How does the Spectrum of LED Grow Light Affect the Plant Growth?

The interaction between light and plant growth is a fascinating and complex subject that has been studied extensively, particularly with the advent of LED grow lights. These lights have revolutionized indoor gardening by offering precise control over the wavelengths of light plants receive. Below, we explore how the spectrum of LED grow lights influences plant growth, drawing from detailed insights into the science of light and its effects on various stages of plant development.

Understanding Light Spectrum and Plant Growth

Light spectrum is the range of wavelengths produced by a light source. When discussing light spectrum, the term ‘light’ refers to the visible wavelengths of the electromagnetic spectrum that humans can see from 380–740 nanometers (nm). Ultraviolet (100–400 nm), far-red (700–850 nm), and infra-red (700–106 nm) wavelengths are referred to as radiation. As growers, we’re most interested in the wavelengths that are relevant to plants. Plants detect wavelengths that include ultraviolet radiation (260–380 nm) and the visible portion of the spectrum (380–740 nm) which includes PAR (400–700 nm), and far-red radiation (700–850 nm).

Plants utilize a wide spectrum of light, ranging from ultraviolet to far-red and everything in between. Unfortunately, many grow light manufacturers only account for specific wavelengths of visible light, between 400-700 nm. This spectrum of light is referred to as Photosynthetically Active Radiation (PAR), so-called because it is the spectrum of light that plants can use for the process of photosynthesis or the conversion of light energy into growth. Chlorophyll, the molecule responsible for photosynthesis, tends to absorb light from the blue and red spectrums best, both present in PAR lighting. There is mounting evidence that light beyond PAR wavelengths is also critically important to plant growth and health - far red as an example can help trigger flower development when used in the right ratio to red (660nm).

The Role of Different Wavelengths

Ever since NASA began experimenting with LEDs for growing plants in the 1980s, we have known that different light spectrums have widely varied effects on plants. Some spectrums stimulate vegetative growth and others increase the yield in flowers and fruits. Other spectrums seem to have very little effect in plant growth. Thanks to the variable light spectrum available from full spectrum LEDs we are finally starting to understand the relationship between light spectrum and plant growth.

Blue Light (400–500 nm)

Blue light is connected to growth and flowering and can improve the quality of certain types of plant. When used in conjunction with other grow light spectrum wavelengths, blue light can help you tightly manage growth cycles. Blue light triggers the opening of stomata by activating specific receptors in the guard cells surrounding the stomata, allowing for the uptake of carbon dioxide and release of oxygen, and water vapor loss through transpiration. A larger proportion of blue light has an inhibitory effect on cell elongation, which leads to shorter stems and thicker leaves. Conversely, a decrease in the amount of blue light will cause a larger leaf surface area and longer stems. Too little blue light will negatively affect theട

Red Light (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. Red light primarily supports the growth of stems and expansion of leaves and regulates flowering, germination, and dormancy. Red photons are the most photosynthetically efficient of all and therefore indoor growers want to maximise the amount of red in the grow light spectrum.

Green Light (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. Green light takes part in regulating plant architecture by promoting the growth of shoots and inhibiting root growth. This effect can be beneficial in controlled environments like hydroponic or aeroponic systems, where space for root expansion is limited.

Ultraviolet Light (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. Ultraviolet (UV) light plays a vital role in triggering metabolic changes in plants, promoting the accumulation of beneficial compounds such as carotenoids. Research shows that moderate UVA exposure can increase plant dry weight by 5% to 15%.

Far-Red Light (700–850 nm)

Due to the stretching response of plants to far-red light, growers who benefit from more compact growth must cautiously add far-red to their lighting regimen. Make sure to consider the ratio of far-red light to other wavebands, and the crop variety when adding far-red light to your lighting recipe. While scientists do not fully understand how far-red light modifies plant growth, it plays a critical role in photosynthetic efficiency. The “Emerson Effect” is the notion that two photosystems, one that is most sensitive to 680 nm photons and one to 700 nm photons (and far-red light up to 850 nm), work together to optimize electron transport and photosynthetic rates.

Spectrum Effects Across Growth Stages

Research shows that tailoring the light spectrum for specific growth stages can not only enhance plant growth but also increase yield and reduce energy consumption. As plants mature and go through their growth cycle from seedling, to adult, and then flowering and fruiting they use different color spectrums so the ideal LED light is different for each stage of growth.

• Seedling Stage: Cultivators initially want a plant to establish a strong root structure during its germination and seedling stage. That root structure can be enhanced with different ratios of red and far-red light with wavelengths at 660nm and 730nm. With more far-red light, plants will grow taller and have fewer leaf nodes.
• Vegetative Stage: Blue light (400-500nm) helps establish a healthy root and stem structure for your plants during the vegetative stage. Light intensity should be maintained at moderate levels (around 250-350 µmol/m²/s) to encourage vigorous vegetative growth without causing excessive stretching.
• Flowering Stage: Increase the proportion of red light (610–700 nm) and far-red light (700–800 nm). Red light is essential for flower initiation, while far-red light influences flowering timing and the size of buds. During flowering, a 12-hour light cycle with higher intensity red and far-red light (up to 600 µmol/m²/s) is optimal. This promotes both quality and quantity of flower yield.
  

Full-Spectrum vs. Targeted-Spectrum LED Grow Lights

Full-spectrum LED grow lights are designed to mimic natural sunlight by covering a broad range of wavelengths, making them a versatile choice for various growing stages. Full-spectrum lights provide a balance of blue, red, and green wavelengths, supporting plant growth from seedling to flowering. They are ideal for growers who want a single light source that works throughout the plant’s life cycle. The main advantage of full spectrum grow lights is that growers will not have to switch out bulbs or bring in different grow lights for the different stages of plant growth.

Targeted-spectrum grow lights, on the other hand, are designed to emit specific wavelengths that support different plant growth stages, like vegetative growth or flowering. These lights mainly focus on blue and red light, with minimal green or yellow. Some models also include UV or far-red wavelengths, which can further influence plant development. Targeted-spectrum LED grow lights limit the amount of yellow and green light a fixture emits, which decreases the amount of energy wasted by the fixture, and consequently, the amount of heat produced. This in turn reduces the need for additional cooling measures needed in your grow environment, and can lower utility bills significantly.

Practical Implications for Growers

The best LED grow light is the one that produces the best crops and highest yields with the greatest efficiency. While lighting must be combined with many other elements for a successful grow, choosing the best lights possible makes it easy to maximize your production. LED grow lights can use different white LEDs ranging from warm white to cool white, in effect adjusting the percentage of blue in the spectrum. Deep red, UVA, and Far Red LEDs can also be added to change and ‘broaden’ the spectrum as needed by the grower. LEDs are much more efficient than any other lighting technology and allow growers to reduce running costs and heat levels in the grow area.

Different plants and plant growth stages have unique light spectrum needs—what works for tomatoes, for example, may not be as effective for other species. This is where adjustable spectrum grow lights become invaluable. These lights allow you to customize the wavelength output to suit both the type of plant and its growth stage, from seedling to flowering, ensuring the best possible development throughout the plant’s life cycle.

FAQs

Q: Why is red light so important for plant growth?

Red light (600–700 nm) is highly efficient for photosynthesis because it’s absorbed well by chlorophyll, the pigment that drives this process. It promotes stem and leaf growth and is especially critical during flowering, as it helps initiate and regulate bloom development.

Q: Can plants grow with just blue and red light, or do they need full-spectrum light?

While plants can grow with only blue and red light—since these wavelengths are the most effective for photosynthesis—full-spectrum light provides a more balanced environment. Green light, UV, and far-red wavelengths contribute to canopy penetration, metabolic changes, and flowering triggers, leading to healthier, more robust plants.

Q: How does UV light benefit plants if it’s outside the PAR range?

UV light (100–400 nm) isn’t part of the PAR range (400–700 nm), but small amounts can enhance plant quality. It boosts the production of compounds like carotenoids, improving flavor, color, and nutritional value, though too much UV can damage plant DNA and membranes.

Q: Should I change the light spectrum for different growth stages?

Yes, adjusting the spectrum can optimize growth. Blue light is ideal for seedlings and vegetative growth to build strong roots and stems, while increasing red and far-red light during flowering enhances bloom size and yield. Adjustable-spectrum LEDs make this easy.

Q: Are LED grow lights better than traditional lights like HPS?

LEDs are more energy-efficient, produce less heat, and allow precise spectrum control compared to high-pressure sodium (HPS) lights. This means lower costs, less need for cooling, and the ability to tailor light to your plants’ needs, often resulting in better yields.