by Ed Rosenthal
DLI: The day’s total of quantum light received is called the Daily Light Integral (DLI).The DLI for outdoor gardens varies considerably depending on latitude, season, and weather. For example, in the middle latitudes of the U.S. a sunny summer day will produce a DLI of roughly 26 moles/day; if it’s cloudy, the DLI drops to about 12 moles/day. In the winter, a sunny day yields a DLI of approximately 9 moles/day, and cloudy conditions will reduce that to a mere 3 moles/day.
LIGHT UNIT CONVERSIONS
In addition to using light for energy, plants use it to regulate growth.
Plants use blue light to determine what direction to grow, an effect called phototropism and heliotropism (solar tracking).
Whether stems are elongated or stout is also determined by light. Far-red promotes elongation; red and blue promote stout stems.
Red and far-red light play a role in controlling flowering and other developmental processes red light at 680 nm stops plants from flowering. Far-red light at 730 nm just beyond visible red on the spectrum, promotes flowering in the absence of red light.
UVA light is at the wavelength of the invisible portion of emissions from black-lights. It helps reverse damage done to plant DNA by UVB light, as well as stimulating the production of anthocyanin and other flavonoids.
UVB light affects the potency of high-quality plants. The amount of THC a plant produces increases as it receives more UVB light. This light can be provided to indoor plants with proper lighting. Outdoors, the amount of UVB light is highest at the beginning of summer. By late September, the amount is a fraction of summer levels. (For more information on using UVB light to increase potency, see Flowering.) In humans, UVB causes tanning and sunburn.
Visible and near visible light ranges from about 200 nm in the UVC region to about 800 nm in the far-red region, with human vision able to detect light from about 400 nm in the blue-violet to about 730 nm in the far-red, with a peak sensitivity at 550 nm in the green region. Ozone in the atmosphere filters out the very high-energy photons coming from the sun in the UVC region below 200 nm to 290 nm. UVB from 290 nm to 320 nm penetrates the atmosphere and, while it causes sunburn and skin cancer, it is useful to plants that use it to produce flavonoids and terpenes, including THC in cannabis. UVA from 320 nm to 400 nm is also very important for plant growth and is able to contribute to photosynthesis as well as other responses such as phototropism.
The most important region of the light spectrum for plant growth is what is known as Photosynthetically Active Radiation (PAR), between 400 nm and 700 nm. PAR is typically measured in terms of how many total photons reach an area. The units for measuring it are typically expressed in millionths of a mole per square meter per second (µmol/sq m/s), with full sunlight registering at about 2,000 µmol/sq m/s, depending on latitude and season.
When a photon is absorbed by a chlorophyll molecule, its energy is transferred to an electron and it ceases to exist. Several things can happen to this excited electron. The energy absorbed can be lost either as heat, when the excited electron drops back to its ground state, or as light, causing fluorescence at longer wavelengths. Most importantly, it can be transferred to another molecule in close proximity causing a chemical reaction that results in photosynthesis.
Chart shows photosynthesis results at each wavelength along the PAR spectrum
A light meter reads the foot-candles in the space; there are sensors atop the meter that detect and measure the light. Be sure to take measurements at canopy level, where your plants should be getting the most light possible.
UVC is germicidal; the sterilizing light is used in some hydroponic systems (see Hydroponics). It is harmful to humans and other animals.
Since lumens, lux, candles, and foot-candles are all measures of light in the narrow range of human sensitivity, they don’t accurately measure all the light to which plants are sensitive.
The better measure of light in relationship to its usefulness to plants is Photosynthetically Active Radiation (PAR), which includes the range from 400-700 nm. PAR is typically measured in terms of how many total photons reach an area. The units for measuring it are typically expressed in millionths of a mole per square meter per second (µmol/sqmsq m/s), with full sunlight registering at about 2,000 µmol/sq m/s, depending on latitude and season.
Nonetheless, PAR measurements do not measure far-red and UV light. Nor do they take into account the differences in how plants absorb and use various wavelengths. For example, plants use light in the red range almost as efficiently as blue light, though blue light is more expensive to produce. Still, PAR is a more meaningful measure than the others.
LIGHT METERS
Most gardeners probably will not purchase a meter because a great garden with excellent buds can be grown just by calculating the watt input of the particular garden lights you use. But foot-candle and quantum light meters are a great source of information and come in very handy. Rather than guesstimating the light reaching the garden, the meters provide you with an accurate reading. A light meter lets you double-check your calculations and helps you ensure that you have set up a garden with light distributed evenly throughout.
OUTDOOR GROWING
The best source of light is the sun. It requires no expense, no electricity, and its bright beams do not draw suspicion. During the summer, the sun is brighter than artificial lighting and is self-regulating. Outdoors, on a clear day at the beginning of summer, when sunlight hits Earth at the most direct angle, the light’s intensity can reach 15,000 foot-candles (161,000 lux) at noon, the brightest part of the day. Plants may not be able to process all the light at its peak, since marijuana plants are probably not able to use more than 7,000-7,500 fc (75,000-80,000 lux). Measured throughout the day from dawn to dusk, the average intensity is much less, probably 1,000-2,000 fc (10,700-21,500 lux). Of that, only about 20% of sunlight is PAR. The rest of the light spectrum is not used by plants. The excess light is converted into heat and then dissipated through transpiration, re-radiated as infrared heat, or dissipated using bio-chemical processes.
Marijuana is a sun-loving plant. It grows fastest and is most potent when it gets unobstructed light all day. Plants in containers can be moved back into bright light when they become shaded.
These plants were given enough room to grow into giants. They were grown with 6’ (1.82 m) of space between plants. Sailcloth was used to diffuse the sunlight.
This chart shows how much light plants receive daily over the course of a year. During the fall and winter the amount of light is based mostly on latitude. Notice that light levels drop dramatically during September, just as the plants are ripening. By October light has dropped in half as compared with June. By December it is down by nearly two-thirds. During the summer light intensity varies more by longitude. For example, the western half of the United States receives 50% more light than the eastern half. The eastern portion receives the least light, while intensity is higher in the Midwest and even more intense in the west. The southern portion of California and Nevada receive the most light.
Marijuana plants do best under full light all day. Gardeners can use the sun as the primary source of light if they have a garden, greenhouse, terrace, patio, roof, or skylights or even a directly lit window. Bright spaces that are lit from unobstructed sunlight at least five hours a day usually need no supplemental light during the summer.
Autumn light can be more problematic. If the garden continues to receive direct sunlight, there is usually enough light for the buds to mature. However, if the light changes in the fall so the plants get little direct sun, they will need artificial light to supplement the weak sunlight, overcast conditions, and oblique angles that create shadows. Without the additional light, buds do not develop properly. They grow loose and airy, and not particularly potent. Natural light can be supplemented using the same kinds of lights gardeners use for indoor gardens.
To find out exactly how much your plants are getting, use a light meter. If the light is close to 4,500 fc (48,375 lux) for five ho
urs or more, the space is bright enough. Lower light levels result in less growth and slower ripening, and lower yield and quality.
USING A CAMERA LIGHT METER
You don’t have to use a special meter to measure foot-candles. The light meter in a 35mm camera will work, too—provided it’s a camera with adjustable settings. All you need is a large sheet of white paper or cardboard. To calculate the number of foot-candles of light reaching your garden, prop the paper or cardboard at a 45-degree angle at the height of the plant canopy. Set the camera’s ASA (film speed) dial at 100 and the shutter speed at 1/15 of a second. Then adjust the f-stop for proper exposure, as if you were taking a photo. The f-stop reading correlates to foot-candles, as listed below.
AT ASA 100 AND 1/15 SECOND:
f4 = 10 fc
f5.6 = 20 fc
f8 = 40 fc
f1 = 80 fc
f16 = 160 fc
If you’ve got more light than that, use higher settings on your camera. With the ASA speed set to 200 and the shutter speed at 1/125, the f-stop for proper exposure foot-candles are as follows:
AT ASA 400 AND 1/125 SECOND:
f4 = 128 fc
f5.6 = 256 fc
f8 = 512 fc
f11 = 1024 fc
f16 = 2048 fc
f22 = 4096 fc
f32 = 8192 fc
In higher latitudes, plants must be harvested early in the fall to accommodate climactic conditions. Unfortunately, during autumn, when the plants are finishing flower growth and ripening, both the length and intensity of light diminishes. This reduces yield from its potential. There are three possible solutions: force the plants early, supplement the sunlight with electric lighting, or grow early-maturing plants.
In the Daily Light Integral (DLI) chart you can see how much the moles per day changes from August through October. In August, the DLI ranges from 35 moles per day in the eastern U.S., to 45-50 in the west. In October the patterns change between north and south. The southwest receives 35-40 moles per day, but the rest of the country, except for the far north, receives only 20-30 moles per day. In addition to the low light levels, the sun delivers very little UVB light by mid-autumn.
To convert µmol/sq m/s to daily light integrals in µmol/sq m/day multiply by the number of seconds of light applied. For example, if you are growing plants in a garden with 1,000 µmol/sq m/s under continuous light (24h), you would have 1,000 x 3,600 s/h (seconds in an hour) x 24 hours or 86.4 µmol/sq m/day. For a 12-hour photoperiod, this would be 43.2 µmol/sq m/day. What could be easier? It should be noted that the average daily light integral in the summer in Arizona is about 65 µmol/sq m/day, while in New York during the winter it is only about 25 µmol/sq m/day.
Sunlight at noon on a clear early summer day produces about 2,000 µmol/sq m/s (10,000 fc or about 110,000 lux) at latitude 39˚ in Maryland, which lies in a middle latitude of the U.S. At higher latitudes, the light is weaker; at lower latitudes, it is stronger. It is stronger west of the Mississippi River in North America than east of the river.
HPS lamps’ amber color reflecting from a white wall.
Most gardens with MH or HPS lamps produce about 1,000 µmol/sq m/s (5000 fc, 55,000 lux) at the top of the canopy.
Gardens with fluorescent lamps seldom produce more than about 300-500 µmol/sqmsq m/s (1500 fc, 15 lux).
During autumn, gardens located in windows and terraces may receive direct light as the light comes from a more oblique angle. If they receive direct sunlight, the plants are probably getting enough light. Sometimes window spaces that are shaded in the summer get direct light in the fall as the sun’s angle changes seasonally. If plants get only indirect light, even bright light, they require supplemental lighting. Using artificial lights to supply light to plants on a patio or terrace, or in a greenhouse or a window, need not cause suspicion. Use a metal halide (MH) lamp, which emits a clear light that blends in with natural light.
One rule of thumb is to supplement autumn’s low light levels with about 20-30 watts per square foot (215-320 w/m) for about five hours a day during daylight. A 1000w lamp enhances the light of a square with 6-7 foot (7.8-2.1 m) sides. If the lights are on during the brightest part of the day, their light will go unnoticed by passersby, but not the plants. You can safely use metal halide lamps or fluorescent tubes designed for reptiles to provide UVB spectrum light. HPS lamps are available in many wattages and are very efficient. However, they should be used with caution because they emit a distinctive amber light that may be noticed by neighbors or passersby. MH lamps produce a “white” light that is less noticeable and also contains helpful UVB light. The lights also supply heat to the plant, which can be helpful in autumn.
INDOORS
Plants in indoor gardens require very bright light to grow well and yield a good crop. However, varieties differ in the amount of light they require to support fast growth and high performance flower development. Sativas require the most light, followed by sativa-indica hybrids, indica-sativa hybrids, then indicas.
Yield and quality increase—and ripening time decreases—when plants are grown under bright lights and provided with their other needs.
Sativas require the most light. They evolved below the 30th parallel, near the equator, and are adapted to long periods of intense sun.
Sativa-indica hybrids need less intense light than sativas, but still do best with light on the high range.
Indica-sativa hybrids are more light-forgiving than sativa-based plants. They can function in the mid-to-low light range.
Indicas need the least intense light of any of the varieties. They evolved in northern latitudes and are the best bet for low-light gardens.
During the growth cycle, most varieties will do well with 2000-2500 fc, (21,500-27,000 lux, 400-500 µmol/sq m/s) although the plants can efficiently use 5000fc (54,000 lux, 1000 µmol/sq m/s) or more. The more light they receive during vegetative growth, the faster their growth and the sturdier their stems. When grown under low light, or under a leafy canopy or when shaded by trees or other tall plants, all varieties develop long internodes (spaces on the stem between the leaves) due to the enhanced far-red light; plants with equatorial genetics are more affected by this.
Equatorial sativas need intense light and do best with between 70-80 watts per square foot (w/sq ft) (750-860 watts per square meter (w/sq m)). With less light the buds will be loose and lanky.
Sativa-indica hybrids require bright light. They will produce luscious buds when illuminated with as little as 60 w/sq ft (640 w/sq m).
T-12 fluorescents are tubes that have a diameter of 1.5” (4 cm) are the least efficient. They will soon be phased out. T-8 tubes are 1” (2.5 cm) in diameter. T-5 tubes are 5.8” (15 cm) in diameter and are the most efficient fluorescents.
Fluorescent bulbs are classified according to the size of the tube. The diameter is listed as its “T” number, such as T-5, T-8, or T-12, with the number signifying how many eighths of an inch they are. So T-5 tubes are 5⁄8 inch; T-8 are 1 inch (8⁄8”); and T-12s are 11⁄2 inches (12⁄8”). One problem with T-8 and T-12 fluorescent tubes is that their large diameters prevent them from being spaced as closely as T-5s, so it is difficult to get the light intensity necessary to produce the highest-quality bud.
Indica-sativa hybrids require less light and can produce very good buds using about 50-60 w/sq ft (535-640 w/sq m).
Indicas need the least amount of light to thrive. Some indicas produce well starting at about 40 w/sq ft (430 w/sq m), though others need 50 w/sq ft (535 w/sq m) to produce nice, tight buds. More light, 60 w/sq ft (640 w/sq m) doesn’t hurt; at 60 w/sq ft (60 w/sq m), the buds will be larger, tighter, and more potent.
Gardeners have a wide selection of lights to choose from. These include fluorescents, metal halides lamps, high-pressure sodium lamps, and LEDs. Cultivators rarely use incandescent or quartz halogen lights. These lamps are inefficient, converting only about 10%–20% of the energy they use to light and wasting the rest creating heat. If you are planning to use an in
candescent lamp to light a “dark spot,” don’t. Use a compact fluorescent or LED instead.
FLUORESCENT LIGHTS
Growers have used fluorescent tubes to provide light since the early years of indoor cultivation in the 1960’s. They are inexpensive, easy to set up, and are fairly efficient, though they do require special fixtures and ballasts. Plants grow and bud adequately under them. However, fluorescents do not create the intensity of light emitted high-pressure sodium (HPS) lamps, so they usually don’t produce the large, tight buds the more powerful lamps do.
Fluorescents come with various spectral outputs, which are determined by the type of phosphor used to coat the surface of the tube. Each phosphor type emits a different set of light colors, identified as “warm white,” “cool white,” and “daylight” or “natural white.” These names signify the kind of light the tube produces, with daylight or natural white coming closest to approximating the sun’s spectrum. Lamps of different spectrums can use the same fixtures.
Vegetative garden growing under fluorescents. The tubes were installed in two light fixtures. Three tubes lit each foot (30 cm) of width of the garden.
Fluorescents are excellent tubes to use for cloning because the light is delivered evenly over a long space and the tubes don’t produce too much heat.