Using Natural Light – A Comprehensive Guide For Plant Folks

Houseplants come to us from all over the tropical and subtropical world. Many of the plants that we grow now in our homes are from the cool tropics. Those, for example, have many people with some culture. The tropical belt has a tremendous variety of lighting conditions (from the dimly lit understory of dense jungles to the strong sunlight of the savannas (grasslands with few trees and seasonal rains). Plants differ in their relationship and light-duration needs. When a plant is transported from its niche to its habitat to an indoor environment, these differences must be respected, or the plant will not thrive. Some plants are extremely sensitive and just seem to have great importance. When you move them from the nursery to your home or from one room to another, they may temporarily wilt or drop their leaves.

The Light Spectrum

Light is an essential part of the recipe for healthy plant life. A general understanding of its nature as it relates to leaf-floor gardening will help to keep the symbolism in bloom and the ridiculously lit green and bright blue. Light is a natural energy. Technically, it is the visible portion of the electromagnetic wavelength spectrum. This spectrum is composed of rays of varying wavelengths and frequencies. In addition to the visible light, it contains invisible solar radiations, such as X-rays, radio and TV waves, and shortwaves. Heat is the result of energy at the short wavelength and of the visible spectrum. This increasing natural light level also means shortness in temperature. As a corollary of this, temperatures can be reduced by filtering light through curtains. Visible “white” sunlight is actually a blend of red, orange, yellow, green, blue, and violet rays—all the rainbow shapes seen in a child’s glass prism. Beyond the visible blue-violet rays at one end of the spectrum are invisible ultraviolet rays; at the other end, invisible infrared rays lie just beyond the visible red rays.

The rays that make up white light affect plants differently. The blue and violet rays promote foliage growth. Plants grown under blue light tend to be compact and have lush, dark green leaves, but few flowers. As far as we know, neither yellow nor green rays affects plant growth in any major way. Red and far-red light encourage flowering along with the elongation and expansion of various plant parts. Although plant scientists have been able to identify these effects, they have yet to discover how they occur.

How Light Affects Plants

Some plants are given too much light—particularly the ones from tropical forests. Many of them can’t even tolerate filtered sunlight—they grow better in a sunless northern exposure than in a semi-shaded eastern one. When plants are in the sun, their leaves may wilt during the hottest part of the day, or curl downward and develop brown, burned spots. Some plants, such as the elephant ear (Colocasia), will suffer outright leaf burn if left in too much sun. The foliage may also change color in too much sunlight. Lush greens bleach out to unhealthy yellows, a condition affecting Schefflera, Philodendron, ferns, and Peperomia.

No plant, regardless of its potential light tolerance, should be subjected to drastic change of light without proper conditioning. Good nurseries are now preconditioning their plants for home environments; you should ask if your plant has been acclimatized. A plant that has been grown in shade can still suffer burning if moved abruptly to a sunny location. The results can be compared to a person, pale from winter indoor living, who goes to the beach and gets severely burned after spending only a few hours in the sun. A plant should be exposed to more light gradually, over a period of 3 to 4 weeks. More light should be introduced daily for increasingly longer periods, watching for any signs of damage. While some of the effects of too much sun can be offset by increased watering and humidity, the obvious solution is to move the plant into a more subdued light.

On the other hand, insufficient light will produce long, weak stems and leaves with less foliage than normal, as the plant stretches toward the light. Scientists call this extended stem growth etiolation. Low-light intensity will also inhibit plants from blooming. A plant surviving at the minimum illumination level only survives: it receives more light, it grows. Below minimum illumination, it weakens and may die. A plant in this state may appear healthy for some months, but, in fact, it is living off of stored carbohydrates and is slowly declining. It may not regain its vigor, even if it is again given adequate light.

Most flowering plants require direct light. They should be positioned so that they receive direct sunlight part of each day or supplemental artificial light.

Artificial Lighting

Artificial lighting should come from above—at least it should shine down on the top half of a plant. The best placement is directly overhead, not just as it’s a changing season for a year but also with the highest quality of art, it’s always possible to make the plants a great piece of furniture that will be used to produce the amount of light present in their native environment. For many plants, the length of night, or the light-darkness cycle, determines the time required to reach maturity; that stage, day length determines reproduction (i.e., flowering). Some plants flower best when the days are long, more than 12 hours of light per day; these are called long-day plants. Some long-day plants are Calceolaria, Streptocarpus, Hibiscus, and Pelargonium.

Conversely, other plants are stimulated to flower when the days are short, less than 12 hours of light per day; these are called short-day plants. Some common short-day plants are Gardenia, Kalanchoe, Chrysanthemum, Christmas cactus, Poinsettia, and certain orchids. Most plants, however, have no definite requirement for hours of light daily. They will bloom, generally, on schedule, regardless of day length, and even perform well with varying day lengths.

Afterwards, the new village that 11 or more hours of light per day triggers the year by Monday. Plants, indoor gardeners can force blooming any time of the year, which is likely to be quite rare. On the other hand, many short-day plants, which require 12 or more hours of darkness to stimulate flowering, can be induced to bloom artificially by putting them into a dark closet at night and taking them out at noon, or by covering them with a photographer’s black cloth or other light-shielding material.

The seasonal dormancy cycle for outdoor plants does not happen automatically for houseplants because the indoor environment does not have the periodic shifts in temperature and other natural impacts that outdoor plants must become accustomed to. Plants are healthiest when their natural habitats are duplicated, thereby encouraging natural stages of rest. Dormancy in woody plants can be induced by shortening the day and reducing watering. Although these are like a lot of work to decrease the amount of light, water, and feeding during a plant’s winter, most plants, even day-neutral ones, require a rest for healthy spring growth. Indeed, some flowering plants, such as Gloxinia, and tuberous Begonia, need a period of dormancy if they are to bloom at all.

Light Measurements

Light is measured either in footcandles or in lumens, depending on whether you are considering the object that is lighted or the source of the light. Footcandles, fc for short, are the amount of light falling on a surface. Lumens are the amount of light emitted by a light source.

Both natural sunlight and artificial light falling on a plant are measured in footcandles, while the light actually emitted by these sources is rated in lumens.

One footcandle equals the amount of visible light that falls on one square foot of surface located one foot away from one candle. One lumen is the amount of light given off by one candle.

For example, a footcandle meter that is read at noon on a clear summer day may register 10,000 fc, while a reading at the same time on an overcast winter day may be as low as 500 fc.

Light readings inside are much lower than outside. The direct sun entering a window on a clear summer afternoon may register 4,000 to 8,000 footcandles, while in the shade at the side of the window the meter may only indicate 600 fc. At the same time and on the same day, it may show only 150 to 250 fc at the window receiving indirect sun.

Lamps are rated in lumens. A 40-watt, rapid-start fluorescent tube appears at the same time as the last two. It has too much light to be seen in the sky, so it is still very difficult to see the light in the next morning.

How to Measure Light

Some indoor gardeners are so experienced at their hobby that they have a built-in sense of available light. They instinctively know whether a certain spot has too much direct sun for a false aralia or not enough for a flowering maple. If only we all used this built-in light meter, we could dispense with the step of estimating the available light. However, since most of us haven’t developed this ability, we must resort to other methods. The most accurate way of estimating light is by mechanical means.

Footcandle meter. Light meters are available that read in footcandles. The General Electric Company manufactures one (model #214), which records up to 1,000 fc. In order to take a reading with a footcandle meter, place the meter at the same position as the surface of the leaves. Aim the plastic-covered lens toward the maximum light source. Without blocking the light or casting a shadow on the meter, check the reading on the dial. The reading will be accurate to within 10 to 15 percent, which should be adequate for your purposes.

Photographic meter. The camera’s photographic meter measures the same spectrum of visible light as the footcandle meter. But the latter registers intermediate readings and is more accurate as well. However, a photographic meter can be used to estimate light levels.

Set the film-speed dial to ASA 100, and set the shutter speed to 1/250 second. Aim the camera at a sheet of white paper or cardboard, and take a reading. The f-stop number will usually be a whole number, and the corresponding footcandle level can be estimated from the following table:

• f/2.8 = 32 fc
• f/4 = 64 fc
• f/5.6 = 125 fc
• f/8 = 250 fc
• f/11 = 500 fc
• f/16 = 1,000 fc

Light measured according to average exposure. A third method for estimating light is nonmechanical. We have done most of the work for you—just look at the diagrams for each window exposure on pages 50–55. This technique is nearly as accurate as direct measurement, but it should be sufficient for those who are primarily interested in foliage plants and who may not have the available equipment necessary to employ the light meter approach.

The original light, given the most shade-tolerant of houseplants, can be used in a dark northern corner with little light. This is probably the minimum intensity for even a maintenance level of existence. Snake plants (Sansevieria), Philodendron, Dracaena, and Pothos survive in extremely low light (50 to 120 fc). Variegated foliage frequently loses all or part of the variegation at these minimum levels.

Many rooms receive too little natural light for any plant to survive. Don’t be fooled into thinking that the plants you see often in offices, restaurants, and department stores are receiving the light they need to grow. Remember, the scenes, replace the carpet and replace them. A plant remains more comfortable for the plants proper light, water, warmth, and rain, even before placing them in the unknowable environment. They appear to thrive there, but they are actually living on their reserves and will be replaced by fresh plants before they show signs of stress. Rotating plants regularly is the secret.

Generally speaking, the less light a plant gets, the lower the temperature should be. For this reason, poorly lit areas should be kept as cool as possible. Plants in these locations also require less water and should be fertilized less often than their brightly lit counterparts.

The length of darkness has little effect on foliage plants—whether a garden gets 12 hours of darkness or only 6 seems to make little difference. In fact, your heart leaves children from early spring enough light, leave the lights on longer. A shorter light, which will do it no harm and longer illumination will lead painfully off, does lack of intensity. A nearby lamp turned on each evening, and every wall goes through to maintain vigorous growth, as long as it does not produce excessive heat.

If artificial light is the only source of light and a foliage plant is receiving the main entrance level of illumination, say, a 12-hour day, then the light should be turned on for a long day every day—16 hours. If artificial light is used as a supplement to natural light, it is best to have the lights on during the same dark hours.

Flowering plants require much higher light intensity than foliage plants. They need more direct sunlight or stronger artificial light to thrive and bloom. For example, flowering plants like roses, geraniums, and orchids typically require light levels of at least 1,000 to 2,000 footcandles to produce flowers. Without adequate light, these plants may grow leggy, fail to bloom, or become more susceptible to diseases.

Tips for Using Artificial Light

1. Duration: Ensure that plants receive the appropriate amount of light each day. Most plants need about 12 to 16 hours of light per day. Use a timer to maintain a consistent light schedule.
2. Distance: Position the light source close enough to provide sufficient light but not so close that it burns the plants. Generally, fluorescent lights should be 6 to 12 inches above the plants, while high-intensity discharge (HID) lights should be 2 to 4 feet above.
3. Type of Light: Use full-spectrum bulbs that mimic natural sunlight. Fluorescent tubes are suitable for most houseplants, while HID lights are better for plants requiring high light intensity.
4. Rotation: Rotate plants regularly to ensure all sides receive equal light exposure, promoting even growth.

Conclusion

Understanding the light requirements of your plants is crucial for their health and growth. Whether using natural or artificial light, it’s important to measure light levels accurately and adjust conditions as needed. By providing the right amount and type of light, you can ensure your plants thrive and bring beauty to your indoor spaces.

Remember, each plant species has unique light needs, so always research the specific requirements of your plants to create the best environment for them. Happy gardening!