Since LEDs are relatively new to the indoor horticulture industry, there are many false misconceptions that are important to recognize before evaluating or purchasing LED grow lights.
Color Temperature (Kelvin)
Many people use color temperature (Kelvin) as a main factor when evaluating grow lights, however, Kelvin only measures how light looks to the human eye— i.e. whether white light has a warmer or cooler color temperature to it. Color temperature tells us very little about the actual color spectrum distribution of a light, which is what actually impacts plant growth. This color spectrum cannot always be perceived accurately by the human eye, which is why lights with the same Kelvin that look identical in color temperature can have different spectrum profiles which will have different effects on plant growth. The popularity of using Kelvin comes from a time when most grow lights used were HPS or CMH, both of which are high-intensity discharge (HID) lights. These types of lights produce fixed spectrums of lights which is a result of them all using the same gases in their lamps, so there is not much variation between lights. That made it okay to speak about the color temperature in much less precise terms. LEDs however can produce almost any spectrum of light in any distribution depending on the quantity and type of chips used.
The color spectrum profile of a light is important to consider because each wavelength (color) of light is responsible for a different aspect of the plant’s growth. For example, blue light drives stockiness, while red light helps plants stretch, while green light helps penetrate plant canopies. Plants use all wavelengths of light for well-rounded growth, which is why it is optimal to get a full-spectrum light that uses a combination of all colors. Most high-end LEDs use a full-spectrum white light, similar to the light from the sun that the plants evolved to grow under. This is why color temperature is not the most accurate way of evaluating the spectrum of LED lights and you should instead be considering the specific light spectrum distributions of each light.
When considering different grow lights, many people look at wattage to determine how powerful the light will be. However, wattage only specifies how much power the light uses, but not how much light output it actually produces for plants. The light output, or PPF, is what ultimately determines how effective the light will be for growing plants. Using wattage to evaluate how much PPF a grow light produces is another hold over from when HPS and CMH lights were the standard. They had relatively fixed efficacies. Meaning regardless of the brand a 1000W DE HPS light was always going to be producing a very similar amount of light. However, this is not the case for LED grow lights.
What is more important for LEDs is to consider is the efficacy of that specific light, which is how efficiently a light can convert that power (wattage) into light for your plants. Efficacies differ depending on the quality of the LED grow light, which means lights with the exact same wattage could have very different light outputs. For example, since LEDs are much more efficient than HID lights, our 720W FL-1 LED Light has a higher light output than the average 1000W DE HPS Light while using less energy. Even among LEDs there is a large variation in efficacy. An example of that is how our 440W VL-1 LED Light has an efficacy of 2.8 µmol/J is able to produce a higher PPF than HLG’s 480W grow light since it only has an efficacy of 2.5 µmol/J. You want to look for lights that have high efficacies, so you can get the highest light output while also using less electricity.
Calcium Magnesium Deficiencies
LEDs causing Calcium Magnesium deficiencies is a rumor that started when people began noticing purple stems when using LED lights but not HPS. While purple coloring can be a sign of magnesium deficiency, if your plants do not have any other signs of distress this coloration means the plant is producing natural purple pigments (anthocyanin) in response to UV light. This is only true under high-quality full spectrum lights that produce UV light, and since many artificial HPS and LED lights don’t produce UV, this purple coloring is unfamiliar to many growers. Therefore, if there are no other signs of distress— such as yellowing blotchy leaves— purple stems are not necessarily a negative sign, and in fact, is a positive sign if you are using a high-quality light.
Once you decide to switch to LED grow lights, it is also important to note you cannot run them exactly as you would HPS/HID lights. Read our article “Common Mistakes To Avoid When Growing With LED Grow Lights” to know what new factors you need to take into account in order to maximize your LED grow.