If you’ve ever noticed that your plants grow poorly and stretched in the winter, you are noticing the effects of light on them. There are three primary ways that light affects your plants: day length (how long it is light for), intensity of light (brightness), and the quality of light (colour).
Day length is how long the plant gets light for each day. Actually, more properly, it is the uninterrupted darkness that plants measure. It works like this: when plants are in the dark, they begin produce a particular hormone-like substance. When that substance gets to a certain concentration in the plant, the plant responds to it. What level that substance is at when a plant reacts, and also how the plant reacts are both species specific. Some plants begin to flower when that concentration is high (these are called short-day plants, because they flower when there are long nights and short days), others flower when this concentration is low, (these are long-day plants because they flower when the days are long and nights are short). Day length also affects root, stem and leaf growth in many species, although this is less studied than the flowering.
Plants do not need much light in order to perceive it to be daytime. Most plants need less light than you would need to read a book, and they only need that light for a short time. This makes it quite difficult to force certain plants to bloom at certain times. Pointsettia and Phalaenopsis orchids, for example, both need long nights to flower. If you have light–even from another room–reaching these plants during their long night, they may never initiate flowering. Greenhouses growing these plants often use black-out curtains to keep all light out of their building to ensure that the plants will flower on time.
Likewise, long-day blooming plants can be forced to flower at almost any time of the year if you can make sure that they are exposed to artificial light several times during their night period. You do not need to use grow lights for this. Even small, incandescent lights will be enough for changing the plant’s perception of the time of year.
Light intensity is the amount of light that illuminates the plant at any given time. Many places are quite cloudy during the winter severely limiting the light that plants receive. In my area, 52 degrees north latitude, we do not get much cloud, but the light comes in through the atmosphere at only 14 degrees above the horizon at noon on December 21. Because of this angle, some of the light is reflected away as it passes through the atmosphere, and so we get less bright light mid-winter than people who live closer to the equator.
Since the main way that plants eat is through photosynthesis (where light and carbon dioxide are converted to food energy), low light means little plant growth. This can be mitigated, though, in two ways.
First, you can add light to your plants by using grow lights. There are many types of lights available commercially, and you can also rig something up with simple florescent lights if you need to.
Second, you can increase the carbon dioxide level near the plants. More carbon dioxide means that the plant can use whatever light they get more effectively. People and other animals will naturally increase the carbon dioxide levels, but you can also get carbon dioxide generators (these are not appropriate for inside a home–the risk of accidental carbon monoxide poisoning is too great–but you can use them in a greenhouse.) Many chemical reactions give off carbon dioxide, and in China you can buy packets of material that will react slowly over time and raise your carbon dioxide levels.
The colour or wavelength of light greatly impacts plants. Plants mostly use blues and reds for growing, for example, so these colours of light are particularly important. In the winter the problem for your plants is a reduced amount of red light.
As I mentioned earlier, as light travels through the atmosphere, some of the light is reflected off. The light that makes it through has less red in it than light in the summer would. More red light is lost as the sunlight comes through the atmosphere than far-red light. This is significant because plants continuously measure a ratio of far-red light to red light. In other words, plants notice when there is more red light than far-red light and it affects their growth.
Plants always want to know when they are under a canopy. Some are happy there, but many want to be above the other plants. Plant leaves absorb red light (that’s why they look green–they reflect the green light and absorb the red light). So in a canopy, the red light is absorbed. Plants do not absorb very much of the far-red light so that light will make it through a canopy easier than the red light does. If the plant senses that the ratio of red light is reduced compared to far-red light it will reduce the size of its leaves, and put a lot of energy into making more stem. If, however, there is more red light and less far-red, it will assume that it is above the canopy and put out large leaves, and not put much energy into building a tall stem.
Because the winter light red to far-red ratio is wrong, plants will assume they are under a canopy and grow tall stems with small leaves. In a greenhouse this can be a problem, because we have limited space both for growing plants, and for shipping them. Some greenhouses actually buy plastic film that filters out some of the far-red light to allow plants (even in the winter) to grow more compactly. More often, though, greenhouses will use grow lights that have a spectrum which increases the red light. The stretching of plants is less of an issue for growing in one’s house, but if you ever wondered if there was a way to alter this, changing the colour of the light will do it.
Plants are dramatically affected by light. Whether you are talking about the amount of light, its colour, or its duration, plants will respond in surprising ways if the light is wrong. If you want to take your growing to the next level, understanding how light affects your plants in the most challenging season for your indoor garden is critical for ideal results.
For more information on growing your winter indoor garden, check out part I.