In this guide, we will walk through lighting options, best-practices and schedules. There are several lighting options available to farmers these days and the right choice depends on your situation. At the end of the article, we break it down by type to help you find the perfect light.
What makes a good light?
Heat Generation - If you are growing in a tent, closet or other confined space, heat can be a major issue and the type of lighting you choose can have a major impact on keeping your space at a comfortable temperature.
Energy Consumption - Growing cannabis indoors comes at an ecological, environmental and financial cost and there are major differences in how much energy different lights use,
Energy Output - Cannabis needs energy from direct light to produce to their full potential, so you will want to provide enough energy to reach your growth goals.
Natural sunlight is a great option if available - it's free and renewable. Sunlight through a window results in low yields, so you will need to live in an area where it's safe to have the plant outside during the day. You can augment sunlight with artificial light sources.
Pros: Free and your plants are naturally optimized to take advantage.
Cons: Not very stealthy and you can't control the weather or the seasons.
Compact Fluorescent Lights (CFL) and fluorescent tube lights can be great options for your plants. The upside is that they're cheap, burn relatively cool and are easily available - you most likely have some in your home right now. The downside is that since they don't transfer as much power to the plants as LEDs, they can burn hotter.
You will need around 50 watts per square foot (0.3 Meters) during the vegetative phase and 75 watts per square foot (0.3 meters) in the flowering phase.
Pros: Cheap and readily available to get started
Cons: Burn hot compared to LED and not as efficient as other methods
Over the past years, LEDs have overtaken almost all other lighting types for home grows. They are relatively inexpensive to purchase and operate and burn very cool compared to traditional grow lights. In almost all cases, we recommend LED lighting to other sources.
LEDs wattage vs lighting capacity is a confusing topic. Manufacturers attach a wattage number, but that number refers to the full capacity if every LED chip burned at maximum capacity (e.g. A light with 100 chips and each is at 10 watts max rating has advertised wattage of 10x100=1000 watts). Since LED chips don't burn at their max capacity, the actual wattage pull at the wall is generally about half the advertised wattage.
The most important number for LED grow lights is the photosynthetic active radiation rating, or PAR rating. PAR isn't a measure of brightness (like lumens, which measures human visible light) but is a measure of the light ranges that are specifically beneficial to plants. Quality light manufacturers publish PAR ratings at set distances to establish coverage area per light.
Pros: Most efficient and still fairly inexpensive to get started
Cons: Less available than CFL
High-Pressure Sodium lights are extremely effective lights and have been successfully used to grow amazing buds for decades. They do burn hotter than LEDs, which can cause temperature problems in tents and other confined spaces. Commercial operations use HPS because they are more performant for large areas. This is not only true in the cannabis industry, but also for indoor tomato growers and many other commercial crops. HPS is effective at a wider angle than LED lights, so they can provide broader coverage for large areas.
Pros: Fairly efficient and productive
Cons: Expensive, burns hot and not readily available locally
When should I turn the lights on or off?
There are a couple of things to consider on lighting schedules. To keep photoperiod strains from entering flowering, vegging plants need more than 12 hours a day. Light cycles are generally denoted as light/dark hours. Standard options are 18/6 (18 hours of light followed by 6 hours of dark), 20/4 (20 hours of light followed by 4 hours of night) and even 24 hours of light without any darkness.
During flowering, photo-period strains require over 12 hours of uninterrupted darkness each day. Even a single flashlight sweeping over can be enough to stop some strains from flowering, so light control during flowering is essential. For auto-flowering strains, this isn't an issue since their flowering is controlled by an internal timer that starts ticking at germination.
For outdoor growing, the seasons control the light schedule. For indoor growing, tents, closets and grow boxes have good light control and most farmers use inexpensive electronic timers to turn lights on and off on a schedule. Keeping a tight schedule with automatic timers are good for the plant and easy for the grower.
The best way to keep your plants on a good schedule is to pick up an automatic timer. These are inexpensive and readily available online or at local hardware or department stores. The important note is to make sure that you get a unit that can handle the wattage your light setup pulls. You also need to get a grounded unit (3 prongs in the US) - many of the cheaper analog units are made for household lamps and the like, so they only have two hole plugs. It's worth spending a few extra bucks for safety.
How high should I hang the lights?
To get the most benefit and avoid injury to your plants, pay careful attention to your light placement. Always hang the light directly over the plant(s) you are looking to benefit from it. That part’s easy though, the hard part is how close to the plant to keep it. If you place it too far above the plant then it loses its benefit to the plant. If you place it too close to the plant, then the plant can be burned or bleached by the light. LED add another complexity because they require a minimum distance for the colors from the various diodes to ‘mix’ so they can be absorbed by the plants.
A good rule of thumb is to avoid getting closer than 10 inches (25cm) with any light. Put your hand at the edge of the growth and if it is too hot for you then it will be too hot for the plant. We’ll provide some guidance. Always follow the manufacturer's guidance on the proper distance for your plants, but here is a quick rundown on light placement at each phase of a plant’s life cycle.
Seedling - Need lower level of light because excessive light will stimulate vegetative growth before the plant has developed adequate roots to feed the growth. Keep strong lights at a good distance but not so far that it causes the young seedlings to stretch to reach it. A good approach here is to stay at the high end of the manufacturer's recommended distances, moving a little closer every couple of days as the roots become established.
Vegging - Vegging plants need more light than seedlings, but still less than flower plants. Keep your light in the middle of the manufacturer's suggested range.
Flowering - Flowering plants need a lot of light energy to grow fat buds, so put the lights a the low end of the manufacturer's suggested range.
How many lights do I need?
This is another one that is totally dependent on your goals and setup. Here are some basic metrics to help you select the right lights for your setup and desired outcomes.
50 Watt LED Panel - 6 to 9 seedlings for the first weeks or one small vegging plant,
75 Watt CFL - 6 to 9 seedlings for the first weeks or one small vegging plant. 2 x 75 Watt CFL will grow a small autoflowering plant through flowering. More CFLs equals more plants.
100 Watt LED - 6 to 9 seedlings for the first weeks, 2 small vegging plants or a single small auto-flowering plant to flower, covering approximately .4 square meters of canopy in flower.
200 Watt LED - Small to medium sized plant through its lifecycle, covering, covering approximately .8 square meters of canopy in flower.
500 Watt LED - Large plant in flower or two plants in vegetative growth, covering approximately 1.5 square meters of canopy in flower (approximate size of a 4’x4’ tent)
Next Up: Learn about containers and soil.
Learn about the different container and soil options available, and all about transplanting marijuana plants as well.