Lighting, Watering, & Fertilizing Indoor Cannabis

As you're setting up your cannabis grow room, with the right humidity, ventilation, and other necessary climate conditions, you'll also need to consider lighting, watering, and fertilizing.

Lighting is a key factor in growing healthy cannabis indoors. The types of lights, the way you set them up, and other pieces that control and direct them are the keys to your yield and the flavor of your end product. Here, we guide you through the process of choosing and installing your grow lights.

Before you head out to your local nursery or hardware store to shop for grow lights figure out how much light you need. In general, a standard 1,000 watt grow light will cover four plants that have a fully grown diameter of about 3 feet, depending on strain. If you set up your grow lights and plants and notice that some parts of one or more plants aren’t receiving light, you’ll need to add one or more lights.

Choose light fixtures and bulbs

Most standard household light fixtures and bulbs are insufficient for growing cannabis. They don’t provide the intensity and quality of light the plants need for optimal growth. The exception is fluorescent lights (typically T5s) or compact fluorescent lights (CFLs), which are okay, but result in smaller, lower-quality buds. We don’t recommend fluorescent lighting.

After ruling out fluorescent lighting, your choice of grow lights depends on your goal and the stage of growth:

• If your goal is high yields, choose a high intensity discharge (HID) bulb — metal-halide (MH) for the vegetative stage and high-pressure sodium (HPS) during the flower stage. These bulbs emit a lot of light and a lot of heat, so you need to position them at a greater distance from the plants.
• If you’re looking for better terpene yields for extraction, use light-emitting diode (LED) or ceramic metal halide (CMH) bulbs, because these preserve the terpenes without bulking up the flower weight and density the way high-intensity discharge (HID) lighting does.

Your choice of light fixture depends on the bulbs you want to use. After choosing a bulb type, shop for grow light system that includes all the lighting components you need, including the bulbs. Components/features of a grow room lighting system include the following:

• Fixture with reflector hood: The fixture holds the bulbs, and the reflector hood directs the light down to the plants. Reflector hoods come in different types:
• Closed hood: Shaped like a box, a closed hood reflector creates a more focused beam of light (and heat).
• Vented hood: Similar to a closed hood reflector but with openings on the ends for connecting the hood to in-line duct fans for cooling.
• Wing: Typically a curved and textured aluminum sheet that provides a less focused beam of light than a closed hood reflector. The light covers a greater area but is less intense (so is the heat).
• Parabolic: Shaped like an umbrella, a parabolic hood distributes light like a wing but in a more circular pattern.

Your choice of hoods is a personal preference. Go with a closed hood if you’re concerned about heat or with an wing or parabolic if you’re not.

• Ballast: The ballast provides control over the current that the lightbulb draws from the power source. The following two types of ballasts are most common:
• Magnetic: Less expensive, heavy, hot, potentially noisy, susceptible to flicker, and supports only bulbs of a certain wattage. If you want to change from a 400W bulb to a 600W bulb, for example, you need to replace the ballast.
• Digital: More expensive, smaller, lighter, cooler, quieter, less susceptible to flicker, more efficient, may be equipped with a dimmable option, may cause radio frequency interference.
• Hooks and pulleys: Grow light systems often include hooks and pulleys for hanging the light fixtures in your grow room. Pulleys enable you to more easily raise and lower the light fixtures to place them at the right distance from the tops of the plants.
• Timer: Grow light systems typically come with a timer, or you can purchase a timer separately, which automates the process of cycling the lights on and off on schedule.

Mount your light fixtures

Mount the light fixtures to the ceiling of the grow room above the plants, positioning the fixtures to ensure equal distribution of light over the entire canopy. How you mount the light fixtures depends on the fixture and how your grow room ceiling is configured. Using hooks, ropes, or chains and possibly pulleys, you can hang your fixtures in a way that you can easily raise and lower them to the proper distance from the tops of your plants.

Position the lights above the plants, so all parts of all plants are receiving light. The light should be as close to the top of the tallest plant as possible without burning it. Keep a close eye on the plants whenever adjusting the lights, and if the top of any plant is getting burned, raise the light.

Don’t place anything flammable close enough to the light that there’s any possibility the light will ignite it.

Set and reset timers

During the vegetative stage, plants require 18 to 24 hours of light. During the bloom/flower stage, they need 10 to 12 hours of light and at least 12 hours of total darkness (for photoperiod strains); auto-flowering strains will flower without 12 hours of darkness. Putting your grow lights on timers greatly simplifies the process of managing the required light/dark cycles, but you still need to manage the changes in lighting over the growth cycle.

If you plan to have a continuous garden with some plants in veg and some in bloom, set up your lighting differently in those two areas. For photoperiod strains, use a separate grow tent or grow room for plants that are in the vegetative stage and those that are in the flower stage.

To monitor your plants through the growth cycle and adjust the lighting, take the following steps:

1. Position the lights at the proper distance above the canopy for the vegetative stage.
2. Adjust your light timer(s) to provide 18 to 24 hours of light. Experiment with different settings in that range over several grows to find the optimum amount of light for each strain you grow.
3. Keep an eye on your plants, adjusting the lighting as necessary to keep the lights the proper distance from the tops of the plants as they grow taller.When your plants are about half the size of full-grown plants, they’re ready to switch from the vegetative to the flower stage. (At this point, you either adjust the lighting, as explained in the remaining steps or move the plants to the flower tent or room.)

The size of a full-grown plant is strain dependent and impacted by light, container size, and other environmental influencers such as CO2. You may have to go through several rounds of growing a particular strain to develop a clear idea what the size of a full-grown plant is and when the plant is ready to switch from the vegetative to the flower stage.

1. If you were using MH bulbs during the vegetative stage, change to HPS bulbs for the flower stage. You don’t need to change out fluorescent, CFL, or LED bulbs.

When changing to the brighter HPS bulbs, shade the plants for a couple days to prevent them from getting blasted by the more intense light. You can place a piece of cardboard between the light and the plants to serve this purpose, but make sure it’s as far as possible from the light to prevent a fire.

1. Adjust the height of your lights to position them the proper distance from the tops of the plants for the flower stage.
2. Adjust the timers, so that the plants receive at least 12 hours of total darkness and 10–12 hours of light. Auto-flowering strains don’t need 12 hours of darkness; experiment with the lighting between grows to determine what’s best.
3. Continue to monitor your plants during the flower stage, adjusting the height of the lights as needed to keep them the proper distance from the tops of the plants as the plants grow taller. When the stigma (the hair-like strands that cover the bud) on half the buds turn orange and red, your plant is ready for harvest.

Measure the light

Light intensity has a big impact on yield. All parts of all plants should have exposure to the light, and the lights should be as close to the plants as possible without burning them. If the top of any plants are wilting or burnt from the light, raise the lights.

For more sophisticated grows, obtain a photosynthetic active radiation (PAR) meter and take measurements at several different locations above the canopy to measure the PAR output of the lights. The PAR measure should never rise above 1,200 PAR.

Watering and fertilizing

Whether you’re growing indoors or outdoors, you need to decide on a system for watering and fertilizing your plants. You basically have two options: manual and automatic. During your first grows, we recommend the manual method as you develop a sense of how much water and fertilizer your plants generally need.

After developing an understanding of your plants’ water and nutrient needs (which may vary depending on the strain), consider installing an automated irrigation system. These systems are equipped with timers that water and feed plants automatically on a pre-set schedule.

Automated irrigation provides the same benefits of lighting systems — the convenience and reliability of automation. However, you still need to monitor your plants to be sure they’re getting enough and not too much water and nutrients.

Using a hydroponics system

In all hydroponics systems, plants are placed in trays or containers that contain a grow medium other than soil, such as pea gravel, expanded clay aggregate, coco coir, or vermiculite. These various systems are then used to deliver water and nutrients to the roots:

• Aeroponic: Plants sit in a tray above a water/nutrient reservoir with their roots dangling down. Solution from the reservoir is sprayed up onto the roots at regular intervals, and excess solution drips down into the reservoir.
• Drip: Nutrient-rich water is dripped slowly at regular intervals into the grow medium where the roots can absorb it. Unused water drains back to the reservoir to be reused or to a waste reservoir and then discarded.
• Deep water culture (DWC): Plants sit in baskets above an aerated (and typically chilled) water/nutrient reservoir with their roots submerged in the solution, which allows for continuous feeding.
• Ebb and flow: Plants sit in pots in a grow tray. Nutrient-rich water is pumped into the grow tray at regular intervals and flows into holes at the bottom and sides of the pots. The pumping stops and water is allowed to drain back into the reservoir from which it was pumped.
• Nutrient film technique (NFT): NFT is like a cross between DWC and ebb and flow. Plants sit in baskets above a grow tray. Nutrient-rich water is continuously pumped from a reservoir into the grow tray and then drains from the opposite end of the grow tray back into the reservoir. This arrangement delivers a continuous flow of nutrient-rich water to the roots.
• Wick: A plant sits in a container above an aerated, nutrient-rich water reservoir, and a rope or other absorbent material (such as felt) is placed through the middle of the growth medium and into the reservoir. Through capillary action, the solution from the reservoir “climbs” the rope, providing the plant with as much or as little water and nutrients as it demands.

Tips for hydroponics systems

Here are a few suggestions for increasing your odds of a successful hydroponics grow:

• Disinfect all your hydroponics equipment with isopropyl alcohol or bleach between grows to kill off any bacteria or other infectious agents. Anaerobic bacteria can build up in dirty systems and kill your plants from the roots up.
• Use clean, pH neutral water. Water from a reverse osmosis (RO) system or distilled water is suitable.
• Aerate the nutrient-rich water solution. You can place an aeration stone in the bottom of the reservoir attached to a small air pump like those carried by local pet stores. Without aeration, your plants may not receive the oxygen they need.
• Replace the water/nutrient solution every couple weeks. Don’t merely add nutrients, because nutrient concentrations may become too high as a result. (Remember to use a fertilizer with a higher nitrogen concentration during the vegetative stage and higher potassium and phosphorous during the flower stage.)
• After dumping the old nutrient solution, run a dilute water and hydrogen peroxide solution through the system to clear out any infectious agents and then rinse with plain water.
• Consider flushing the grow medium with plain water whenever you change the nutrient solution.

When choosing and setting up a hydroponics system, research to find out the type of system that’s best for your grow space and skill level. Simpler is usually better. Use high quality food grade plastics in your system and make sure it’s leak free before starting your grow.

Keep your grow room impeccably clean

At the risk of sounding like your mother, we encourage you to keep your grow room clean. A dirty grow room provides the ideal environment for bacteria, fungi, and pests. Here are a few guidelines for keeping your grow room clean:

• After each use, wash and disinfect plant containers, grow trays, irrigation hoses, and pumps. Use soap and water followed by isopropyl alcohol or a bleach solution (one part bleach to three parts water). Then, carefully rinse everything with plain water.
• Keep your grow room free of any dead plant mater and debris. This is where many pests and pathogens can get a foothold in a garden of healthy plants.
• Watch for common pests such as aphids, fungus gnats, spider mites, and thrips. If you see even one of these nasty critters, identify it and find an effective pesticide. This is where your friendly garden store or grow store staff comes in handy.