Grow lights come in all shapes and forms. High intensity discharge (HID) lights have been used for decades while more recently, light emitting diodes (LED) are becoming a popular alternative. Within the LED category there are different types of lamps, fixtures, and bulbs. There are the standard LED grow light fixtures that typically have a metal or plate chassis, a couple of built-in fans and drivers, and sometimes a heat sink.
In more recent years we’ve seen circuit on board (COB) technology, as well as the quantum boards (QB) (which are essentially an LED panel with a driver and sometimes a heat sink) rise in popularity. As we make technological advances, we achieve more and more efficient bulbs and diodes which means less energy (watts) gets consumed to produce the same amount of light output. Still, it is the consumed or drawn watts that determine how much heat the light source generates as wattage is proportional to BTU (British thermal unit).
A second universally applicable variable for all grow lights is the relationship between distance and PPFD (light intensity). The shorter the distance to the plant, the more intense the light (higher PPFD) but at the same time, the light footprint (coverage area) decreases in size.
When growing indoor plants, they will need different light intensity during their grow stages. Clones and seedlings need much less intense light than mature plants that are producing buds or fruit. Adjusting the lamp up from the canopy will decrease the intensity of light (decrease PPFD). Moving the lamp closer towards the canopy will increase light intensity (increase PPFD).
It’s also important to understand how light spreads over an area: a lamp’s so-called light footprint.
Preparing the Light to be Hung
This one is simple. Unpack the LED grow light and find its hanging points. Typically, a grow light will come with a metal wire hanger that screws or clips onto the lamp’s body. The metal hanger, in turn, hangs from a rope ratchet (typically not included with grow lights) from a grow tent or growroom ceiling. The metal wire hanger can vary in length. If you realize the included hanger with your light is a bit too long for your tent or grow space, shorten it by guiding the metal wire around the frame of the grow tent.
Heat from a grow light, regardless if it’s an LED or an HDI/HPS (high pressure sodium), travels upwards. The more wattage a lamp consumes, the more heat it’ll generate. If you just switched from a, let’s say, 400W HPS to one or two 100W LED grow lights, you'll see a drastic reduction in heat produced. This can be a good or bad thing. For most growers, at least those who live in a warm location, switching from HPS to LED could reduce or eliminate the need to run an AC unit that would otherwise be necessary in order to cool the grow area. On the other hand, those who grow in colder climates may need the increase in temperature to give their plants ideal growing conditions.
Read also: Success With LED Grow Lights
Assuming you grow in a small and confined area, a grow tent, for example, with an ambient temperature of 72°F (22°C), any type of grow light will result in a heat increase. The more watts, the more heat generated. A 100W grow lamp in a 2x2x6-foot tent will increase the temperature by about 10-15°F (5-8°C).
This is a rather insignificant difference and can easily be managed with one or two small USB table fans where the warm air at the top of the grow space is circulated out of the tent.
Getting the Distance Right
This is where it gets interesting. A lamp that is too close to a leaf could burn it, but a lamp that is too distant from a leaf leads to less than ideal amount of light received and could hinder growth.
Plants require a certain amount of light, or rather, light with a certain intensity.
A high number of photons in an area equals intense light. A low number of photons equals low intense light. The amount of photons in a specific area is determined by the total output of photons (light) from a lamp, if the photons are angled with covers, reflectors, or lenses, and, finally, the distance between the light source and said area.
Many LED lights focus their light with either 120-degree or 90-degree lenses or reflectors. This results in photons being more concentrated in the area underneath the lamp but also makes for a smaller light footprint (coverage area). Most lenses and reflectors absorb or block a certain amount of photons, which means a loss in the total amount of light that could reach the plants.
All photons have a natural tendency to move about as they are beamed out of the light source. A lamp with its diodes facing down towards a plant’s canopy will emit the photons towards the plant. However, as the photons are beamed from the diodes towards the plant, the photons spread. If there is no focusing lens or reflector, they could theoretically spread in a 180-degree direction.
As the photons spread and become less dense, the intensity of the light decreases. The further away the lamp is from a certain point, the less intense (fewer photons) the light hitting that point will become.
Here’s an illustration of how light footprint and intensity changes based on the distance between the flashlight and the table.
So, the longer the distance between light source and plant, the less intense is the light received by the plant. If the distance between the plant and the lamp would be zero, or very close to zero, the plant would essentially receive all of the photons emitted by the lamp, so a very intense light.
Now that we understand the theory behind increased distance = lowered intensity, we are ready to adjust the lamp’s distance to achieve desired intensity (PPFD) at canopy level.
Read also: How To Tell When Your Lights Need Replacing
PPFD Levels for Growing Weed
Cannabis plants will need approximately 100 PPFD during their clone stage, 100-300 PPFD during seedling, 300-500 PPFD during vegetative stage, and 600-800 PPFD during flowering. During its flowering stage, it could potentially take even higher PPFD levels if all other conditions (nutrients, CO2, temperature, humidity) are ideal. For beginner growers, 600 PPFD is a good number to aim for.
PPFD is measured with a quantum sensor and the unit is in µmol/m2/s (micromoles [of photons] per square meter per second). Exact quantum meters do not come cheap and are rarely a worthwhile investment for a small-scale grow. Without a quantum meter, one has to rely on data supplied by the grow light manufacturer. Even if these numbers aren’t always exact and could possibly be inflated from time to time, it’s usually the only data to go on.
A video-based PPFD test is much more trustworthy than data sheets but few manufacturers are transparent or thorough enough to record PPFD tests. There are, however, some quality PPFD tests that can be viewed on YouTube.
Point being, use whatever data you have at hand but be critical and know not all data is completely trustworthy. If you have already bought your grow light, get the PPFD sheet from the seller. If you haven’t yet made a purchase, try to find a seller that has both a grow light that meets your requirements but is also able to supply exact and transparent PPFD tests.
If a plant receives too little light (low PPFD), it will stretch towards the light source. A less-than-ideal amount of light will limit photosynthesis and total mass production will not reach maximum.
If a plant receives too intense light, the tip of its leaves will burn (turn yellow). This should also be avoided. Tip burn, however, is a good indicator your lamp is too close to the plants. If you have no tools or data to measure PPFD, move the lamp closer and closer towards your plants until you notice a faint tip burn, which usually happens after 12 to 24 hours after adjusting the lamp. As soon as the tips start to become yellow, you know that the lamp is a bit too close to the canopy. Increase the distance by two inches (five cm) or so, then the light should be at a good height.
Keep in mind that as the plant grows taller, the lamp needs to be raised further.
We want to stimulate our plants’ photosynthesis as much as possible. It is through photosynthesis plants grow and bear fruit (buds). The right light intensity and light quality have a big impact on photosynthesis so it’s important for every grower to understand how his or her grow light should be hung to give plants the best possible conditions for maximum yield.
As practically all grow lamps out there have different output, one distance cannot be generalized for all lamps. As we see in the examples above, even two lights that have vastly different draw power can have similar output and coverage. To optimize your grow, you need to make the calculations for your specific requirements. Use data from the grow light manufacturer to determine your lamp’s output, then adjust the height of the lamp from your plants’ canopy based on the plants’ grow stage.