8/30/2016
Growing With Griffin: Light Well, Grow Well
Tami Van Gaal
Supplemental lighting needs for greenhouse crops generally fall into two buckets:
1. Increasing light intensity to improve growth and quality.
2. Manipulating photoperiod for control of flowering.
Fortunately for most finished growers, the need for supplemental lighting for spring crops is limited to the easier, less-expensive photoperiod bucket. Even for growers in the northern tier of states, the need to increase light intensity is generally restricted to the earliest weeks of the spring production cycle. The need for increased light intensity can change dramatically, however, when a grower chooses to propagate young plants earlier in the season.
The timeline for young plant propagation precedes typical spring production dates by six to 12 weeks (crop dependent), such that plug crops are in production during January and February—months with notoriously short days and low light levels. For many growers, this means that ambient light conditions aren’t adequate to produce a well-branched, well-toned crop. Lighting propagation areas should be considered a requirement, not an option, for most growers during this timeframe.
Lighting production space costs money and nobody wants to spend money needlessly. Therefore, it's important to understand when to provide supplemental lighting, how to determine how much light is needed and which fixtures provide the best solution. When we describe the amount of light that should be delivered to a crop over the course of a day, we’re talking about a cumulative daily light integral (DLI).
Accepted DLI targets for young plant production (from seeds or cuttings) range from 8 to 12 mol/m2/d at crop height. To produce a quality crop, propagation from both seeds and cuttings should occur under no less than 8 mol/m2/d. While ambient, outdoor light levels generally exceed 10 mol/m2/d in January and February for all but the most Northern growers, a significant portion of this light (40% to 70%) is blocked by poly and structures during this same time. Some quick mental math reveals that DLIs in the range of 3 to 6 mol/m2/d can be expected in January at bench levels in a Northern state. A bench-level DLI of 3 mol/m2/d would require an additional 5 mol/m2/d from supplemental lighting to reach the minimum level of 8 mol/m2/d.
Because so much light can be blocked by structures, DLI should be measured at crop level and not assumed based on outdoor, ambient light. A LightScout from Spectrum Technologies tracks DLI for a single day in a simple-to-use, easy-to-place device. For growers wanting longer data collection periods, the Watchdog WeatherTracker (also from Spectrum Technologies) provides additional logging features in a larger format. Using the data you collect from your greenhouse, your supplier should be able to work with you to develop the perfect lighting solution. Options for increasing DLI in the greenhouse include the following types of lights:
High-Intensity Discharge (HID) lamps—Available with high-pressure sodium (HPS) or metal halide (MH) bulbs, the newest HID lighting systems are designed for maximum output with a compact fixture to minimize shading. Greenhouse fixtures are much smaller than their warehouse counterparts. Fixtures are available in wattage ranging from 250 to 1,000W, which means the system can be tailored to your needs. New fixtures and ballasts are far more efficient than older models, with a cost of operation that can be comparable to some LED solutions.
Light-Emitting Diode (LED) lamps—LEDs are garnering much attention in our industry, with rapidly expanding and evolving product options from numerous manufacturers. The range of product options include larger, highly directional assemblies for grow rooms, to smaller fixtures with better light dispersion patterns for greenhouse use. Some growers have seen reductions in crop time and superior growth from LEDs used in young plant production. The wavelength (color) of light is often configurable and lamps can be placed closer to the crop because they run cooler than HIDs. Initial costs vary greatly by product and improved efficiency shouldn’t be assumed.
Visit http://cpl.usu.edu/ for more information regarding operational costs and return on investment. Be sure to focus on products designed for greenhouse use and thoroughly investigate the cost in use of the fixtures, including purchase price, efficiency/energy use and replacement cost.
Southern young plant growers may not require supplemental lighting to increase DLI, but all young plant growers need the ability to provide long days. Long days are provided by either extending the day to a critical photoperiod (daylength extension, 16 hours is typical) or by interrupting the night period from 10:00 p.m. to 2:00 a.m. (night interruption). While the minimum light intensity needed to successfully provide night interruption lighting is 1.27 umol/m2/s
(10 fc) at plant height, daylength extension using full-intensity lighting can contribute to a higher DLI. Young plant crops that require or benefit from long days for proper growth and flowering include dahlias, mums, petunias and tuberous begonias. Growers already providing supplemental lighting to increase DLI will generally extend the daylength past sunset using their HID or LED lights. Growers not needing to increase DLI typically choose from one of the following options:
Incandescent bulbs—Historically, 100W incandescent lights hung 4 ft. above the crop and spaced at 4-ft. intervals were the tool of choice for night interruption lighting. Since incandescents have been phased out of production and sale, growers are turning to other options.
Fluorescent bulbs—Full-fixture fluorescent tubes aren’t a good choice for night interruption lighting due to the structural shading from the fixtures. Compact fluorescent bulbs could be used for night interruption, but the ratio of red to far-red light isn’t ideal and some delay in flowering in some crops has been observed. In the end, few growers choose this technology.
Beamflicker—This innovative solution teams an oscillating reflector with an HID lamp to provide night interruption to a 30 ft. x 90-ft. space. Includes cord and runs on 120 or 240V power. While this is an HID lamp, the fixture isn’t intended to increase DLI.
LED—Lower-output LEDs can provide effective photoperiod control. Plug production utilizing germination chambers brings another case in which specialized lighting is needed. While some crops need dark conditions for germination, many require light, e.g., ageratum, angelonia, gerbera and impatiens. Fluorescent tubes or LED strips are extremely well-suited to this use due to the reduced heat discharged from the ballasts and the low light intensity required. Select fixtures that will perform well under high humidity.
Contact your supplier to learn more about photoperiod needs and general light requirements for your various crops.
What is light quality and why is it important?
Light quality refers to the wavelength of light. In the visible spectrum (the range of wavelengths we can see), different wavelengths of light are different colors. This spectrum ranges from red at 700 nm to violet at 400 nm. Plants use light in this same general range, with different wavelengths impacting growth in different ways. Light in the violet/blue and orange/red wavelengths is more important for photosynthesis. Plants reflect most green light (approximately 510 nm), which is why we perceive the leaves to be green in color. Red and far-red light impact plant recognition of photoperiod and seed germination. Higher ratios of blue light can result in shorter plants. Likewise, high amounts of far-red light will increase cell elongation, which is why we avoid shading crops with hanging baskets.
What is PAR?
Photosynthetically active radiation (PAR) includes light with wavelengths between 400 and 700 nm, which is the range of light used by plants. Measure-ments of PAR indicate instantaneous light intensity in this specific range, essentially answering the question, “How much usable light is hitting the crop at this second in time?” The most correct way to measure PAR is by using a meter at crop level and reporting the reading as micro-moles of light per unit area per unit time, generally reported as umol/m2/s. A PAR reading would indicate if there was adequate light to manipulate photoperiod, but wouldn’t necessarily confirm if there was adequate light over the course of a day to support plant growth.
What is DLI?
When we want to understand the total light reaching a crop over the course of a day, we turn to the daily light integral (DLI). DLI measures accumulated light over the course of a day and is reported as mol/m2/d. DLI is best measured at crop level with a meter intended for this purpose, though some extrapolation could be made using existing greenhouse monitoring equipment.
Ambient DLI varies widely across the country throughout the year. Jim Faust at Clemson University led work to create national DLI maps, which can help growers understand the ambient DLI before entering the greenhouse. Remember that greenhouse coverings and structures will reduce the light entering the greenhouse, so the Outdoor DLI Maps shouldn’t be used as an indicator of bench-level DLI.
Much work has been done to understand the minimum DLI needed to produce various crops. The Purdue Extension service offers a bulletin that outlines the DLI needs of a wide assortment of crops. This information is valuable in determining how much supplemental light to provide to a crop for economical production. GT
Tami Van Gaal is a GGSPro Technical Support Specialist for Griffin. She can be reached at ggsprotech@griffinmail.com.