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Cannabis Cultivation Tips: How To Set Up Indoor Grow Lights

Excess light can damage your cannabis plants as much as too little light. Learn all you need to know about light as it relates to cannabis, and how to properly set up your grow lights for massive indoor harvests.

Learn to assess and position your cannabis grow lights for maximum effectiveness.

  • 1. The 3 most common types of modern grow lights
  • 2. Light spectrum, PAR, and lumens
  • 3. PAR and PPFD in watt computation
  • 4. Sun, bulbs, and proper photonic pressure
  • 5. How much light does your grow room need?
  • 6. The watts per square metre rule: how many plants can you grow?
  • 7. HID (MH, HPS): rules for the right distance
  • 8. LED: rules for the right distance
  • 1. The 3 most common types of modern grow lights
  • 2. Light spectrum, PAR, and lumens
  • 3. PAR and PPFD in watt computation
  • 4. Sun, bulbs, and proper photonic pressure
  • 5. How much light does your grow room need?
  • 6. The watts per square metre rule: how many plants can you grow?
  • 7. HID (MH, HPS): rules for the right distance
  • 8. LED: rules for the right distance

THE 3 MOST COMMON TYPES OF MODERN GROW LIGHTS

Contemporary indoor cannabis cultivation usually involves at least one of three artificial light sources: HID, CFL, and LED. Frequently, growers use a combination of grow lamps. Of course, there are other, more antiquated lighting technologies, but they are all pretty much “Betamax” lamps. This blog will focus on the most common lighting used by the 21st-century cannabis cultivator. We are going heavy on the practical grow room advice without relying on the pseudo-scientific stats.

HID or high-intensity discharge lamps have been the grow lights of choice for every kind of indoor grower from beginners to pro cultivators since the early 1990’s at least. Over the years, lamps have become more efficient, ballasts have gone digital, and reflectors have gotten bigger and better. Growers that favour HID are big believers that lumens are the only output statistic that matters when it comes to lights.

Sure, CMH or ceramic metal-halide has come on the scene and the 315W CMH bulb is impressive, but a rather expensive upgrade for a standard MH or metal-halide lamp. Moreover, 315W CMH is not quite as high performance as a high-end dual spectrum/agro 600W HPS or high pressure sodium lamp. Similarly, CFL shows some promise, but always runs a distant second to the tried and trusted HID.

LED or light-emitting diode is the up-and-coming breakaway technology that looks to end the reign of HID for cannabis cultivation. The latest generation of LED systems are now capable of producing comparable, and in some cases, superior quality marijuana than the average HID set-up. PAR or photosynthetically active radiation is the metric that LED zealots consider most important.

It’s best not to get too obsessed about the various new measures of light and modified light spectrums. Keep it simple. Think of light in terms of watts and electricity bills. Next is the question, “How much power consumption?” Followed by, “How much heat will the lamp produce and how much light will it emit?”

OLD SCHOOL HID

In ordinary decent stoner terms, HID has gotten some new bells and whistles, but the core principles of cultivating cannabis with this kind of artificial light remain the same. MH for vegetative growth and HPS for flowering. Although, the dual spectrum HPS lamps are a popular alternative and probably even better for autoflowering strains.

400W and 600W bulbs are the most suitable for indoor cannabis cultivation. Lights need to be hung level. The optimal light height or OLH is between 30-50cm above the plant canopy. This means carefully fastening the reflector to the ceiling or roof of the grow tent with either easy roll hangers or rope ratchets.

It’s best to use fixtures that can be easily adjusted and repositioned as the plants mature and grow taller. However, seedlings and cuttings may find a 600W MH too intense and a 60cm+ distance overhead might be more appropriate.

Don’t mess with light rails unless you’ve got the engineering credentials. Stationary lights that don’t wobble with a level, horizontal bulb housed in a clean, open reflector is professional, standard cultivation. Ideally, use a large reflector, or as big as will squeeze into the grow space, with a wide spread to utilise the entire area.

Glass filters, light, and air-cooling lamps are expensive. Therefore, cool tubes are really only suitable for closet grows as they have some of the worst reflective properties of any reflector on the market. Adding more lights and more fans means higher power bills.

LIGHTING AND GROW SPACE

Generally, 400-600W per m² is plenty of light for a home grow show. Commercial growers may push this to 1000W+ for maximum yield. Packing as many lamps as possible into the grow-op is not a fast track to a heavy harvest. More HID lamps means more light, but also a lot more heat. These babies run hot. 250W lamps are for micro-growers only.

The only advantage the 250W lamp offers is that plant tops can get as close as 20cm to the bulb. Heat output is far less than a 600W, but light will not penetrate as far. Limited light will reach the lower branches of taller plants.

The addition of just one extra 600W HPS can massively increase grow-op temps. It’s often a less costly and more effective solution to invest in upgrading the grow room instead. Maybe cover the walls in Mylar or upgrade from a magnetic ballast to a dimmable, digital ballast, or go all the way to LED.

HOW MANY PLANTS?

The answer is as many or as few as the grower prefers. You can pack 16 cannabis plants in 11l square containers into a 1m² grow space in a SOG. Alternatively, a grower can fill the same space with one large plant using the ScrOG method. Either way, the yield will be comparable. Again, it’s all about how you make use of the light and the grow area.

SUPPLEMENTING WITH CFL

CFL or compact fluorescent light is really effective in the cool white spectrum as an alternative to an MH bulb. A 250W CFL can deliver comparable results to a 400W MH lamp. This is achieved by hanging the CFL as close as 10cm above the plant canopy. CFL runs cool and can get really close to plants without burning foliage. Plus, CFL lights will only marginally increase grow-op temps. This makes CFL perfect for seedlings, cuttings, and for use during vegetative growth. CFL lights are sometimes the only option for micro-growers.

Unfortunately, CFL boasts neither impressive lumen nor PAR output figures. Worse, CFL lights are not very effective for flowering marijuana. Cannabis plants need far more intense illumination during bloom than CFL can supply. Expect lightweight yields and loose buds from CFL lights in the flowering phase.

NEXT-GEN LED

The latest LED systems have finally achieved parity with traditional HID lighting systems. At present, the situation is similar to when flat screen televisions first became available at the turn of the century. The benefits and advantages were plain for all to see. However, the limiting factor was prohibitively high prices by the leading manufacturers.

High-performance LED kits that draw 350-400W and can outperform a 600W MH and HPS configuration with a full spectrum array don’t come cheap. There are some models that really can do it all. The very best systems can run cool and are incredibly economic in the long-term. Hanging distances vary widely, so buyer beware and make sure to check the manufacturer’s instructions.

Below, we dig into the science behind light output so you can assess the ideal wattage and placement of your cannabis grow lights. We’ll cover terms like photons, lumens, and PAR, as well as the main types of grow lights and how they differ.

LIGHT SPECTRUM, PAR, AND LUMENS

Light is made of photons, which are tiny particles that inevitably travel at the speed of light. Photons in light beams vibrate with different frequencies and wavelengths. Humans can see photonic radiations within wavelengths from 380–680nm, while plants’ sensitivity to light ranges from 200–800nm. Not all wavelengths within this spectrum produce the same effects on photosynthesis; those that most activate biochemical processes in plants are called PAR (photosynthetically active radiation), which has a spectral range of 400–700nm.

Light can be measured in photometric quantities derived from the human eye’s sensitivity to colour, or else with radiometric quantities related to the energy transported by light beams. Lumens and lux are commonly used photometric units that refer to light perceived by the human eye.

However, photometric measurements do not indicate how much light energy a lamp sends to a plant for photosynthesis because they don’t include the PAR variables. Despite that, lumens and lux can provide an initial indication on a specific bulb’s output, and we can compare different products by measuring their photometric efficiency with the lumen:watt ratio, regardless of the eventual differences in light spectrum output at different wavelengths.

Lux and lumens work well in measuring MH, HPS, CFL and T5 lights, but they won’t accurately measure the efficiency of an LED light when growing cannabis.

PAR AND PPFD IN WATT COMPUTATION

Broadly speaking, the minimum amount of wide-spectrum light needed by a cannabis plant is around 9,000lm per square metre, while the optimal is over 20,000lm/m². However, a vigorous indoor plant’s growth and flowering depends on the light thresholds reached at specific wavelengths that trigger photosynthesis. That’s why LED grow lights use radiometric systems in order to measure the quantity of photons irradiated in the PAR colour frequencies we mentioned before.

The most widely used radiometric measurement in horticulture is the PPFD (photosynthetic photon flux density), which measures the flow of PAR photonic micromoles per second in a square metre (μmol/m²/s). Assuming the PPFD of a grow light is provided, we can calculate its radiometric efficiency and compare different lighting systems using the PPFD:watt ratio.

SUN, BULBS, AND PROPER PHOTONIC PRESSURE

Midsummer midday sun reaches the Earth at latitudes of around 45° with a photosynthetic power of 1,200–2,000 PPFD. Yet, cannabis seedlings, clones, and mother plants are happy with a PPFD of just 200–400μmol/m²/s. Cannabis plants in their vegetative phase will need 400–600μmol/m²/s, and flowering plants normally require 600–1500μmol/m²/s PPFD with atmospheric levels of CO₂. A study found that the most productive PPFD for cannabis is 1500–2000μmol/m²/s at a temperature of 25–30°C with natural CO₂ increased to 750ppm.

Even if cannabis is a demanding plant, irradiation above the limit threshold for each variety, stage of life, or environmental conditions doesn’t necessarily increase yield. Conversely, an excess of photonic pressure can cause damage to leaves and flowers. In other words, flower production increases when the cannabis plant receives 20–30 moles of PAR light per day, then levels out between 30–40 moles, and reduces at over 40 moles.

HOW MUCH LIGHT DOES YOUR GROW ROOM NEED?

To establish the correct amount of light for your crop, you need to multiply your grow room length by its width to obtain the growing surface, then multiply the obtained figure with your desired PPFD level.

If your grow box is 250cm long and 80cm wide, your grow space will be 2.50m × 0.80m = 2m². If you aim for a flowering plant canopy of around 2 square metres and you want to experiment with a PPFD level of 500μmol, you simply need 1,000μmol/m²/s. This result should then be divided by the PPFD per watt of light to ascertain the wattage needed.

THE WATTS PER SQUARE METRE RULE: HOW MANY PLANTS CAN YOU GROW?

LED systems produce higher PAR than MH, HPS, and other kinds of lamps. Despite that, your plants will need about the same amount of watts per square metre to achieve a harvest similar to that of traditional HID bulbs. Also consider that LED light systems are often advertised as being more powerful than their actual electrical power draw, but this doesn’t mean that a 400W LED can cover the same square footage as a 1,000W HPS bulb. Here are some general charts of the power needed to grow vigorous cannabis plants using different lighting systems. During the early vegetative stage, they will need around half the power.

As a reference, HPS lights can approximately cover the following canopy areas:

250W ≈ 0.5–1m² ≈ 2 plants
400W ≈ 1–1.5m² ≈ 4 plants
600W ≈ 1.3–2m² ≈ 8 plants
1000W ≈ 1.8–3m² ≈ 10 plants

LED lights can approximately cover the following canopy areas:

120W ≈ 0.5m² ≈ 1 plant
200W ≈ 0.8m² ≈ 2 plants
280W ≈ 1m² ≈ 4 plants
350W ≈ 1.5m² ≈ 6–8 plants

To get an idea of how much light intensity you are providing to your plants, you can buy a lux meter and measure the light intensity over different points of the canopy. Alternatively, you can do a theoretical calculation that considers the lumens emitted by the lamp in relation to the grow surface. In order to calculate the lux pressure on the canopy, just divide the lumens of your lamp by the square metres of the illuminated surface. If you position a lamp that produces 100,000lm at a distance of 1m from the plant’s top, it will illuminate an area of 1m² with an intensity of 100,000lx (100,000lm ÷ 1m = 100,000lx). If the lamp is only 0.5m away, it will receive 100,000lm ÷ 0.5m = 200,000lx.

Unfortunately, the luminous intensity decreases by a factor equal to the square of the source distance from the illuminated object, therefore your same lamp placed at two metres will increase its beam width, covering 4m², but with only 25,000lx. Consequently, you would need four lamps to reach your hypothetical 100,000lx target on your whole grow area.

HID (MH, HPS): RULES FOR THE RIGHT DISTANCE

HID bulbs come in a metal halide (MH) version with cold light suitable for the vegetative phase, and a high-pressure sodium (HPS) version with a much wider spectrum for the flowering stage. These lamps run hot and require a proper cooling and exhaust system. They also emit less light over time, thus new bulbs should be kept further away than older bulbs, and after a couple of years, you should consider replacing them.

The proper distance for both MH and HPS bulbs depends mainly on wattage. Always begin with the light at the upper end of the range and gradually lower it. Assuming the ventilation in your grow room is properly set, with a smaller 250W bulb you can start at a distance of 35cm, progressively descending toward the canopy until you reach 25cm from the buds at the end of the flowering stage. With a 600W bulb, you might start at 50cm, descending down to 30cm; if you picked a 1000W lamp, you better allow at least 80cm of distance.

Common practice has you hold the back of your hand facing the light just above the top of your plants; if it feels comfortable, so will your plants. But don’t trust general rules too much. Always check your plants for any signs of overheating or excessive photonic pressure.

How High Should You Hang MH and HPS Grow Lights?

Popular, widely used, and arguably the best for higher yields, many growers are committed to their MH or HPS grow lights.

While MH and HPS are suitable for different stages of the growing cycle, you can generally hang them at a similar distance from your plants. Both lights function best when closer to the crop, but it’s good to test them out if it’s your first time.

Whether you’re a novice grower or are switching things up, we recommend starting at the upper end of the light’s range. This way, you can gradually lower it until you’re satisfied your plant is getting enough energy. We recommend keeping your light at least 30–38cm (12–15″) away from the top of the canopy at all times. Remember, looks can be deceiving, and you don’t want to heat up the leaves too much.

If you’re unsure how much heat the plants are getting, do a quick hand test. Simply place your hand at canopy-height under the light and see how hot it is. If it feels too hot for you, it’s also too hot for your plants. Therefore, just move it further away.

One last thing to take into consideration are your bulbs. Not unlike those in your home, a bulb’s functionality decreases over time. Since bulbs give off less energy as they age, an older bulb will need to be closer to your plant than a newer one. Similarly, a brand-new one should be kept as far away as possible without sacrificing plant development. Most people replace their MH bulbs at least once a year, whereas HPS users can replace them once every couple.

MH/HPS Grow Light Distance Chart
STRENGHT DISTANCE
150W 20–30cm (8–12″)
250W 25–35cm (10–14″)
400W 30–48cm (12–19″)
600W 35–64cm (14–25″)
1000W 41–79cm (16–31″)
MH/HPS Grow Light Distance Chart

LED: RULES FOR THE RIGHT DISTANCE

LED systems run cooler than HID bulbs, but they still need some cooling to prevent accidental burning of your plants. LED lights give off a great amount of light pressure, even at relatively low temperatures. This amount of light itself, not the heat, can cause light burn and leaf bleaching.

Different models of LED grow lights have different optimal distances from plants, and manufacturers usually include recommendations alongside their products. Just start at a medium distance within the recommended range and observe your plants over the coming days. Back the lights off if you notice any leaves turning pale or with burned tips. If your plants are happy, move the LED panel closer, but never to the point where the upper leaves start bleaching, yellowing, or turning brown.

High-wattage LED panels with more than 300W should be kept at least 70cm away from the canopy, then eventually moved closer during the flowering stage. As a general reference, 200–400W LED lights should be positioned 30–70cm away from plants, while 450–600W lights require a distance of 50–80cm. Higher wattage systems must be located even further away. Remember to keep a close watch over your plants whenever you start using a new light.

How High Should You Hang LED Grow Lights?

Go-to grow lights for gardeners, LED lights take a lot of fine-tuning to find an ideal hanging height. However, with so much power, under the right circumstances you can get some amazing yields.

LED lights are tricky. There’s so much variety on the market that there’s no one-size-fits-all option. As a result, there’s no standard distance for them. Basically, whichever unit you choose, manufacturers will provide recommended heights and you’ll still wind up fine-tuning.

Another way LED grow lights can trip you up is they don’t always look as powerful as they are. Lights appear dim, so it’s easy to think they’re not emitting enough energy. On the contrary, they’re deceptively strong, and you’ll need to keep an eye on things. If you don’t, your plants could end up bleaching, or you’ll end up with an underwhelming crop.

Although every LED grow light is different, as a general rule, hang them further away than you would other lights, and adjust. You can then keep adjusting throughout the plant’s grow cycle, moving them closer to the plant as it flowers.

LED Grow Light Distance Chart

Again, all LED grow lights are different, but the following chart should help with some general rules.

The simple guide on how to hang your grow lamps that you’ve been searching for. Illuminate your indoor cannabis grow-op like a pro.

How To Select the Best Grow Lights for Your Marijuana Grow

Throughout our articles we have discussed several success factors of an indoor production. Here is a short summary: Growing indoor will provide you with the opportunity to influence all environmental factors. This way you can optimize soil, air, water and grow light to create the perfect conditions for your plants. The more you take care of them, the better your yield. And with light it is easy. Always remember: More light = bigger yields!

Lights are essential because they largely determine your plants’ grow cycles, their photosynthesis and therefore their health and their buds. Lighting is the food of your marijuana plants. When first creating your grow room make a blue print to decide what system for water management, light and air flow you want to use. In this piece we will discuss the details about lighting.

Which Lighting is Best for Growing Weed?

There are 4 main types of lamps that you can use for growing cannabis:

Fluorescent grow lights (T5 and CFL)

Metal Halide grow lights

High Pressure Sodium grow lights

While each of these options has pros and cons, as well as different stages of growing cycle and different sizes of operation they work best for, in this article we’ll focus more on the types that will be your bread-and-butter for a commercial-scale grow operation.

Pro Tip 1

Normal light bulbs won’t work for your plants 🙂

LED Grow Lights

LEDs are super easy to set up. Small ones are literally plug and play. Once you have plugged them into a wall you may just hang them above your plants. They are the way to go for an easy setup that still grows great yield.

They don’t run very hot but for a professional production you still will have to set up exhaust fans and organize airflow and temperature throughout your growing facility.

Make sure to keep enough distance to your plants with LEDs to avoid light burn. Also, be sure to get a light that includes green and white light for full spectrum. Only red or blue light will not be enough for your plants.

LEDs are the way to go for small production up to 1 ounce of cannabis per month, but produce slightly lower yield per watt than HPS and Metal Halide lamps, and are generally less optimal for professional grows.

Fluorescent Grow Lights for Cloning Young Plants

Fluorescent lights will be your go-to for the very first stages of your plants’ lives. They don’t use much electricity, they’re cheap, and they’re also very popular with many hobby gardeners and so are very easy to find.

Compact bulbs (CFL) that you can find in any hardware store can great for small or narrow areas, or grow tent set ups for beginner grows. For your professional production, however, you are going to want to look for T5 Grow Lights (the long tube-like ones) which can be found in a home and garden center.

Those lamps are best used for cloning, seeding and young plants. Without burning the plants you are able to place them close to the plants and save electricity. Also, they do not produce a lot of heat, and the purple-white spectrum is ideal for seedling plants.

However in the vegetative and flowering stages, keep in mind that fluorescents will lead to smaller yields. While T5 lights are great for small plants, in the later stages you will want to use a higher powered light like HPS or Metal Halide.

Pro Tip 2

Use T5 flourescent grow lights for cloning, propagation, and seedling stages

Metal Halide Grow Lights for Vegetative Stage

These lights are one of two types of High Intensity Discharge (HID) grow lights, and are incredibly efficient. Metal Halide (MH) lights typically come in an integrated fixture with an external ballast and reflector hood, and will need proper ventilation due to high heat output.

These bulbs, along with High Pressure Sodium (HPS), the other type of HID grow light, produce the highest yields per watt of electricity out of any grow lights available. For this reason, they are the go-to for professional growers in the later stages of plant development.

Metal Halide produces a bluish spectrum which is ideal for the vegetative stage.

Pro Tip 3

In the vegetive stage switch from T5 fluorescents to HIDs. Use a ratio of 2 Metal Halide lights to 1 HPS light for maximum yield.

HPS Grow Lights for Flowering Stage

Simlar to Metal Halide, HPS grow lights must be used with reflector hoods and ventilation. The fixtures used for Metal Halide bulbs and HPS are typically the same, so you can use the same fixture for both vegetative and flowering stages, and simply swap out the bulbs.

HPS produces a more yellowish/full spectrum that promotes budding and is ideal for the flowing stage. These bulbs are an absolute must-have for a professional grow.

Pro Tip 4

In the flowering stage, switch to using only HPS bulbs to promote budding and maximize yield.

Pro Tip 5

HPS and Metal Halide lights will get very hot and you want to use them with an exhaust fan to not burn your plants.

To understand the climate and temperature that your plants need, check our article on air flow and ventilation which will teach you how to keep the right temperature in your sealed room.

The lamps are well suited for growing marijuana and if you are looking for the highest yield HPS/Metal Halide is the way to go. Just consider the extra amount of work and cost for setting them up including cables, exhaust fan and fixture. Make sure to get a digital ballast. They use less electricity and bulbs will live longer.

Pro Tip 6

High quality HPS and Metal Halide grow lights will typically create 2 to 4 times more yield than fluorescents.

Why Your Plants Love Light

Your plants ability to perform photosynthesis is the key to your commercial success. Make sure to put some thought into your lighting before getting started. Shining just about any light on your plants is not going to do the trick.

Light waves come in a colour spectrum. The sun is a full spectrum light. A low light intensity might lead so long plants because they stretch to receive enough light. When having high intensity your plants may turn our shorter. You can adjust the light intensity by choosing the distance from your plant.

In the vegetative stage your plants will want up to 18 hours of light. In the flower stage they grow best in 12 hours of light and 12 hours of darkness. Your ability to choose the rhythm of lighting lets you create the perfect light timing for your plants.

Make sure you provide a minimum of 6 hours of darkness for your plants. The 12/12 hours flowering cycle will make your plant believe it is late summer which is their time to develop buds. You want to start this after 2 to 4 weeks of growth in the vegetative phase. This is the time where you can switch from fluorescents to HPS grow lights.

Pro Tip 7

Gently switch from one cycle of light to the other. Take a couple of days to switch from 18/6 to 16/8 to 14/10 to 12/12 hours.

Pro Tip 8

Use a timer to do the switching. This way your lights are always on schedule and you do not have to worry about it. Timers can be easily shopped online or in any store that has electronics for home improvement.

Grow lights are the heart and soul of your plants’ health. If you are small or on a budget you might start out with fluorescent lights but keep in mind that this will affect your yield. Use a mix of fluorescents and LED grow lights for vegetative phase and HPS for flowering stage for professional purposes.

Updated: December 17, 2019

Lights determine your marijuana plants' grow cycles, photosynthesis, health, and the quality of their buds. Which Lights are Best for Growing Weed? There are 4 main types of lamps that you can use for growing cannabis: LED grow lights, Fluorescent grow lights (T5 and CFL), Metal Halide grow lights, and High Pressure Sodium grow lights.