Worried your LED fixture is burning your crops? Or perhaps, you’re wondering if your grow light is even strong enough? It’s time to stop stressing out about light stress!
No matter what kind of crop you’re growing here is how to easily ensure your plants are getting the correct amount of light.
What is Light Burn / Stress?
Our crops love light, but just like us, they can receive too much and be burned by it. Thankfully, there are many ways to easily prevent light burn. And even if you don’t have the tools available (which we will give you), you can catch light burn before it happens by knowing the symptoms of light stress.
Both low and high light environments cause stress on our lovely little crops.
Low-light environments cause our crops to excessively stretch and become leggy in pursuit of light, creating weak stems and overall health that leads to sickness, stem snaps, and death.
High-light environments, at first, cause light stress, which most often results in leaves curling up and in together — as if praying — in an attempt to close themselves off from the light. This works but only for a short time, giving the gardener a short interval to correct the issue before light burn occurs.
Light burn causes DNA damage that bleaches a crop’s leaves, permanently turning them white or yellow. It also causes damage to secondary metabolites like antioxidants and terpenes, leaving a crop susceptible to diseases while hurting the quality and quantity of its yield.
Light burn can look similar to nutrient burn and other fertilizer issues. To tell the difference, besides looking for leaves that are “praying” to indicate the issue is indeed light burn, you will note where the issue is occurring as leaves at the top of the canopy will show the most stress/damage.
A sure-fire way to tell if you’re giving your crops too much light is by learning how much light is coming out of your fixture, how much is actually reaching your plants, and how much light your plants want. When you know these, you can ensure your crops are in the safe zone, meaning it is highly unlikely they will ever burn.
How High Should I Hang My Lights?
Many factors go into how high your LED lights should be over the top of your plant’s canopy. Without a doubt, the two biggest factors are going to be the strength/intensity of your grow lights, followed by the type of plants you’re growing.
It’s common to see 6″, 12″, 18″, and 24″ as suggested hanging heights. This worked well in the past, but due to all the different types of grow lights to how well they perform to advances in meter technology, these general recommendations are obsolete for most. I couldn’t tell you how high my lights are currently hanging over my crops, but I know exactly how many photons they are receiving down to the second.
How Strong Should Your LED Grow Light Be?
While there are many ways to work around lights that are either too weak or too strong, it’s often not very fun. Thanks to many LEDs having dimming capabilities along with the ability to raise or lower their height to change their intensity on your crops, you don’t have to be absurdly precise when selecting the right light for your garden. You do want to make sure it physically fits, though!
While outdated and tricky to apply to LEDs due to the range of efficacies and spectrums between different types, watts can still be used to help select a light that adequately lights your growing space. Thanks to LEDs’ ability to convert the watts it pulls from your outlet into light vs. heat, our grow rooms only need about half the amount of watts to light compared to the old HIDs. New full spectrum LEDs like the Growcraft Ultra also perform a third to twice as well as old “blurple” LED fixtures.
General Grow Tent Size Guide For White (Full Spectrum) LEDs
- 2×2 tent/mini garden: 50-100 watts
- 2×4 tent/small garden: 50-300 watts
- 3×3 tent/medium garden: 150-400 watts
- 4×4 tent/large garden: 600-1000 watts
- 5×5 tent/giant garden: 1000+ watts
As you can see, you have a wide range of watts to choose from when it comes to your grow space. When in doubt, staying at the top of that range is going to be your best bet. Especially because the light will likely be able to move up to a bigger tent/space successfully. When your LED fixture has dimming capabilities, you can really go big or go home. LEDs with dimmers are wonderful! Now, they won’t run as efficiently as possible when dimmed, but they are still more efficient than old grow lights. You can also simply hang them higher than normal to decrease their intensity.
How Much Light Do Your Plants Want?
It’s often helpful to imagine that your crops are batteries that need a certain amount of light to charge them. Because the crops we grow indoors come from all over the world, different plants have various daily light needs (DLI) that need to be satisfied within a certain number of hours within a 24hr period.
Most crops have a day-neutral photoperiod and aren’t terribly fussy about how fast you charge their batteries within 24hr, just that it gets charged. It’s good practice to still try to give them a light on/off period that’s close to what they’d naturally received outside — giving between 8 and 20 hrs of light for these crops is usually fine.
Some plants, however, have a photoperiod that’s either long-day or short-day and won’t flower or produce fruits until they get the right amount of hours of light.
Take cannabis, a short-day crop. To keep it in the vegetative phase, where it grows and grows, you just need to ensure you’re giving it more than 12-14ish hours of light. Some growers run their lights for 16 hrs, others do 20, and some even run their lights 24/7 in the vegetative stage. To ensure the plants don’t burn under these various hours of light, growers just need to know their crop’s DLI (daily light integral) to properly adjust the intensity of light their crops are getting. DLI is the max amount of photosynthetically active photons (PAR) a plant can tolerate over 24hr period.
PAR just defines the section of the light spectrum that crops use for photosynthesis, PPF (photosynthetic photon flux) tells you how many PAR your fixture produces, then PPFD (photosynthetic photon flux density) is the measurement of PPF that is reaching your crops for them to use.
After their seedling stage, the DLI you give your crop will probably stay the same. Those that grow short-day crops increase their lighting intensity by about a third for the flowering stage because they reduce the hours the lights run by about a third.
To find if your crops are getting their prefer DLI, all you do is take the hours of lights you run them for and combine it with the intensity of photosynthetic light at the top of your crop’s canopy (PPFD). Because of the inverse square law, this won’t be exact, but it should put you in a safe zone to work up from if you so choose.
General Target PPFD & DLI Chart
|Target PPFD (umol/s/m2)
|Avg. DLI (umol/day)
Are Your Plants Getting The Right Amount Of Light?
There are many ways to calculate your DLI, from using a Lux meter to a phone app. And there are several ways to increase and decrease it, from adding/removing lights to using their dimming function to raising or lowering their height. Thanks to the inverse square law, when you double the distance your lights are from your plants, you decrease the intensity of light on them to 1/4th of its original intensity.
Ways to calculate your DLI:
ChilLed’s LEDs all feature a dimming function along with hangers that easily allow you to raise or lower your lights. You can use those two methods along with our phone app light meter to lock in a pretty optimal PPFD and DLI.
Additionally, we have PAR Maps on each of our lights’ product pages that show you how much PPFD will fall on a certain area at 6″, 12″, 18″, and 24″ of height, especially, helpful when you don’t have a light meter of some kind.
If you want to be absolutely precise, you’ll want a PAR meter! A lux meter is a cheaper option, but it involves an additional calculation and knowledge about your grow light’s spectrum.
Now that you know how high your lights should be over your crops and how to avoid light burning them, the question to ask yourself is, do you really want to run your lights at your crops’ max?
Our plants have a min-max range of the light they can optimally receive, and while the max DLI will produce the fastest growth, it will cause nutrient and other stress issues to appear and progress faster since the plant is working so hard. It is often why nutrient issues and light stress appear together.
Many new to indoor gardening may want to start around the middle of their crops’ optimal DLI range while working on developing a keen eye for the symptoms of light stress (which is important if you grow outdoors as well). Once you feel you feel comfortable with everything from feeding the right amount of nutrients to providing optimal temperature and humidity levels, you can start to increase your lights to your crops max DLIs.