How to Use Indoor Grow Light for Every Stage of Weed Plants

How to Use Indoor Grow Light for Every Stage of Weed Plants

Every living organism follows a pattern or series of phases to complete its life cycle. And we all know what happens at the end stages of lifecycle.

The same goes for weed plants; they grow and follow the distinct phases of the growth cycle, and each phase requires different caring than the other, including light ratios, nutrients, lights for growing seedlings, water and so on.

The stages of weed plants are similar to other crops except for the harvesting period. The weed plant can take up to 4 to 8 months to complete its life cycle, depending on the type or strain and place you are growing.

In the indoor environment, with all the resources and equipment, you can boost the cultivation by half compared to traditional growing.

The light cycle is another crucial factor in indoor weed cultivation for producing a favorable yield that most beginners ignore.

That’s why we’ve prepared this guide, especially for beginners, explaining the importance of light ratio for growing weed during the different stages of plant growth.


Stages of the Weed Plant Growth Cycle


The Average Full Grow Cycle of Weed Plant

Generally, weed plant growth is categorized into 5 different stages, from germination to harvest.

A weed plant's average complete growth cycle depends on the variety you grow. Different strains, such as Indica, Kush, White Widow, Purple Haze, Hemp, Afghani, etc., are available.

All of them grow differently but follow the same principles of the growth cycle.

A typical weed plant outdoors can take up to 4 to 8 months to reach the harvesting time.

In an indoor environment, you can cut off the harvesting time by half of the outdoor because growers have complete control over the climate and plants.

The lifecycle of some fast-growing weed varieties, such as Auto Blueberry or Auto Blackberry Kush, could be a minimum of 9 weeks to harvest.

If you grow these varieties in the outdoors, the harvesting period could take up to 4 months.

The extension of stages of weed depends on the options of seeds (feminized or autoflower) and regardless of whether you cultivate them inside or outside.

Average Duration of Photoperiod Feminized Seeds (Indoor):

  • Germination Stage: It takes 2 to 10 days to begin the germination process.
  • Seedling Stage: After 1 to 3 weeks, the seedlings begin to pop out from the seeds.
  • Vegetative Stage: It takes 1 to 15 weeks to grow bigger before buds start to appear.
  • Flowering Stage: Anywhere between 7 to 14 weeks, the buds begin to form.

Different Stages of Weed Growth

Like any other plant, the growth of a weed comprises 5 different stages, i.e., germination, seedling, vegetative, flowering, and harvest. Each stage is different from the other and requires additional care and support.

 germination stage for weed plants

1. Germination Stage

Germination is the first stage of the plant’s growth cycle, where seeds need to sprout in a warm environment. This phase occurs when you plant the first seed into the soil.

The weed seeds are usually small, dry, and hard—light and dark brown. During the germination phase, the hard seeds slacken by damp conditions.

Sowing seeds in a damp environment is critical to achieve the maximum germination rate. The moisture in the soil softens the weeds' outer shell and makes an easy way out for seedlings.

The germination process is usually done in a dark environment and doesn’t need any nutrients. Make sure to water enough to keep the soil moist for a few days.

As time passes, the tap root will come out from the weed seed and grow in descending. After that, the first embryonic leaves will start to come out, and the weed life cycle will begin.

In indoor, the germinations usually take place around 2 to 10 days to occur. The outdoor germination process can take up to 2 weeks.

Finally, the seedlings will start to form true leaves with saw-like edges. That’s where the germination stage stops and starts a new phase, i.e., the Seedling stage.

Light Cycle for Germination

The light cycle is another term for a plant's lighting conditions throughout each growth cycle stage.

The light is the most critical factor that determines the quality and quantity of yield. Ignoring this factor may lead to plant abnormalities, i.e., less growth, less productivity, and stopped photosynthesis.

That’s why following a consistent light schedule while cultivating plants indoors is important.

Generally, the germination phase requires a higher proportion of blue light to promote healthy growth and suppress excessive stretching.

Blue light is responsible for enhancing plant quality. Despite the fact that the germination stage isn’t affected by the light spectrum compared to other stages, you still can’t ignore the light spectrum completely.

Light Requirements for Germination

In the germination stage, the light shouldn’t be a concern as seeds only grow in dark or low light.

But you can promote the sprouting process with the help of fluorescent lights or LED grow lamps. Fluorescent and LED lights produce less intensity (blue light), which is perfect for the germination stage.

indoor lights for weed plants

Light Ratio for Germination Stage

Since the germination stage doesn’t usually depend on the different light spectrums, you can go with 90% blue light and 10% ultraviolet light.

There’s no need to use full spectrum and red light in the germination stage.

Key Factors in Weed Germination

Everyone knows the importance of water, light, temperature, humidity, nutrients, etc., for optimal growth, but they ignore the quantity required for different stages of weed plants. Here are some growth factors that are necessary for the germination stage:

  • Light (PPFD): The germination stage usually performs well in dark or low-light environments, so using the range between 200 to 400 µmol/m²/s is recommended. Too much light can hinder germination and damage the airy seedlings.
  • Temperature: The ideal temperature for weed germination is between 24 to 26°C (75-79°F). Anything above or less would do permanent damage to your seeds.
  • Relative Humidity: Since weed seeds are harder from the outside, you should maintain the moisture level to 70-80% to soften the outer shell.
  • Nutrients: At first, the germination phase doesn’t require additional concentrated fertilizers. But you can use light or low-concentrated fertilizers to speed the germination process.
  • Wind Speed: Set the wind speed at a low level so the moisture in the soil doesn't dry up quickly.
  • CO2 Concentration: CO2 concentration at this stage may not significantly affect the germination process. Still, you can maintain CO2 concentrations to 400-500 ppm. You can also increase the limit with partial CO2 supplementation.
  • O2 Concentration: O2 is very important for the seeds. It supplies energy by oxidizing the stored nutrients within the seed, facilitated by oxygen. Approximately 21% of air oxygen concentration is recommended for the germination stage.
  • Daily Light Duration: Generally, light isn’t necessary for germination, as seeds perform well in low light. Keeping the light duration to 18-24 hours with a blue light spectrum won’t damage the seeds.


2. Seedling Stage

The seedling stage begins after the successful germination process of sprouting leaves from the seeds.

After a few weeks of germination, the triumph sprouted seeds are moved to a new pot or container where they spend the next 1-4 weeks, depending on the strain and growing conditions.

The plants with delicate leaves are prepared to develop their robust root system at the seedling stage. In this phase, the newly sprouted plants look for nutrients to produce healthier leaves.

Sometimes, growers think they can boost the growing process by excessive watering and nutrients, resulting in stunted growth and lower production. Seedlings may require nutrients, but they can absorb a limited amount.

So, avoid overfeeding the plants at this stage of the weed plant.

Light Cycle for Seedling

Consistent light scheduling is as essential as consistently monitoring the light and temperature when we talk about indoor growing.

The seedling stage requires a high proportion of blue and a medium proportion of white light.

Outdoors, the seedling phase usually falls in the summer, with longer days and more light hours.

That’s why the recommended light cycle for seedlings is 18 hours of light and 6 hours of complete darkness, mimicking the outside environment.

A mixed blue and white light spectrum ratio for 10-14 days is enough for weed plants to develop vigorous leaves.

Once the plants emerge, more leaves, i.e., three or more nodes, move to the next stage.


seedling stage for weed plants

Light Requirements for Seedling

Unlike the germination stage, seedlings require light—a mix of blue and white light.

The germination process performs well in low to no-light environments, but when a plant emerges from its shell, it requires light to develop healthier leaves.

Light intensity should be moderate, as seedlings can't handle too much intensity.

That’s why using those grow lights that produce less heat is critical. Fluorescent, LED, or T5 grow lights are recommended for optimal heat intensity.

Light Ratio for Seedling Stage

Since seedlings are tiny plants and can only withstand adequate heat intensity, blue light is important at this stage.

However, incorporating full-spectrum white light helps provide a more comprehensive illumination, promoting healthy growth.

Regarding the light ratios, 70% of blue and 30% percent of white lights for growing seedlings.

Key Factors in the Seedling Stage

The following growth factors in the seedling stage determine the optimal growth of weed plants. If any of these are suboptimal, it could hinder the plant’s growth:

  • Light (PPFD): The seedling stage requires more light intensity compared to germination—not that high, but a little. The recommended PPDF range for seedlings is between 200-600 µmol/m²/s. Anything above this range results in stunted growth.
  • Temperature: The ideal temperature for seedlings is around 20-25°C (68-77°F).
  • Relative Humidity: The humidity is important for seedlings to develop a strong root system. It requires comparatively less humidity compared to seed germination. The recommended humidity level is between 60-70%.
  • Nutrients: Seedlings require nutrients when they are at least 3 inches tall. At this stage, you can give light to moderate ratios of NPK, i.e., 2-2-3.
  • Wind Speed: Air circulation is crucial and directly impacts weed growth. Wind increases the presence of atmospheric turbulence, enhancing the supply of CO2 to the plants and increasing photosynthesis. Gentle air circulation is enough for the seedlings just to promote the airflow.
  • CO2 Concentration: Maintaining the CO2 concentration between 400-500 ppm is considered sufficient for average growth.
  • O2 Concentration: A moderate percentage of O2 concentration is enough to oxidize the stored nutrients. Approximately 21 percent of air oxygen concentration is recommended for the seedling stage.
  • Daily Light Duration: Seedling requires more light than bigger plants; a photoperiod of less than 14 hours stops the plant’s growth. Extending the light hours to 16-18 with low-intensity lights would do the job.


3. Vegetative Stage

Vegetative is the middle and the most critical stage in the life cycle of plants. The future of your weed plant depends on this stage.

If everything goes well, you’ll be rewarded with a fruitful yield at the end of the plant life cycle.

During this phase, plant stems grow bigger and thicker, producing more leaves and setting themselves for the perfect canopy.

The vegetative stage starts as soon as plants develop more leaves and lasts 4-8 weeks.

During this phase, growers perform different training techniques, such as topping, LST, super cropping, SOG, etc., to reduce the spacing between densely populated nodes.

Light Cycle for Vegetative

When plants start to take off, the weed plants need adjustments in their light cycle. At the vegetative stage, weed plants need light for 18 hours, mostly the full spectrum, and 6 hours of darkness.

Some indoor weed growers express the idea of 24 hours of light, but that idea would lead to insufficient nutritional elements and stunted growth at the later stages of the weed plant.

The period of darkness is also critical to weed plants as they need rest to relocate the nutrients and energy stored as sugar, preparing for another day of growth.

Light Requirements Vegetative Phase

Since plants proliferate in the vegetative phase, they feed on light more than usual. Photosynthesis becomes faster in this stage; plants must adapt to the environment under high light intensity.

It’s best to use high-intensity lights such as high-pressure sodium (HPS) or metal halide (MH) are recommended for vegetation.

You can also increase the intensity of LED lamps by setting them close to the plants, aware that this could cause light stress on plants.


veg stage for weed plants

Light Ratio for Vegetation

As discussed earlier, plants need high-intensity light during vegetation, and full-spectrum white light is crucial for supporting the rapid growth of stems and leaves. However, grow light red-blue ratios are also important, along with full-spectrum white light. Regarding the light ratios, 80% of full-spectrum white light and 10% each of red-blue ratios are recommended for the vegetative stage.


Key Factors in the Vegetative Stage

The following growth factors are mainly responsible for the rapid and healthier weed growth:

Light (PPFD): High light levels during this stage enhance the growth and strength of stems. A range of light PPDF 400-800 µmol/m²/s is considered optimal for the vegetative stage.

Temperature: Since this stage supports high intensity, you can set the temperature between 22-28°C (72-82°F) for ideal growth.

Relative Humidity: During this stage, leaves get thirsty and require moisture to thrive. Anything between 40-60% humidity would be enough for weed plants.

Nutrition: For best vegetative growth, it is important for plants to consume all the available nutrients present in the soil. Primary nutrients are essential, as well as the other traces of elements. For primary nutrients, use moderate or adjust the NPK, Ca, Mg, and S ratios for secondary nutrients and Mn, Zn, Fe, Cu, B, and Mo for other trace elements.

Wind Speed: Keeping the humidity levels in check, a moderate or subtle air movement is enough for vegetation.

CO2 Concentration: You would expect a high concentration of CO2 in the vegetative phase because plants have yet to produce flowers. The recommended CO2 for this stage is between 400-800 ppm.

O2 Concentration: High oxygen levels always promote vegetative growth. It also decreases the ammonium toxicity in eggplant. A 21% of the air oxygen concentration is recommended in vegetation.

Daily Light Duration: The more light plants get in the vegetative stage, the more they expand and grow quickly. The recommended photoperiod is between 18-21 hours for the vegetative stage.

 weed plants under white light

4. Flowering Stage

The flowering stage is considered to be an exciting phase for every grower as they start to see the prosperous fruits of their labor.

It happens when weed plants begin to produce flowers and typically lasts 8 to 12 weeks. The duration of this stage depends on the type or strain of weed you grow.

Strains like Bubba Island Kush bloom within 7 weeks as it’s one of the fast-growing strains of weed.

During this period, restricted light hours are observed carefully to achieve the full potential of the flowering stage.


Light Cycle for Flowering

The flowering stage requires more light than the vegetative stage and mostly depends on red light to stimulate flower growth.

However, full-spectrum white light still helps maintain the overall plant health. That’s why the usage of full-spectrum white light is more than that of any other light in this phase.

As mentioned before, this phase follows strict light cycles to produce a well-defined odor.

During this phase, it’s recommended to provide 12 hours of full-spectrum light followed by 12 dark hours.


Light Requirement for Flowering Phase

To speed up the maturing process, you need brighter lights to darken the pistils and trichomes to turn whiter, keeping the dark hours in mind as they are also crucial to stimulate flower production.

For 12 hours of light and 12 hours of dark environment, HPS and LED lights are recommended for necessary illumination.

 flowering stage of weed plants

Light Ratios for Flowering Stage

This particular stage is the one that uses different colors of light, including UV, full-spectrum white light, 660nm red light, and 730nm deep red light. All of them act differently according to the conditions and plant’s requirements.

Regarding the light ratios, 10% of UV, 20% of white light, 60% of 660nm red light, and 10% of 730nm deep red light are recommended.


Key Growth Factors of Flowering

Let’s explore the key growth factors of the flowering stage to achieve fruitful results:

Light (PPFD): At the flowering stage, plants need even higher levels of light intensity compared to other stages. The recommended light PPFD of weed plants is between 600-1000 µmol/m²/s.

Temperature: The temperature should be lower than the vegetative stage—even more in dark hours. The recommended temperature for flowering is around 20-26°C (68-79°F), with slightly lower nighttime temperature.

Relative Humidity: Decreasing the temperature means there’s extra humidity already dropping on the plant’s surface. It’s important to maintain the humidity levels between 40-50% during the flowering phase.

Nutrition: At this point, weed plants need more potassium and phosphorus rather than nitrogen content. Therefore, increasing the K and P content is recommended.

Wind Speed: A sturdy air circulation is recommended for healthy buds and ventilation.

CO2 Concentration: Higher CO2 concentrations help plants at this stage to increase the quality and taste of the buds. That’s why maintaining the CO2 concentration in a range of 800-1200ppm is endorsed.

Daily Light Duration: 12 hours of light and 12 hours of darkness should be strictly monitored during the flowering stage.


weed plants under red lighting

5. Harvest Stage

Finally, harvest is the last stage of the plant's growth cycle that brings smiles to every grower's face.

Once your weed plants are ready to harvest, they need to be cut down in half or in small branches to proceed to the drying process, where they hang upside down in a room that is cool and devoid of light. This process of hanging continues for 7 14 days straight.

After drying, it transfers to the next step, i.e., curing. Curing involves maturing weed in a sealed environment where the temperature ranges are maintained between 10-15 °C (50-60 °F).


Light Cycle for Harvest

During harvest time, plants need more far-red light to promote the maturation of chemical compounds while retaining some white light to observe plants' conditions.

You can continue the previous stage light cycle, i.e., 12 hours light and 12 hours dark during the harvesting process.

 weed plants ready for harvest

Light Ratio for Harvesting Stage 

The harvest stage mainly depends on two lights, i.e., red light and far-red light, to remove the excessive chemical compound.

The ratios usually contain 20% of red light (660nm) and 70% of far-red (730nm) during harvest.


Key Factors While Harvesting Weed

A few key factors need to be considered while harvesting the weed plants. Those key factors are as follows:

Temperature: After finalizing the package, the package needs to be stored in a cool and dark place where temperature ranges from 18-24 °C (64-75 °F).

Humidity: There’s no need for humidity at the harvest stage. All the excessive humidity should be removed entirely.

Nutrients: There’s no need for any nutrients at this phase. In fact, those present nutrients or any other chemical residue should be flushed or cleansed properly.

Wind Speed: A good air circulation is also necessary to dry up the excessive humidity from the harvest.

Light Duration: Red light is commonly used in the harvesting stage for the maturation of chemical compounds and should be followed by 12 hours of light and 12 hours of dark.


Conclusion on how to use indoor grow light indoors

Well, cultivating weed in an artificial environment is not an easy task to do unless you have proper knowledge and gardening skills.

It requires dedication, passion, and hard work to carry on that struggle into weed cultivation.

Being a weed grower isn’t a big deal if you know the stages of weed and the light ratios for growing weed. We’ve already covered that topic above.

Every stage is different and demands different sets of skills and atmospheric requirements.