LED Grow Light Distance Chart: How to Position Your Cannabis Plant

cannabis under the indoor grow light

Let's grow together!

Growing cannabis today has become a lot easier than it used to be. Growing them indoors has even been proven to be better than growing them outdoors, whether you are growing them for medical or personal use. Some advantages of growing cannabis indoors are that you can control the conditions required for optimal growth of your plants like the amount of light and you can easily protect them from pests.

This article will help you understand the conditions required for the indoor production of cannabis and how you can achieve the correct placements for your grow light. 

Cannabis seedling in production

Different stages of cannabis growth

Cannabis growth has different stages and each stage has its separate requirements which include different levels of water, nutrients, and different light requirements. There are different stages of cannabis growth; seed germination, seedling stage, vegetative stage, and the flowering stage.

The seeds are small, dry, and brown in colour. 

Cannabis seeds

1. The seed germination stage

     The Seed Germination stage takes about 3 to 10 days. The seeds need to be fully formed and mature before they can germinate. The seeds must be kept in a warm, dry, and moist area. When these conditions are met, the first root sprouts from the seed, and its first two leaves called the cotyledons are produced. It is then transferred into the planting soil. 

2. The seedling stage 

     The root system grows during the seedling stage and this takes about two to three weeks. This stage does not require as much water and nutrients as the other stages. After the cotyledons grow, the plant grows a pair of saw-like edged leaves, then they start to grow their characteristic digitated leaves known as the cannabis leaves. As these characteristic leaves grow, the plants enter the vegetative stage.

Cannabis seedling with round leaves (cotyledons) and small serrated leaves.

3. The vegetative stage

     The Vegetative stage occurs in 3 to 16 weeks. Nutrients can be introduced during this stage, especially nitrogen rich nutrients. During this stage, the branches start to grow, more roots and more leaves grow. The plant grows in different heights  and widths as much as they can and they usually require extra space at this stage. Light and water requirements also increases in this stage.

Cannabis vegetative stage – week 4

4. The flowering stage

   The Flowering stage follows the vegetative stage. This stage takes about 7 to 14 weeks. During this flowering stage, the female plants start to produce buds and resins. This stage requires more water, more nutrients and more light. Nutrients containing more phosphorus are also required. The flowers are usually fully developed 8 weeks into the flowering stage. 

Cannabis late flowering – week 8

What are the conditions required for the growth of your cannabis plants?

    Generally, different conditions contribute to the growth and development of plants. The absence of one or more of these conditions can lead to poorly developed plants or even dead plants, the same goes for cannabis plants. The different stages of plant growth also have different requirements which include;

1. Space/Room to grow

      Plants like to have their space, they need room to grow, especially the parts that are above the ground. The leaves need to be properly developed because they are responsible for making food for the plant. Poorly developed leaves mean the plant will be poorly developed. Mature plants usually require more space. Although, you can use an indoor garden if you do not have access to an outdoor field or a grow room if you do not have access to an indoor garden. 

2. Time

     All plants require a long time to grow. Cannabis plants usually require about from 3 to 6 months to grow indoors and this varies depending on the conditions met. 

3. Temperature

     The right temperature is needed for plant growth. According to research, Plants have been found to grow faster in warmer temperatures. At the same time you do not need much  heat. 

4. Water

     Plants need a steady supply of the right amount of water. You can water your plants manually with a hose or a watering can and you can also make a drip irrigation system. You can even make use of a timer with the drip system so as to water your plants at the right time, even when you’re not around your plants. 

5. Nutrients

     Plants need a certain amount of nutrients to grow. They get these nutrients from the soil, water, and air. However, certain soil types can contain an imbalance of nutrients and this can be made up for with natural compost, animal dung or fertilizers. Fertilizers are however not the best option because they only contain nitrogen, phosphorus and potassium. 

If you’re unsure of your soil quality, you can click here to read on how to improve it. 

6. Air

     You need to ensure that whatever space you are growing your cannabis in has to be properly ventilated. Plants take in CO2 and release O2, so without proper ventilation, the CO2 level drops and this can kill the plant. 

7. Light

     Different plants require different light levels. Plants adapt to different light intensities in the environment in which they grow. Some plants have been found to grow under high-intensity light and some other plants have also been found to thrive in places with low wattage  like under a plant canopy where natural sunlight cannot penetrate through the leaves of the plant above. 

Indoor plants also require enough lightning, if you’re growing your cannabis in an indoor garden, you have to ensure that your plants are placed under the natural light coming in through the roof or a window or an artificial source of light. If you are growing your plants in a grow room then you need an artificial light source. 

Natural sunlight or grow light, which is better?

Sunlight is an important factor for healthy plant growth, you can grow plants using direct sunlight or an artificial light source. 

Light occurs in a spectrum of various colors, plants primarily require the blue light frequency and the red light frequency. There is a popular saying, ‘blue light is for veg, red light is for fruiting and blooms’. The red frequency is important for fruiting and flowering while the blue frequency is important for foliage and leaf development. 

The sunlight contains the full spectrum and is more powerful. While some artificial light sources contain specific light frequencies, some also contain the full light spectrum. Experiments have been carried out where vegetative plants were grown under different light frequencies. Those grown in the absence of blue light were found to have grown slowly and developed less mass than those in the presence of blue light. 

The advantage of grow light is that you can control the light intensity by adjusting the light height. You can also control the temperature of the light. Certain plants thrive in cooler temperatures and so will not do well under high-intensity light from the sunlight. Grow lights with less energy levels than the sunlight can be used in situations like this. 

You can read more about sunlight and artificial grow lights here.

What are the light requirements for your cannabis plants?

The different growth stages of your cannabis plants have different light requirements. The germination stage most often do not need so much light to germinate. 

The seedling stage requires about 18 hours of lighting and 6 hours of darkness. During the vegetative stage, your cannabis plants require at least 16 hours of light and 8 hours of darkness. 

The budding process needs 12 hours of darkness and 12 hours of light. This stage uses more of the red frequencies. Knowing the right type of grow light to use in your grow room can be difficult so we have listed out the different types of grow lights and their functions;

Types of grow lights to use:

With so many varieties of grow lights and systems on the market, It can be hard to figure out which one suits your needs the best. while considering growing cannabis indoors. It is important to know the different types of grow lights and which is the best way for you.

Here is an overview of the most popular grow lights, including their features and benefits. 

  1.  High-Intensity Discharge (HID) Lamps

     HID lamps (also known as incandescent bulbs) are a group of electric gas discharge bulbs and lamps, they work when electricity is passed through tungsten electrodes placed in a transparent tube. They usually produce very bright lights.  

Of all the common artificial light sources used for indoor growing, they have been used for the longest period of time. An HID lamp is a good idea for growing cannabis but they tend to get really hot and produce a lot of heat. They can also generate exorbitant power bills when left on for the amount of time needed for optimum growth. 

  1. HPS (High-Pressure Sodium) lamps

HPS lamps are the common orange-colored lamps we see in street lights, they are good for growing cannabis especially in the flowering stage because they have low blue light frequencies. 

3. Metal Halide lamps 

These are the blue light counterparts of the orange HPS bulbs. They are good for the vegetative stage of plant growth because of their blue light frequencies.

4.     Fluorescent Lights

     Fluorescent light bulbs are tubes filled with low pressure gases. They use less light energy than incandescent bulbs and so they produce less heat than incandescent bulbs. They come in different shapes and sizes. 

5. Compact Fluorescent Lamp (CFL) bulbs 

    Compact fluorescent bulbs are spiraled and designed to fit into small spaces. They are energy efficient, have a long life and low operating cost. They however do not have a high penetrating power. 

6. T5 lights 

     These light tubes are 5/8 inches in diameter. They are good for the vegetative stage because they produce blue light. They have low penetration power and are not effective for high yields. 

7. LED lights

     A LED is a light-emitting diode. Led lights are known to produce light more efficiently than any other light bulbs. They have been proven to be the most effective for growing cannabis. They are also the most advanced and they have lower power consumption. 

How do you measure the amount of light your plant is getting?

    Powerful lights can cause light burn in your plants. In order to avoid this, you can measure the amount of light your plant is getting with a light meter, like a lux meter. Also, measure the photosynthetic photon flux density (PPFD). 

The PPFD is the amount of photosynthetic active radiation (PAR) photons that land on a specific surface area of a plant canopy per time. The quantity of PAR a plant receives daily is measured by the Daily Light Integral.

PAR is the type of light that assists plants in photosynthesis and this falls within the range of wavelengths 400 to 700 nanometers (nm) on the light spectrum. Different parts of a plant canopy have different PAR photon readings. PPFD is measured in micromoles per square meter per second. 

How do you know the best distance for your LED grow light fixtures?

     The general rule for LED grow light distancing is that you should be between 18-42 inches on top of the plants for the best results, however, you can place the light lower than 18 inches and higher than 42 inches. It’s all about getting the right amount of light. You also have to consider certain factors that can affect your light placement. 

    Stages of plant growth

    For the different stages of the growing process, you need to closely monitor the light placement for excellent results. During the seed germination stage, your plants do not really need any intense light. The seedling phase requires about 200 PPFD, the vegetative stage requires about 400 PPFD and the flowering stage requires about 600 PPFD.

Below is a LED grow light distance chart;

LED light current (Wattage)Flowering StageVegetative stage 
20010-16 inches12-27 inches
40013-21 inches20-27 inches
60018-30 inches30-38 inches
80020-34 inches 32-42 inches
100021-36 inches36-46 inches

Source: Led grow light different distance chart

How long should you leave your LED grow lights on?

LED lighting should not be left on for the whole day because plants have their dark cycle or resting cycle. During the dark cycle, they rest and transfer nutrients to their apical parts. Plants also need different light levels during their growing process. 

How do you know which LED grow light to buy?

Having known about LED grow lights, knowing which grow light to buy can also be challenging. While selecting the LED light, ensure they are durable and have proper cooling systems. Make sure they meet the PAR photons requirements, manufacturers indicate the PPFD values on the packs of the lights. It is important to also take note of the programmability. Programmable LED lights are preferable because you can schedule when the lights go on and off especially when you are not around your cannabis plants. The intensity can also be programmed. 

Conclusion

Grow lights are a great tool for your home garden, whether you’re an experienced gardener or a beginner who wants to learn how to grow their own cannabis. 

No matter which variety of grow light you decide to go with, be sure it emits the appropriate light spectrum for the plants you intend to cultivate. 

Let's grow together!

Brent Hellendoorn

Brent is excited about all things minimal, and thus environmentally sustainable. From kitchen-scrap composting to indoor herb gardens and air-purifying houseplants, he enjoys continual learning and innovation. In simple, eco-conscious living, there is always room to… grow!

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