In general, blue light spectrums encourage vegetative and structural growth and red light promotes flowering, fruit, leaf growth, and stem elongation.
Red light stimulates vegetative growth and flowering (but if a plant gets too much, it will become tall and spindly). Blue light regulates plant growth, which makes it ideal for growing foliage plants and short, stocky seedlings (but too much will result in stunted plants).
Metal Halide (MH) lamps provide a large amount of blue light – the spectrum considered best for the vegetative stages of growth (3). High Pressure Sodium (HPS) lamps are ideal for both vegetative and flowering. Both of these produce significantly more heat than LEDs but are relatively inexpensive to buy.
Blue Light
Blue lights with a Kelvin color temperature from 5000K to 6500K or wavelength in the 400-500 nanometer range are best for the vegetative stage of the grow cycle and for starting seeds indoors.
Although this curve is sometimes over-interpreted, it does show that red light is at least as effective as other colors of light at promoting plant growth. Therefore, while red light isn't the most efficient color for general illumination for people, it is among the best colors of light to stimulate plant growth.
Blue light helps plants produce chlorophyll, the pigment they need to grow. It also helps encourage germination and root development in young plants and seedlings. Red light regulates plant growth and helps plants produce flowers and fruit. Green light helps maximize photosynthesis.
Blue light is essential during a plant's germination phase. Stronger concentrations of blue light will encourage sprouting and development of strong roots.
Light color
If you are just using grow lights to start seeds or to grow leafy vegetables, stick to lights that are labeled either as blue-green spectrum or balanced light spectrum. It is increasingly common for lights to be labeled “for greens and seeds” or “for flowers and fruit”.
Most LED grow lights you can find today are “full spectrum” lights, which is sort of a buzzword that means you can use them for vegging and flowering.
Plants grow best when they are exposed to light that is as similar to natural sunlight as possible, which is between 2,700 and 7,000 Kelvin. In the old days, growers would use red- and blue- colored LED's to give plants this full spectrum of light that they need to grow.
Green light is the least effective for plants because they are themselves green due to the pigment Chlorophyll. Different colored lights help plants achieve different goals. Blue light, for example, helps encourage vegetative leaf growth. Red light, when combined with blue, allows plants to flower.
Given a choice between these limited options, the blue-heavy light clearly works better for vegetative growth, and the red-heavy light works better for flowering. But this doesn't mean that the plants don't want more red light during vegetative growth, or more blue light during flower.
Far-red light makes plants think they're not getting sunlight. As a result, too much far-red on your seedlings may prevent germination altogether.
Red and blue lights are often used during the vegetative stage when plants grow leaves and stems. Yellow lights are typically used during the flowering and fruiting stages when plants produce flowers and fruit.
A more correct statement is that, generally, green light is less efficient at stimulating photosynthesis than blue or red light. In some situations, the greater reflection and transmittance of green light by leaves can be desirable. Green light can better penetrate a plant canopy and thus reach lower leaves.
Telltale Signs of Too Much Light
The most apparent sign is leaf burning. This typically causes the yellowing of leaves at the top of the plant but the veins stay green, and the leaves take on a yellow or brown, burnt look.
While some plants need less darkness time than others, too much light will impede any growth schedule over time. Too much light can also refer to the intensity of the light. Intense, direct light is great for some plants, but it will damage (or even kill others).
The light intensity isn't strong enough. In comparison to specialized grow lights or the light emitted from the sun, regular LEDs are incredibly weak. Far weaker than most plants need for healthy growth. They just won't be effective enough.
Vegetable seedlings and other “full sun” plants require about 2000 to 3000 lumens (at minimum) per square foot of growing space. So, if you're growing just a single standard tray of seedlings, using a grow light that emits 3000 lumens is sufficient.
' plants do require a daily respiration period of at least 6 hours per day (for seedlings) and ideally 8-10 hours for more mature plants. Therefore we do not recommend providing more than 14-16 hours per day of light, even if you are growing long-day-loving varieties and trying to induce flowering/fruit from them.
Use a light timer to leave your grow lamps on for an average of 16 hours per day for young seedlings. If your light garden is next to a bright window, you can run your lamps for 14 hours. If your space doesn't have much natural light, you can run your lamps for up to 18 hours.
Ranging from 600-700 nm, red light wavelengths encourage budding and flowering. Along with blue light, red light wavebands are considered one of the most important for photosynthesis and biomass growth. Indoor growing environments typically use a balanced combination of red and blue light for best results.
Red and blue light are the most important for plant growth impacting photosynthesis and flowering. A mix of 90% red light and 10% blue light provides the balance of light colors needed for most plant growth.
White LED fixtures create a more pleasant environment, and the spectrum is similarly useful to plants as purple light, but fixtures are usually less efficient.
Blue light provides chlorophyll production (the most active pigment for photosynthesis) in plants. Plants that receive plenty of blue light will have a strong, durable, and healthy stem and leaves. 400-500 nm wavelength range blue light is used to support stem and green leaves development.