.
Light Intensity
Lux is a unit of measurement for measuring the intensity of light. It is defined as the amount of luminous flux (measured in lumens) that is spread over a given area (measured in square meters), or 10.7 square feet.
Note: some references may refer to Foot-candles (FC) - this is essentially the same as Lux, but factored for square feet. In order to calculate the number of FC from Lux, you divide the Lux figure by 10, or if you want to calculate Lux from FC then multiply by 10.
Different plants have different light requirements; depending on the reference source indoor plant requirements may be divided into groupings according to their light requirements. The generally accepted "ranges" of lighting for indoor plants
Very Low (indoor) <500 Lux
Low (indoor) 500 - 2,500 Lux
Medium (indoor) 2,500 -10,000 Lux
High/ bright direct sunlight (indoor) 10,000 - 50,000 Lux
High/ bright direct sunlight (outdoor) 80 -100,000 Lux
Remember that indoor Lux levels in the UAE may differ from those in the USA and Europe due to the amount of tinting we have on our windows.
A Lux meter is a really useful tool, as it can help identify problem areas in your home with respect to the location of indoor plants or help you correctly locate/ orientate a new plant in the home. Traditionally light meters were instruments used in the horticultural interior decorating industry, and even by photographers.
Luckily you already probably have one close at hand even if you didn't know it, your modern smartphone. Smartphones are filled with sensors including a light meter which helps to adjust your phone display to dark or light conditions, as of course as with everything, "There is an App". I like to use a fairly simple one called Lux Light Meter, it gives you basic readings, which is all you really need. The free version comes with a few pop-up adverts, but nothing that detracts from the overall use of the application. Readings are displayed "live" and you can select between output in Lux or Foot-candles.
Use your hand to cover portions of the phone and screen to locate the position of the sensor on the phone - normally at the top of the phone; when the sensor is covered, the Lux reading will go down to almost zero.
If you get a continual 0 reading, this means that your phone cover may be covering the sensor, or the sensor is damaged or disabled.
Once you have the correct orientation, position the phone next to the plant and orientate the sensor to the nearest point of light (either artificial or natural); take a number of readings over the height of the plant (on the sides) and from the top.
This will give you a very good indication of the light (in Lux) that your plant is receiving.
Unfortunately, Lux does not describe or indicate the frequency of the light that the plant is receiving, only the intensity. In order to check only the natural light levels make sure all indoor lights are turned off when conducting the readings.
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