Long days for greenhouse crops

Fall is upon us! Poinsettias will be starting to show first color in greenhouses, a sign of the shorter days of fall. Long days can have several effects on plant growth and development. Most relevant to greenhouse crop producers is the effect of long days on flowering. Long days promotes flowering of species with facultative and obligate long day flowering responses. Alternatively, long days will inhibit flowering of plants with facultative and obligate short day flowering responses. Another effect long days has on woody and herbaceous perennial crops is to delay the onset of dormancy and keep them actively growing. Since long days can be required for some crops or production strategies and growers will have to take steps to create long-day conditions during the short days of fall, winter, and early spring.

When are days “long”? Well, that depends on the plant. Whether a day is “long” or “short” depends on the response of specific species and their critical photoperiod. For long-day plants, the critical photoperiod is the photoperiod at or above which flowering occurs. For short-day plants, the critical day photoperiod is the photoperiod at or below which flowering occurs. For some plants, 14-hour days may be “short,” while to others it may be “long.” To err on the safe side, a 14 to 16-hour day (or an interrupted night) is usually a safe bet when trying to create a long day.

Creating long days

There are two primary methods of providing long-day lighting in greenhouses and controlled environments: 1) day extension; and 2) night interruption (Fig. 1). Both of these methods can successfully elicit long-day responses, whether it is flower induction for long-day plants, vegetative growth for short-day plants, or delaying the onset of dormancy and promoting active growth.

Day-extension lighting is the use of lights to extend the natural day length or photoperiod. One approach is to have lights begin operating at just before sunset and continue running until the target day length has been met. Alternatively, lights can come on in the early morning when it is dark outside, extending the day in the morning as opposed to in the evening. The other method for providing long days is to use night interruption lighting. As the name implies, night interruption lighting breaks up the long, uninterrupted dark period so plants perceive a short night, or a long day. Sometimes referred to as “mum lighting,” night interruption traditionally operates from 10 p.m. until 2 a.m. Whether using day-extension or night-interruption lighting, the minimum light intensity required for plants to perceive “day” is 2 µmol·m^–2·s^–1; higher light intensities can be used, but this is the minimum for effective photoperiodic lighting.

Lights for long-day lighting

There are a number of different lights that can be used for long-day lighting, including incandescent, fluorescent, light-emitting diodes (LEDs), and high-pressure sodium (HPS) lamps. Each of these has their own advantages and disadvantages.

Incandescent lamps have been the most widely used and ubiquitous type of light used for long-day lighting. They are cheap to purchase and easy to install. Incandescent bulbs also have a ratio of red (R) light to far-red (FR) light that is effective for inducing flowering. While the FR light is essential for flowering, it can promote internode elongation or “stretch.” Additionally, for anybody who has ever touched an incandescent bulb while it is on or shortly after it is turned off, you know that these bulbs can get hot; the production of radiant energy increases the power consumption of these bulbs.

When compact fluorescent bulbs were being introduced, there was lots of interest by producers looking to use less energy than traditional incandescent. While they do consume less power, there is much less FR light in compact fluorescent bulbs, which can make them less effective than other lights with more FR light. One way to realize some energy savings while not delaying your crop is to alternate compact fluorescent and incandescent bulbs in your greenhouse; the incandescent bulbs will provide adequate FR light, while the compact fluorescent bulbs will use less energy.

A part of the explosion of LED lights that have come onto the marketplace recently is the “flowering lamp” (Fig. 2). These are low-intensity LED bulbs that can screw into the same fixtures incandescent and compact fluorescent bulbs screw into. In addition to being completely interchangeable with traditional bulbs, they have a range of different “recipes” or different combinations of light spectra like red, far-red, and blue. While the initial capital cost for these bulbs is more expensive compared to incandescent and CFL, the low energy consumption and long lifespan will more than pay back the investment.

A newer style of HPS lamp is being used for long-day lighting. While it looks similar to traditional HPS lamps at a glance, these new fixtures have a reflector that oscillates, changing the directionality or orientation of the light over the crop. Since the lamp location is stationary, the intermittent lighting from the oscillating makes this lighting strategy a type of cyclical lighting, where intermittent — not continuous — lighting is used.

Take-home messages

Before you plant, check the photoperiod requirements of your crops. You can improve timing and quality of photoperiodic crops by using effective lighting strategies and equipment. Knowing when, how, and with what to provide long days in your greenhouse will liberate you from the natural day length and expand your production possibilities.