It’s hard to emphasize how important light is for a successful cannabis crop. Light fuels photosynthesis, which fuels growth. A happy cannabis plant is one that’s exposed to lots and lots of light. The problem is that there are so many lighting options on the market that it’s hard to find the option that works for you. Here’s how to
Choosing the Right Light for Your Grow Room Size
When choosing a grow light, you want to make sure it’s the right light for your space. Regardless of how bright it is, there are other factors that affect whether the light will work for you.
When you’re comparing lights, there are a few different terms that you’ll come across. First is wattage – how much electricity is used to power the light. Another common term is the “lumen.” Essentially, the lumen rating is how bright something appears to be from a certain distance. Lumen ratings are more helpful than watts, but they don’t work as well with LED lights.
This is because the color spectrum of LED lights is broader, so they appear brighter than most traditional lights. In general, LED lights will appear slightly brighter than traditional lights measured at the same lumen level.
Finally, the newest term you’ll read about is PPFD, or Photosynthetic Photon Flux Density. This is specifically a measure of the light wavelengths that plants absorb. More and more grow lights are listing their PAR values for a given distance, as it measures what you actually need for your plants.
The fundamental definition of wattage is the number of joules of energy per second are transferred from a source to a receiver. If you learn more about lighting, you’ll notice that PAR is measured in watts/m2. This is due to the source transferring light to the plant.
The old rule of thumb is that you need 100 watts of light per plant, minimum, for a good grow. This rule is fine for traditional lights, like metal-halide lamps. However, it gets tricky with LEDs. The modern rule is that each plant should receive between 400 and 900 w/m2 of light.
Plants in the vegetative stage thrive between 400-600 w/m2 and flowering plants need 600-900 w/m2. Seedlings can be grown on as little as 200 w/m2, but risk getting burnt above 450 w/m2.
In order to compare lights, you need to look for PPFD levels. If PPFD or lumens aren’t listed, wattage can be used as a rough estimate. Some lights are much more efficient than others. LED grow lights are very efficient, meaning that they’re brighter per watt than traditional lights.
With this in mind, LED lights will frequently list their apparent wattage, or how many watts they’d use if they were a traditional light. This can help guide your search if you’re comparing different kinds of grow lights.
Blue light is better for vegetative plants, while red light encourages flowering and fruiting. Many growers continue to use the same amount of blue light for flowering as they did for vegetation growth. To spur flower growth, they supplement with red light to reach the recommended PPFD.
Number of Lights
Once you’ve chosen a certain light, it’s time to figure out how many you’ll need. Since grow lights are usually directional, the light will be brighter in the middle and dimmer at the edges. LED lights are particularly prone to this directionality, shining even-intensity light directly below them but quickly fading at the edges.
If you’re concerned, check the manufacturer’s details to find the PPFD map of the light’s coverage area, to see where the light fades out. Typically, these maps are provided in the spec details.
For this reason, it’s often better to get two 700-watt lights than a single 1400-watt light, or even four 350-watt lights. Your crop benefits from more-even light distribution. The plants at the edge will be able to keep up with the plants at the center. In the end, you’ll get more buds, better buds, and consistent sizing.
How to Measure Light-Plant Distance
Your grow light’s manual should explain the PPFD for a few given distances. If that’s the case, you can hang your lights so the tops of the plants are the appropriate distance away. For example, if you’re in the vegetative stage, hang your lights so the PPFD is about 500-600 w/m2. This distance will vary depending on the light.
If your light manual doesn’t go over this, don’t fret. You’ll just need to get a PPFD meter, which can be found in many gardening stores or on Amazon. You can measure the PPFD for any given distance yourself then. Once you’ve found your sweet spot for distance, you can hang your lights that high above the tops of your plants.
You might be tempted to hang your lights even closer – don’t! Plants can get sunburnt, just like people. If you hang your lights too close to your plants, the top leaves may get burnt. That’s exactly the part of the plant you want to protect. The top of the plant is where your buds will grow, after all. You can also cast shadows on other plants if the grow light is too close, robbing them of the light they need to grow.
How to Hang Grow Lights Properly
You want your lights to be hung safely. A light falling can kill your crop and budget at the same time. However, you also want your lights to be adjustable, so you can move them as your plants grow. What you need is a safe method of raising and lowering your lights.
A rope ratchet uses the same kind of ratchet as rock climbers use to support their weight. You can raise and lower your grow lights by loosening the ratchet and adjusting the length of the rope. If you don’t like the idea of taking your lights off their ropes, this is a good choice.
Chain and S Hook
If you want to go sturdy but low-tech, you can also use chain and S hooks. The chain hooks to the ceiling hook and the S hook, and the S hook attaches to the light. It’s heavy-duty and great for heavy grow lights. The downside is that you’ll need to be able to lift the light on your own in order to raise it or lower it.
Automation: How to Set It Up Right
Once you’ve got your lights hung properly, it’s time to set up some automation. It’s much easier to have a timer turning your lights on and off than it is to do it yourself. There are timers to fit any budget, from cheap analog timers to advanced lighting systems with built-in timers.
Analog timers are simple to use. The most basic ones are similar to egg timers, with a face that rotates. They run on a 24-hour clock, and you simply press down the switches for the times when you want the lights on. You use them by plugging the timer into the wall socket, and your lights into the timer.
When the timer is set to on, it allows electricity to flow through. When it’s off, the electricity cuts off too. It’s like unplugging your lights. This is great for low-tech setups but can cause problems with fancier light systems.
Digital timers are a little (or a lot) more advanced. They can range from barely a step up from analog timers to in-depth control panels. Some grow light kits even come with control stations that include timers. The basic idea is the same, though – when the timer says it’s time to go off, electricity going to the light gets cut.
Be careful if you’re using third-party timers on metal-halide or HPS lights. These lights are rated for the wattage they consume on average. When running, an HPS light uses the wattage it’s rated for. However, whenever it gets turned on for the day, it uses a significant extra amount of electricity. If you have a timer rated for 1500 watts, and a 1500-watt HPS light, the first time you turn them on together will probably fry the timer.
If you’re worried about this, get a controller that can handle 20% more than you plan to use. This gives enough of a buffer than you won’t bust a brand new rig the first time you turn it on.
How to Space Plants
Cannabis plants need room to expand. This is even truer if you’re planning on training them to have multiple colas. A good rule of thumb is that each cola should have at least two square feet of space. How you achieve that can vary.
Some growers focus on growing many smaller plants or use low-stress training techniques, which require less space per plant. The Sea of Green method is one such growing style. With this technique, instead of allowing plants to grow large, you only leave them in the vegetative stage for two weeks. You switch to flowering very early, keeping your plants small. This method is only worthwhile if you have a large number of plants, so on average, you should fit about one plant per square foot.
Low-stress training methods involve training the plant with guides, instead of more dramatic cutting methods. This takes a little more space than the sea of green method because you’re letting plants get bigger. You gently guide the plant to grow in a circle by using stakes and string, so lower branches receive light too. Low-stress training usually requires about two square feet per plant and a lot of attention.
Others top their plants, or force them to grow on a screen, which can require four to six square feet per plant. These methods are more dramatic for the plant. Topping involves cutting off the top half of the plant one or more times, in order to force more colas to grow. The more colas the plant has, the more tops there are on which bud can grow.
The great thing with a measurement like PPFD is that it measures light intensity over square footage. No matter how many plants you have in a given space, as long as the foliage is receiving the correct light intensity, they’ll grow. The biggest problem is making sure no plants overshadow any others. Methods that produce big plants fit fewer plants into the same space to avoid this.
Signs of Insufficient Grow Lighting
Since light is the fuel for growth, if your plants don’t get enough light, they won’t grow. There are a couple of tell-tale signs that your plant isn’t getting enough light:
- Legginess: If there’s a lot of space between leaf nodes or your plant seems to be “stretching” or leaning towards light, it’s leggy. That means that it’s trying to grow its stem until it reaches more light. It’s a sign of light starvation in plants, and means you need brighter lights.
- Dropping leaves: If a leaf doesn’t receive enough light, it will slowly fade from green to yellow and then fall off. This is most common in lower leaves that are blocked by higher branches.
- Slow growth: This is the biggest sign that a cannabis plant doesn’t have enough light. Cannabis can grow up to ten feet tall, depending on the strain. If you’re in the middle of the vegetative stage and not seeing several inches of growth minimum every week, then your plants need more light.
Lighting is probably the most important factor for a good marijuana harvest once you’ve chosen a strain. There’s a lot to consider when you’re setting up your lights. It’s not as complicated as it may seem, though. Get some quality grow lights with good reviews/ratings and hang them a foot or two above your plants, and you’re most of the way there. Cannabis grows like a weed, after all. Working with specific light intensities, colors, and distances are simply ways to maximize your harvest.