One of the most critical challenges—whether for greenhouse managers or for horticulture aficionados—is to provide plants with enough photoperiodic sunlight for effective photosynthesis, so that they can grow optimally regardless of the geographical location and climate. Winter months even in supposedly warm-weather California can have a Daily Light Integral (DLI) of 10 to 20, which is insufficient for optimal plant growth. As a result, supplemental electric lighting is required in most cases to accelerate flower development, create hardier stems, increase chlorophyll content, and also increase leaf count.
Light acts as a key environmental signal and a critical source of energy for plant growth, with plants using light for both photosynthesis and development. Lighting parameters influence germination, seasonal and diurnal time sensing, plant stature, growth habits, and transition to flowering and fruit ripening. It is therefore important to control the quality, quantity, intensity, direction, duration, and wavelength of the light reaching the plants, in order to ensure effective growth, sustained development, and maximized crop productivity.
Main Reasons Why Controlling Light Is Vital for Plant Growth
Light Uniformity
Light uniformity refers to how evenly the light is distributed across a given growing area, and should be an important consideration—just as light intensity and quantity is—for all types of plant lighting installations. Light uniformity can regulate crop growth, plant development, flowering schedules, and water distribution. If the illumination system for a growing area is not designed to distribute the light in a uniform manner, the crops will dry out or develop at different rates depending on whether they are getting access to more or less light across the same area. If some plants receive more light than others and exhibit uneven growth patterns, that in turn can lead to uneven shading.
Light uniformity is affected by a number of factors, including (but not limited to) the light source used, the reflector design, the type of fixtures, the light distribution, beam angle, fixture quantity, fixture spacing (how close together they are), and the distance of the fixtures from the plants themselves.
A uniform blanket of light can be achieved by equipping the light fixtures with light bars, which can be easily arranged according to desired spacing to achieve effective intra-canopy light penetration. The luminaires with their source type (ideally LED) and light bars should be mounted at an optimum height and spacing—either via calculations or by following manufacturer recommendations—to deliver a uniform layer of light (without creating hazardous light intensity levels or hot spots) over the full plant canopy, even as the canopy grows and changes over time. These techniques will then translate into increased profits per harvest, and will maximize dry growth yields on a continuing basis.
Light uniformity also affects the efficiency of any prescribed nutrients, since plants receiving lower light annually (compared to the targeted average) will consume more nutrients or dry faster due to uneven water use, and that will reduce profits.
Lower Energy Costs
The electric lighting—used to either supplement the daylighting on the plants or act as their primary source of photosynthesis and development—can be a significant portion of the total energy use, and will impact your bottom line accordingly. In addition to the investment in the lighting system itself for optimal crop growing needs, it is important to choose a light source for not just its high output, but also for its maximized energy efficiency. The efficiency of the lighting system is also negatively impacted by the amount of heat it produces.
The ability of LEDs to produce a lot of light at low cost makes them the ideal lighting source for all kinds of horticulture and crop growth systems. The efficacy (lumens per watt) of LEDs has increased dramatically in the last decade, whereas the cost per lumen has decreased significantly at the same time. In addition, the small form factor of LEDs allows a wide variety of optics, reflectors and housings to be designed around them, enabling much more precise light generation with greater efficiency and at a lower cost.
The plants’ leaf surface temperature (LST) is very important to measure accurately, and is typically warmer than the ambient air temperature. A 75ºF ambient air temperature under HID (MH or HPS) systems generally leads to an LST of about 85º-88ºF. The higher energy efficiency of LEDs ensures that they are much cooler (irradiating much less heat) than their HID equivalents—resulting in the ambient air temperature becoming about 10º cooler than comparable HID lighting systems. Cooling costs are therefore typically lower when using LED fixtures, especially when integrated with automated controls and ventilation strategies. When the fixtures run cooler, less air conditioning is required for the space, less water gets evaporated due to excess heat, the plants will retain moisture better, and they will be protected from “light burns”. At the same time, the LST should measure the same regardless of whether the plant is being lighted by HID or LED systems. In order to enable optimized metabolic rates with the cooler-running LED systems, the temperature set point at the grow facilities should be raised by about 9°F to achieve the same optimal LST (about 82-85º for cannabis plants). LED systems can therefore further reduce cooling-associated costs by requiring warmer ambient growing conditions.
Since LEDs produce much less waste heat compared to HID lamps, they can be placed much closer to crop surfaces without the risk of overheating and related stress for the plants, while still ensuring uniform light distribution. This means that LED systems can be designed with a lot more flexibility—such as horizontal, vertical, multi-layer, intra-canopy, or inter-crop lighting layouts.
The unique energy-efficient features of LED enable innovative strategies that were hitherto not easy to achieve with traditional sources, and provide better uniformity, higher quality, and increased fruit yield for the growers.
To be continued…