Fraser Horton
Fraser Horton
Last Updated on April 28, 2021

Ensuring your cannabis plant has enough light (as well as enough hours of darkness) in the grow tent during a 24-hour period will enable it to efficiently conduct photosynthesis and produce healthy buds. For that matter, full spectrum grow lights are an integral part of your grow room if you’re planning to become an indoor grower.

While there’s no right answer to how much light weed plants need, since marijuana plants need different amounts of light during each growth (or growing) stage, there are some general guidelines as to how much light they need in each stage. In order to help you grow healthy plants, we’ll go over how you need to take care of them, and what light schedule you need to maintain.

Optimal Light Schedule for Growing Marijuana Plants (Amount of Light vs Hours of Light)

From germinating to growing flowers and buds, your plant needs to have the ideal growing conditions in order to thrive, and one of the most essential is light. Marijuana plants go through the following four stages on their journey to growing resinous buds:

  • Germination stage (3-10 days);
  • Seedling stage (2-3 weeks);
  • Vegetative stage (3-16 weeks);
  • Flowering stage (8-11 weeks). 

Each stage needs specific light and nutrient conditions which marijuana growers tailor to the specific strain they’re cultivating. Whether it’s a Sativa or an Indica, an autoflowering plant or a photoperiod plant, a high in CBD medical marijuana strain or a strain high in THC, each one requires specific growing conditions that will determine the quality of the buds, as well as the yield.

Seed to Harvest Light Guide for Indoor Growers of Cannabis Plants

Germination Stage

Germination can happen anywhere between 24 hours and 72 hours, and during this stage the cannabis plant develops a taproot. During this period, seeds should be kept in a cool dark place, away from direct sunlight or artificial light, until they reach the seedling stage.

Seedling Stage

The seedling stage lasts between 2 and 3 weeks, and this is when the plants start to develop leaves and new fan blades. The plant should have 16 hours of light per day during this stage, and 8 hours of darkness. Seedlings might benefit from fluorescent CFL blue lights since they don’t produce a lot of heat and have a lower wattage. 

Vegetative Stage

The vegetative stage lasts between 3 and 16 weeks during which the cannabis grows stems and leaves, but no buds. This is when your cannabis plants should have between 12 to 18 hours of light.

Flowering Stage

The flowering stage can last from 5 to 16 weeks and the duration of this stage depends on the type of strain you’re cultivating. This is when flowers start to form, which then produce the resinous buds that give us our favorite cannabis products. During this stage, the marijuana plant needs 12 hours of light and 12 hours of uninterrupted darkness in order to induce flowering.

Outdoor Plants (Photoperiod vs Autoflowers) Light Cycle

While indoor plants need light changes in order to induce flowering, outdoor growers have nothing to worry about since photoperiod cannabis plants have an internal process that allows them to detect when they need to move on to the next growing stage

Photoperiodism refers to short-day plants which produce flowers when the days are getting shorter, meaning their life cycle is nearing the end. Therefore, in the northern hemisphere weed plants grow from April/May to September/October, while in the southern hemisphere they grow from September/October to April/May.

On the other hand, autoflowering strains start to flower automatically instead of waiting for a specific light cycle. Meaning, after the short vegetative stage of about 2 to 4 weeks, they begin to produce flowers.

Different Types of Grow Lights for Your Marijuana Plants

When growing marijuana indoors, you’ll need to make sure that you do everything to adjust the light cycles (length of sunlight hours each day) according to the needs of the cannabis plant. There are different types of grow lights that’ll get the job done, so you’ll have no problem finding quality grow lights that are effective but don’t cost a fortune.

Fluorescent Grow Lights (CFL) 

Compact fluorescent lights, or CFLs, are the most common grow lights that you can see on the market and in grow tents. They’re budget friendly and suited for small grow tents of 1-2 plants, which makes them one of the best grow lights for these conditions (price vs performance). They go well with standard light fixtures, save energy, and are available in various wattages and color temperatures. 

Daylight spectrum CFL lights are suitable for the vegetative growth phase and warmer CFL lights are better for the flowering phase. A standard 40W CFL bulb is expected to last for about a year, and the expected cannabis yield is about 0.3 grams per watt (or a yield of 12 grams per standard light). Keep in mind that if you have a bigger yield, CFLs might not be the proper lighting system for you.

High Intensity Discharge HID (MH and HPS) Grow Lights

High-intensity discharge or HID grow lights are the most popular in the cannabis cultivation community. Growers believe that these lights produce the best and largest yields. The two main types of HIDs are MH (metal halide) and HPS (high pressure sodium), with the former producing a cooler, blue light, and the latter a warmer, red light. Therefore, MH lights are best suited for the vegetative phase, while the HPS for the flowering phase, so growers often do a mix of both.

You can expect a yield of about 0.5–1g+ per watt (300–600 grams per standard HID light). Keep in mind that they can increase the temperature in your grow room or even burn plants, so check your plants regularly. The bulbs are also likely to increase your electricity bills on account of them being power hungry. 

The lifespan of the bulbs is about one year, and you should replace them annually in order to maintain optimal light input. In order to implement these types of lights you’ll need the bulbs, a ballast, and a reflector, that will cost about $180+ (€150+), but this is still cheaper than high-end LED lights.

LED Grow Lights

LEDs are the most energy-efficient grow lights on the market. Their light is suitable for both the vegetative and flowering phases, and some even have the function to change the light spectrum according to the growing phase. Even though they’re the most expensive, they can also save you money over time. They’re much cooler compared to HID lights which reduce the risk of burning plants, while also decreasing the cooling bill. 

Commercially available LED grow lights don’t need a special ballast, you can just plug them and start growing your plants. Low-quality LEDs are about $140 (€120) for a single light, while high-quality LEDs can cost up to a few hundred euros. But as we said, this is actually a budget-friendly option, as one light has a lifespan of 5–10 years. Compared to HID grow lights, they may have a lower yield, although the standard yield is about 0.5g–1.8g per watt with LED lights.

LEC Grow Lights

LEC lights, ceramic metal halide (CMH), or ceramic discharge metal halide (CDM) lights, use a ceramic arc tube that projects a more natural color compared to the quartz tube in the MH lights. They have a built in ballast which makes setup really easy, more lumens per watt, and a longer lifespan compared to HID lights.

The downside to LEC lights is that they generate lots of heat and emit UV-B rays that improve trichome production (but are harmful to our skin and eyes), so be careful when going in your grow rooms while they’re on. Standard LEC lights cost around $300 (€250), while high-quality models can cost up to $1200 (€1,000). Their lifespan is approximately two years, and you can expect a yield of 1.5g per watt.

Final Thoughts on the Amount of Light Weed Plants Need

Light is integral to the overall growth cycle of the cannabis plant, so the appropriate amounts of full spectrum light, as well as a good amount of uninterrupted darkness, are both essential for proper cannabis growth. No matter if you’re growing indoor or outdoor plants, autoflowering or photoperiod cannabis, you’ll need to adjust the light in order for your plants to thrive. 

And when picking out the lights, don’t just look at the wattage, but also make sure that the amount of light they emit is suitable for your particular plant. In the end, all you need to start cannabis cultivation is to choose your preferred strain, pick a cannabis growing method, whether it’s SOG, SCROG, defoliating, trimming, or something else, and equip your grow space with enough lights.

Disclaimer

The information presented on this page is provided as a public service to aid in education and is derived from sources believed to be reliable. Readers are responsible for making their own assessment of the topics discussed here. In no event shall Leaf Nation be held reliable for any injury, loss or damage that could happen if using or abusing drugs.