Lighting

Light in general is a very important factor for plants' development [31, 59]. It regulates at least the heat balance, is the main energy source and affects the phenotypical expression and the growing habit. For some species light even affects the overall development by influencing the distribution of different substances [59]. An example would be the preparation for fall and winter, for which important substances are secured into the stem or roots, initiated only by light [31, 59]. Hence, you want to make sure your plants get the quality and quantity they crave for! – a rather difficult task, as even modern science is struggling to describe all dependencies between light and plants. Nonetheless, it is a really exciting field of study, for which first results explain the correlation between tastier fruits and lighting. [31, 46]

Some good news for us as non-researchers regarding quality: plants depend for the photosynthesis on lightwaves visible for our eyes. Hence, we can at least see what is going on.

To measure light, the spectrum and the intensity are relevant [31]. Regarding the first, wavelengths around ~360 and 660 nm [59], which is equivalent to the for humans visible blue to red light, are used by plants to get their energy. That becomes more obvious, if we remember that we see objects in the color they reflect the most – the green wavelength mostly being reflected therefore causes the plants' leaves appearing green in our eyes. Photosynthesis is the reason behind this process, which allows the plants with the energy of specific photons (single rays of light) to convert CO₂ into glucose (sugar), a very energetic molecule [59]. For the mentioned heat balance, plants utilize longer wavelengths [31] which correspond to infrared light. Hence, a light spectrum should be provided, which uses wavelengths between ~400 to at least 700 nm, better even higher.

Looking at the intensity, there are several units you may encounter. First, there is the so-called PAR value which measures the available energy per square meter (W/m²) for the photosynthesis [5, 31]. Hence, it does not cover the infrared spectrum and depicts only close to 50 % of the overall energy delivered by sun light [31]. Another method counts the number of light rays received per square meter and second (µmol/(m²·s)). It includes therefore the whole sunlight's spectrum, but is only able to account an energetic average to each light ray [5, 31]. Nonetheless, these units are the ones you should look out for when buying growing lights, as all others (such as Lux, Lumen, Candela, etc.) provide you with no meaningful information as a grower. But when do you actually want to support your plants with artificial lights?

As a rule of thumb, most plants require at least around 12 hours of daylight, including several hours (depending on the variety) of direct sunlight per day. In average, your system should receive around 400 Wh/m²/d [5, 59] measured with a PAR meter, which equals a sunny day in middle Europe in Summer [31]. Hence, living in middle or northern Europe will make it impossible to provide enough light off season, even if you place your hydroponic system at a window looking southwards. Hence, you should aim for artificial lighting, to provide at least 200 Wh/m²/d, but not more than around 500 Wh/m²/d for year round production [59]. Still it should be mentioned, that some varieties (mostly the ones not being common to hydroponics [27]) depend on seasons – hence you have to adapt your lighting for them in order to trigger certain metabolic processes like blooming or fruiting [59]. This applies to the lighting intensity as well as the lighting hours.

If you have questions regarding troubleshooting, check out this section here. Otherwise you may discover some frequently used lighting technologies below.

Diodes are an electronic component, which may be used as well as light source, called LED. Hence, they directly react to the electric circuit and are in theory capable of emitting a good spectrum of light (depending on the physical construction) [5], are long living and a very efficient lighting technology [5, 9]. But they fall short on the infrared spectrum. Hence, if you plan to place your hydroponic system hidden from sun and daylight, you may want additional or other lighting options. Additionally, LEDs come with an high price tag, making it not always recommendable for every project by the way. If you plan to grow plants for a longer period and even to reuse the LEDs for other projects as well, you may be able to make use out of the LEDs advantageous despite the high price [5].

Just be sure to get information on real the spectrum emitted, to make sure you get what you want.

Fluorescent lights use so-called fluorochromes, that emit a photons with a specific energetic value (light rays of a defined color). [9] They are great for beginners, as they are cheaper than other technologies, widely available on the market and don't require special setups to be installed beforehand. [5] They also cover a good spectrum of light, but fall short of infrared radiation like LEDs. Therefore, if you plan to place you hydroponic system hidden from the sun and daylight, you also may consider other or additional lighting solutions. 

HID lights use a pressurized molecule mixture under an high electric voltage in a temperature resistant chamber, to produce a really bright blueish plasma stream. Because the voltage needed to initiate this physical process exceeds the one you get out of the standard power outlet at home and because the plasma stream also emits a lot of heat, you require to tweak your power source and to have a good ventilation. Hence, HID lamps come with the requirement of special setups, but have an overall good efficiency and are often used by professional indoor growers. [5, 9] Be aware, that there are many technologies falling under HIDs such as HPS, MH and CMH lights.

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