October 28, 2012

How To Flower Cannabis Plants With LED Grow Lights

October 28, 2012
LED grow light for veg and flower.

With marijuana legalization spreading throughout the US, many states are allowing for personal cultivation. There are many important factors to consider when cultivating cannabis,  such as choosing seeds and fertilizer, proper watering, and space for growing. Yet, one of the most important aspects of cannabis cultivation is lighting

All plants, including cannabis, require light for survival as light is needed for the photosynthetic process. If you’re cultivating marijuana outdoors, the sun is your light source, but if you choose indoor cultivation, you’ll need some type of HPS or LED grow lights for your weed. Without proper flower lights, your marijuana plants won’t grow properly or produce flower yields to their fullest potential.  

LED Lights for the Flowering Stage: The Best Light Cycle for Flowering

When growing indoors, your marijuana plants do not have environmental feedback telling them which season it is and what they are supposed to do. Instead, you have to tell your plants what to do by utilizing LED flower lights. 

How Long Should I Leave My Grow Light On?

The amount of time you should leave grow light on depends on the growth stage of your marijuana plants. From the seedling to the vegetative stage, you need 18 hours of light with 8 hours of darkness. During the vegetative stage, you need 18 hours of light with 6 hours of darkness. However, during the flowering stage, you need 12 hours of light with 12 hours of dark. Without 12 uninterrupted hours of darkness, your plants won’t begin flowering because it won’t yet feel like fall to them. 

In an outdoor setting, cannabis plants flower in the fall, when days become shorter, and the sun is lower in the sky, signaling to plants that the annual life cycle is coming to an end. After about 8 weeks of vegetative growth, you have to mimic this environmental change in the day cycle by reducing the light time on your plants from 18 hours to 12 hours. 

Along with this change in light time, you’ll need to change the amount of red wavelengths the plants receive as well. This can be accomplished in several ways, but a white, LED veg light alone won’t work. You will also need red LED grow lights as well.

Fixed light spectrum LED grow lights have been designed for both the vegetative and flowering stages. These lights will guarantee maximum intensity during the entire grow season; however, your marijuana plants will produce a better yield if you add more red wavelengths when it’s time to flower. Adding red light will alert your plants to the fall season when the sun is lower in the sky with more red hues. This will initiate the revered flower stage. 

Many claim LED lights for marijuana work well during the vegetative stage but not for flowering. While this is a misconception, it will become a reality if you don’t know how to use the technology correctly.

Can You Keep Grow Lights for 24 Hours?

It is not advisable to keep grow lights on for 24 hours. Marijuana plants need a combination of light and darkness to grow properly and produce expected yields. 24 hours of light can impede your plants’ respiration process. While 24 hours of light won’t kill your plants, it could delay flowering for weeks. 

Light Distance During Flowering

LED Grow Light WattageMinimum DistanceMaximum Distance

Grow Lights for Flowering Plants

As stated, you will need red light wavelengths emitting from your LED flowering grow light. A 90w all red light is an ideal and inexpensive choice for getting the added red wavelengths needed, but make sure you add one to each light setup. There are also LED lights in which you can switch between blue (veg) and red (flowering). The problem here is that you’re only utilizing half of the light’s intensity which translates to a smaller cannabis flower yield. 

Are LED Lights Good for Growing Pot?

LED lights are great for growing pot, and many cultivators have an indoor, LED marijuana grow. If you’ve been using high-pressure sodium lights (HPS), you may encounter the need to use something cooler. While LED lights do produce heat, it will not be as intense as HPS lighting. 

What Light is Best for Vegetative Growth?

The best LED light for vegetative growth is a full-spectrum white light with additional blue wavelengths. This is different from the flowering stage which requires red lights to create artificial autumn conditions, triggering the flowering process in the marijuana plants. 

Best Grow Light for Flowering

The best grow light for the marijuana flowering stage will produce light in every wavelength, but with more red than anything else. In other words, you need full-spectrum white light with additional red wavelengths. You might need to combine two or more lights to do so.  Some cannabis cultivators claim to add some blue wavelengths as well.  If you are using low wattage, your lights should be about 20” from your marijuana plants, but with high wattage increase this to 30”. 

Additional Readings on Cultivation

The Weed Blog is a comprehensive source to find reviews of specific top strains of cannabis and online news resources regarding the legalization of marijuana. We are committed to helping you find valuable information about cannabis on our website. With marijuana law constantly changing, learn about the latest legislation from our website and what you can do to help. Otherwise, consider these other top tier articles regarding cannabis cultivation:

The Secrets of Colorful Cannabis: How to Grow Purple Cannabis Buds

How to Increase Terpenes When Growing Your Cannabis Plants

Top Five Things to Consider When Buying Cannabis Grow Lights


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