All plants respond to light, but some of them respond a little differently than others. Plants, like tomatoes and cucumbers, start flowering and fruiting whenever they can after reaching maturity, but other plants wait until certain signals are sent. The length of exposure to light each day, known as the photoperiod, is one of the most important triggers for desired plant behavior like flowering and fruiting.
Whether it’s called light deprivation, photoperiod manipulation, or photoperiodic control, the technique for controlling plants through light exposure is basically the same. Adjusting the length of time a plant is exposed to both light and dark can affect it in multiple ways, many of which are essential to the greenhouse trade. Timing bedding, and seasonal decorative plants, is practically impossible without the use of light deprivation. It’s also valuable for much more. Explore the concept of light deprivation and photoperiod control with this brief overview.
Photoperiod Control
A photoperiod is simply the amount of daylight in any given 24-hour period. When you manipulate the photoperiod, you’re basically changing the length of the daylight period. This sends signals to the plants to tell them it’s a specific time of year. If you’re growing a crop that only flowers or puts on vegetative growth during certain seasons, you’ll likely need light deprivation to achieve true control. Without the ability to manipulate the length of light exposure, most growers can only produce one to two crops of any specific plant during a year. Photoperiod control can increase that to a monthly cycle of blooming or harvest instead.
Long Days or Long Nights?
Of course, it’s possible to use photoperiod control in both directions. Extending day length is primarily accomplished with artificial lighting, so light deprivation is generally only useful for shortening day length instead. It’s actually the length of the night period that matters the most to the plants and not the specific amount of light they receive. This distinction means that light deprivation is generally a much more flexible technique rather than just light supplementation. Shortening the length of a photoperiod for a short time can trigger better results when long days are restored, making it a useful technique whether used for long or short periods.
Determining If a Plant is Photoperiodic
There are dozens of tested species of plants with well-recorded responses to light deprivation. Yet, you’re not limited to just raising mums or poinsettias after investing in this kind of equipment. Almost any plant that has a photoperiodic response of some kind may benefit from light deprivation. Most annuals have this kind of response, along with many perennials. If a plant is listed in various research as being day neutral, it’s unlikely to respond to any photoperiod manipulation. Short-day plants, in particular, are likely to benefit from light deprivation, while long-day plants generally need artificial lighting instead.
When Light Deprivation Isn’t Enough
Unless a specific plant only needs a brief period of light deprivation to trigger flowering or growth, you’ll likely need to combine this method with artificial lighting as well. Many plants, like poinsettias, need artificial lighting first and then periods of deprivation. Other bedding annuals and indoor tropicals may need the opposite program. Setting up greenhouses to take advantage of both types of photoperiodic manipulation may cost more, but it’s easier than trying to transfer large volumes of plants between two different types of greenhouses.
Methods for Light Deprivation
Light deprivation occurs for plants that need it as the day length changes naturally over the course of the year. If you need to control this process and make days seem shorter at random times instead, blocking out the light is the main method. This can be accomplished by:
- Covering the entire greenhouse with a layer of light blocking tarp, shade cloth, or other materials
- Building smaller enclosures of the light-blocking cover inside the larger greenhouses
- Masking plants individually with caps or covers
- Shutting off artificial lighting inside buildings that don’t admit any natural light.
Light deprivation isn’t useful for every greenhouse crop, but it is widely used for annual bedding plants and seasonal decorations like poinsettias. Tricking plants into thinking it’s a specific time of year is a valuable tool for triggering all sorts of desirable behavior, like flowering. Even if you strictly grow crops that aren’t traditionally manipulated with light deprivation, it’s worth experimenting with a small enclosure just to see what’s possible. Check out our selection of greenhouse products here at BTL Liners to find the total blackout cover or tarp that is right for you.
