Most growers know that fine-tuning their feeding schedules can help improve quality and yield, but did you know it is also possible to fine-tune your garden’s lighting needs?
Researchers in the Netherlands are doing just that, and consultants are now recommending specific “light recipes” for greenhouse-grown crops. Light recipes combine traditional horticultural lighting with specific LEDs (light-emitting diodes) to create a tailored light spectrum for each stage of the plant’s growth.
Natural sunlight contains all of the colors of the rainbow and plants have evolved to harvest specific wavelengths from the sun for growth and reproduction. Plants appear green because the green pigment chlorophyll absorbs red and blue rays, but reflects the green wavelengths back to our eyes.
The light energy absorbed by chlorophyll and its auxiliary pigments is channeled to reaction centers in the chloroplasts to produce high-energy sugars for healthy, vigorous plant growth. Therefore, premium horticultural grow lamps must emit full-spectrum light, rich in both the blue and red ends of the color spectrum.
Blue Light and Plants
The blue end of the spectrum is especially important during vegetative growth. Blue light helps plants stay short and stocky and develop thicker stems and dark green foliage. The waxy covering of leaf cells will also be thicker when given enough blue spectrum, and if you look at the leaf under a microscope, you will see that the stomata (pores for entry of CO2) are more numerous and closer together.
Furthermore, blue wavelengths stimulate chlorophyll production more than any other color in the light spectrum, turning young plants into highly efficient photosynthesis factories.
Blue wavelengths also influence a phenomenon called phototropism—the bending of plant growth in response to light. If you have ever placed a plant on a window sill, you will notice the new growth tends to lean toward the light. If you turn the plant around and come back the next day, the plant will be leaning toward the light again. But in fact, plants aren’t leaning toward just any light source; they are leaning toward blue light at a fairly narrow bandwidth—around the 460-nm wavelength.
The yellow, green and red portions of the spectrum have no effect on phototropism. Why blue? Plants want to see the sky! Plants have evolved to compete for the light. If a seedling is shaded by other plants, the higher proportion of far-red light that filters through the canopy will cause plants to stretch. But after even a short exposure to blue light, the plant will stop stretching.
Red Light and Plants
During fruiting and flowering, plants prefer light richer in the red end of the spectrum. The latest double-ended, high pressure sodium lamps provide high light intensity with an expanded color spectrum for plant growth. The reds and far reds help promote the flowering process, and the enhanced spectrum is more efficient for photosynthesis.
Even so, the best horticultural grow lamps are also enhanced in the 460-nm wavelength, providing just enough blue light to prevent plants from stretching. Lack of blue light will cause plants to become tall and spindly, with pale leaves and poor production, so stay away from cheap warehouse lamps.
Plant Lighting Tips and Tricks
The following are some general lighting recommendations for indoor gardening.
Generally speaking, large seeds don’t need any light to germinate. They have plenty of stored carbohydrate reserves and can germinate in the dark if they are given the right moisture and temperature conditions. Small seeds, on the other hand, often need a little light to germinate. For example, germination of lettuce seeds is stimulated by red light, but inhibited by far-red light.
Starting Clones and Seedlings
New seedlings don’t need high-intensity light, but they do need full-spectrum light. Blue light helps prevent stretching, and red light helps stimulate root development. Most commercial propagators use cool white fluorescent lamps to start their plants, but many growers are beginning to turn to LEDs. LED lamps are more energy efficient, and the reds and blues can be mixed to provide the ideal spectrums for balanced plant growth.
Vegetative Growth Stage
During rapid vegetative growth, plants need full-spectrum light, preferably with extra blue. They also need more light intensity than that provided by standard fluorescents. T5 high-output fluorescent lamps are often a good choice since they are three times brighter than standard fluorescents and they run much cooler than metal halide lamps.
T5 lamps are available in either 3,000- or 6,500-K color temperatures. The 6,500-K lamps are stronger in the blue end of the spectrum, and the 3,000-K lamps are stronger in the red. Lamps can be mixed and matched to provide a wider growth spectrum, and the fixtures can be kept relatively close to the plants for good light intensity.
Supplemental LEDs are also gaining popularity for vegetative growth. New advances in high-output LED technology are providing higher light intensity, and a wider range of frequencies is now available. Supplemental light in the 460-nm (sky blue) range is particularly beneficial during the vegetative growth stage.
Fruiting and Flowering
During fruiting and flowering, plants need full-spectrum light with special attention to the red end of the spectrum. HPS lamps provide the light intensity that plants need, but supplemental red and far-red LEDs are also beneficial. Plants measure their age from dusk to dusk.
As the sun sets below the horizon, there are proportionately more far reds than reds. Special pigments in plants called phytochromes detect the far-red-to-red ratio and tell the plant that another day has passed.
By giving plants a burst of far-red light at the end of the light cycle, the plants will shut down for the night quickly, but if plants go directly to a dark period without receiving the far-red light, it could take several hours for the plant to respond. By successfully manipulating night length with far-red LEDs, better control of the flowering process may be achieved in light-sensitive crops.
Best Grow Lights for Improving Yields
Researchers in the Netherlands are experimenting with “inter-lighting” to improve yields, especially in taller crops. HID lamps are still the primary overhead light source, but LEDs are added lower in the canopy in specific color spectrums. For example, some frequencies of blue light stimulate the opening of the stomata. Green light, on the other hand, causes the stomata to close. As the upper leaves shade out the lower leaves, photosynthesis begins to shut down. By adding high-efficiency LEDs lower in the canopy, it is possible to stimulate more of the plant to become photosynthetically active, resulting in higher yields.
Best Grow Lights for Improving Quality
Some light frequencies, including bands in the ultraviolet spectrum, have a direct effect on plant quality. Most greenhouses designed to grow shade-loving, bedding plants have coverings specifically designed to filter out ultraviolet light. But many herbs and vegetables need higher levels of the blue end of the spectrum for top quality.
For example, frequencies of light in the ultraviolet to blue end of the spectrum stimulate the production of the molecules needed to make purple pigments. When growing plants such as purple basil and red lettuces in a greenhouse, it is important to source “UV-clear” coatings.
UVA and UVB rays also stimulate the plant to make many secondary metabolites as plant protection agents. Some of these antioxidants and secondary compounds can improve the nutritional value of food or the potency of medicinal compounds.
Best Grow Lights for Improving Energy Efficiency
HID lamps are great for growing plants, but they produce a lot of wasted heat. Not only is a significant portion of your lighting bill wasted by producing unusable heat energy, but additional energy is also required for ventilation and air conditioning.
As energy costs continue to rise, growers are looking for more energy-efficient ways to provide supplemental lighting to their crops. By experimenting with new lighting technologies and applying specific light recipes, growers in the Netherlands are already achieving energy savings of up to 30%.
Light is the limiting factor for most indoor gardeners. I wouldn’t recommend throwing away your standard HID lighting just yet, but I’d keep an eye on emerging lighting technologies, such as double-ended lamps and high-output LEDs.
Supplemental LEDs are now available in a wide range of colors, including frequencies in the ultraviolet and far-red bands. Some LED manufacturers even allow growers to specify their own light recipes at no additional cost.
As horticultural lighting becomes more energy efficient and affordable, indoor gardeners will reap greater benefits than ever before.
By Harley Smith