Grow Light Spectrum Explained: Ideal LED Spectrum for Plants (2024)

What is Broad Spectrum Lighting?

Broad spectrum lighting – often referred to as full spectrum lighting, means the complete spectrum of light given by sunlight. This means wavelengths of broad spectrum lighting include the 380nm-740nm range (which we see as color) plus invisible wavelengths too, like infrared and ultraviolet.

One advantage of LED grow lights is they can be set up to produce certain wavelengths for specified periods during the day or night. This makes it ideal for plants because growers can isolate specific spectrum colors depending on crops and growing conditions. Full spectrum lighting can also speed up or slow growth rate, enhance root development, improve nutrition and color etc.

Grow Light Spectrum and Cannabis

The grow light spectrum for Cannabis varies when compared to other plants as growers are focused on maximizing yields, controlling levels of THC and other cannabinoid production, increasing flowering, and to maintain overall uniformity.

Aside from visible colors, Cannabis responds especially well to wavelengths just outside of the PAR range. Therefore, an added benefit of using full spectrum LEDs is the ability to use specific doses of ultra-violet wavelengths (100-400nm), and far-red wavelengths (700-850nm) outside of the PAR range.

For example, an increase in far-red (750nm-780nm) can help stimulate Cannabis stem growth and flowering – something growers want, whereas necessary blue light in minimal amounts, can prevent uneven elongation of stems and leaf shrinkage.

So, what’s the ideal grow light spectrum for Cannabis? There’s no single spectrum since varying light exposure promotes certain plant morphology during different stages of growth. The chart below explains the concept of outer-edge PAR light spectrum use.

Personal vs. Commercial Cannabis

The difference with personal vs commercial grow lights for Cannabis can be determined by a number of factors. Firstly, the available light spectrums in commercial LED grow lights will include the full PAR range and beyond – which is particularly advantageous for Cannabis growers.

Commercial grow lights can be wirelessly configured to put out specific wavelengths and intensities at certain intervals in a 24-hour cycle – grow light settings often work in conjunction with a grower’s HVAC systems too.

With personal LED grow lights, lumens per watt will likely be lower – which makes them less energy efficient with smaller potential yields. Many are not broad spectrum and may only offer small spectrums of blue and red light. Additionally, while personal grow lights will still be inexpensive to run, other factors to be considered include noisier fans, inferior quality plastic casing, shorter LED lifespans and overheating issues.

Should I Use a Different Light Spectrum for Different Plants?

In some crops, blue light can benefit nutritional levels and coloring, and a higher red to far-red ratio can help with leaf size and flowering. It’s why today’s full-spectrum LEDs are so advanced – because by selecting the right quantities of red and blue light (4), chlorophyll pigments absorb more light they need.

Cannabis growers – who pay attention to UVB/blue for its various structural and THC-potency benefits, which we’ll get into, are predominantly concerned with leaf size and flowering. Therefore, far-red and red light is relatively more important to boost their yields.

Other indoor growers are also experimenting with the controlled use of far-red spectrum, like salad leaf farmers for example. Plants associate this spectrum with shading from direct sunlight, which would happen lower down the canopy, causing leaf & stem stretching as the plant reaches out for sunlight.

This means when used strategically, bigger leaves and flowering can occur without unnecessary stress. So while there is no specific LED grow light spectrum for any particular plant, the ratio of red to blue light is very important to maximize growth and the rate of photosynthesis.

Spectrum for Photosynthesis, Growth, and Yield

For photosynthesis to occur and chlorophyll to absorb the maximum amount of light for plant growth, plants use both blue and red light most efficiently. Other spectrums of light, like greens/yellows/oranges, are less useful for photosynthesis due to the amount of chlorophyll b, absorbed largely from blue light, and chlorophyll a, absorbed largely from red and blue light.

It’s worth noting photosynthesis is more complex than just chlorophyll absorption, but it’s important to recognize the fundamental principles.

For growth, blue light is essential to help plants produce healthy stems, increased density, and established roots – all which occur in the early vegetative growth stages. Growth then continues with increased red light absorption, resulting in longer stems, increased leaf and fruit/flowering etc. It’s here that red light plays the dominating role in plant maturity and, therefore, size.

And finally, yield – this comes down to a combination of light spectrums and is often very unique to growers, including growers of several varieties of the same crop (like Cannabis). There’s no one single light spectrum that produces more of a crop – optimal lighting is very much a holistic, ever-changing process.

Grow Light Spectrum by Type

Certain light spectrums trigger growth characteristics in plants. In general, blue light spectrums encourage vegetative and structural growth and red light promotes flowering, fruit, leaf growth, and stem elongation. Each crop type is sensitive to different spectrums and quantities of light at different times throughout a daylight cycle – this directly affects the rate of photosynthesis.

Essentially, we know that controlling grow light spectrum can have a significant impact on areas of growth – like flowering, flavor, color, compactness etc. However, it’s important to recognize that signaling specific growth factors is part of a much larger, complex cycle. Results also vary depending on the environment (indoor or greenhouse), the relative temperature/humidity, crop species, light intensity (lumens per watt), and photoperiod etc.

Let’s look at specific grow light spectrums and their application in horticulture.

UV Light Spectrum (100–400 nm)

UV light spectrum, which is not visible to the human eye, is outside the PAR range (100nm-400nm). Around 10% of the sun’s light is ultraviolet, and like humans, plants can be harmed from overexposure to UV light. Categorized into 3 types, UV-A (315-400 nm), UV-B (280-315 nm), and UV-C (100-280 nm).

While the benefits of ultraviolet light use in horticulture are still being researched, UV light is often associated with darker, purple coloring – in fact, small amounts can have beneficial effects on color, nutritional value, taste, and aroma.

Research shows environmental stress, fungus, and pests can also be reduced using controlled amounts of UV. Research has emerged that suggests an increase in cannabinoids like THC (5) in Cannabis can be achieved using UV-B light (280nm – 315nm).

Blue Light Spectrum (400–500 nm)

Blue light spectrum is widely responsible for increasing plant quality – especially in leafy crops. It promotes the stomatal opening – which allows more CO2 to enter the leaves. Blue light drives peak chlorophyll pigment absorption which is needed for photosynthesis.

It’s essential for seedlings and young plants during vegetative stages as they establish a healthy root and stem structure – and especially important when stem stretching must be reduced.

Green Light Spectrum (500–600 nm)

Green wavelengths have been somewhat written off as less important for plant photosynthesis given it’s (in)ability to readily absorb chlorophyll compared to red or blue light spectrums. Nonetheless, green is still absorbed and used for photosynthesis; in fact, only 5-10% is actually reflected – the rest is absorbed or transmitted lower down! This is due to green light’s ability to penetrate a plant’s canopy

In greenhouses, due to the presence of sunlight, supplementing green light spectrum using LED grow lights would be less important compared to crops are grown solely indoors – like Cannabis or vertical crop farming.

Red Light Spectrum (600–700 nm)

Red light is known to be the most effective light spectrum to encourage photosynthesis as it’s highly absorbed by chlorophyll pigments. In other words, it sits in the peaks in chlorophyll absorption. Red light wavelengths (particularly around 660nm) encourage stem, leaf, and general vegetative growth – but most commonly, tall, stretching of leaves and flowers.

A balanced pairing with blue light is necessary to counteract any overstretching, like disfigured stem elongation. It’s important to consider that while red is the most responsive light spectrum for plants, its efficacy really steps in when in combination with other PAR wavelengths.

Far-red Light Spectrum (700–850 nm)

There are a few ways far-red can affect plant growth – one is to initiate a shade-avoidance response. At around 660nm (deep red) a plant senses bright sunlight exposure. From 730nm and beyond – i.e. a higher ratio of far-red to red light, a plant will detect light “shade” from another plant or leaves higher up the canopy, so stretching of stems and leaves occurs.

Far-red can be very useful to promote flowering, and in certain plants, increase fruit yield (6). In short-day plants like Cannabis, which rely on longer periods of darkness, 730nm can be used at the end of a light cycle to promote flowering. Many growers are experimenting with interrupting the dark cycle with bursts of red light to boost growth and flowering.

Finding the Right Grow Light

There’s a great deal of information and science to take on board as we understand the way plants interact with different light spectrums. Optimizing yield production and consistent quality of plants we’ve learned are attributed to light spectrums used together – much like natural sunlight.

At BIOS we’re constantly developing our knowledge and research of how light spectrums on specific crops and strains work best – and at which time during a plant’s light cycle. Our LED grow lighting systems are designed and developed using detailed scientific research to give growers the control of using the ideal light spectrum for optimizing the yield, quality, and variability of their plants.

See all available Cannabis Grow Lights by BIOS