Home Page
The Book What's New Store About Ed Contact Us Resources Home Page

Experiments At The Cutting Edge

Since humans first noticed that they could alter the outcome of nature by planting seeds, we have been manipulating plant environments. Irrigation, fertilization, pruning and breeding have been included in our bag of agricultural tricks for several millennia.

The use of light and lighting to manipulate plant growth began in the early 20th century with the advent of the modern floral industry. At that time chrysanthemums were available only seasonally in the fall. Researchers learned that their flowering could be delayed or induced by a lighting regimen. Chrysanthemums were discovered to be a short-day plant. SDP refers to the plant’s sensitivity to and measure of the length of the dark period, not to the lit period. Secondly they require a certain critical period of darkness, the number of hours of uninterrupted darkness, to be induced to flower. The number of hours varies by variety.

Marijuana growers use the same techniques as chrysanthemum farmers because cannabis is also an SDP. Indoor growers usually place the plants in flowering by reducing the number of lit hours to 12, resulting in 12 hours of uninterrupted darkness.


The 12-12 formula has been accepted without question by growers all over the world. Probably one of the reasons for this was my discussion of the technique in some of my early books. The 12-12 regimen was selected because my co-author and I reasoned that no matter what critical period a variety might have, given 12 hours of darkness it would flower. However, most marijuana varieties need fewer than 12 hours of darkness to flower.

If marijuana plants grown outdoors required a 12-hour dark period to flower, they would not be induced to start flowering until September 21, the first day of autumn, when day and night are equal length. They would ripen 6-8 weeks later. In real life most modern varieties ripen between early September and late October. Budding was triggered 6-8 weeks earlier. For instance, an eight-week variety that requires 55 days from forcing to maturity, which would normally ripen on October 1st, would start flowering August 1. At that date in San Francisco, sunrise occurs at 5:14 and sunset at 19:18, a total of 14 hours 4 minutes. Dawn and dusk add another 15 minutes of red light. Plants use the absence of this spectrum to measure the dark period. The total lit period came to about 14:20, leaving 9:40 minutes of darkness. The critical period for this plant was 9 hours 40 minutes. If it were given just 10 hours of dark period daily indoors, rather than 12 hours, it would still flower.

Gardeners growing outdoors who discover the critical time needed to induce flowering can use this information to get more efficient use of their indoor garden. Currently, plants grown under a 12/12 cycle spend half their time in the dark. If the plants have a critical flowering time of, for instance, 10 hours of darkness daily, they can be provided 14 rather than 12 hours of light each day so that they receive almost 17% more energy which they use to produce sugars used for more and faster growth.


Sometimes I think nature didn’t get it right as far as marijuana ripening is concerned. If it were determined by human needs, the buds would be ready in late spring, so they’d be readily available in early summer, the party season. If they ripened at the best time for the farmer, it would be in high summer when the weather was warm and the sun strong. Alas, nature has chosen the fall. The harvest can be good if the weather doesn’t attack first.

With a bit of effort you can manipulate the outdoor light cycle to grow and flower the plants at your convenience. I think that the best and most convenient time to harvest is during the summer rather than the fall. Buds that ripen midsummer experience much more intense light and much more UV spectrum than fall ripened buds. The intense light gives the plant energy to grow a bigger bud. The UV light increases its potency.

To force the plants to flower during the summer they need to receive the critical dark period each day. Covering the plants with a blackout cloth each day so they receive 12 hours of darkness pushes the plants into flowering mode. Decide whether you are going to restrict light at the beginning or end of each day. If you restrict light during in the morning, place the blackout cloth over the plants just before dawn; then remove it promptly at the appointed time. For instance, if the sun sets at 20:30 the curtain, which was placed on the garden at 5:00 right before dawn, is removed at 8:30. (If you know the plants’ critical time period for flowering you can adjust the curtain times accordingly.)

If the plants are to be covered in the afternoon then count forward 12 hours from sunrise. If the sun rises at 6:00, the curtains should go on at 18:00. Remove the cover at the end of dusk. The cover is removed because moisture builds up on it from dew and plant transpiration. Removing the cover limits the time the plant experiences a high humidity environment. In late stages, mold prevention measures should be taken. The plants can be sprayed with Serenade biological fungicide, Zero Tolerance herbal fungicide, potassium bicarbonate or other fungicides.

In some parts of the country, such as the gulf coast and parts of southern California, the temperature never dips below 7.2C and plants can be grown all year round. In areas that are slightly cooler but never freeze, a cold frame or unheated greenhouse is all that is required to keep the plants growing. Besides temperature, the problem with growing plants outdoors during the winter is that the dark period is more than 12 hours and the plants are triggered to flower. This is a good time to grow a sativa dominant hybrid because these varieties continue to grow even after they have been triggered to flower.

Another technique is to interrupt the dark cycle with light. The light doesn’t need to light the plant for long, just a few seconds. Think of it as a water spray. You want to get all the leaves “wet” with light, but once they have been sprayed they need no more light. This short interruption of the dark cycle is enough to reset the time count of uninterrupted darkness. By lighting the plants every few hours during the dark period, the plants continue to grow vegetatively, and not flower.

When the light interruption stops, the plants immediately start to flower. Plants can be started in August and planting can continue throughout the winter. To increase growth natural light can be enhanced using reflective material and electric lights. Spring starts on March 22 and on that day light and dark share equal time. At the equinox, or shortly thereafter, the dark period becomes too short to support flowering, and the plants will grow vegetatively.

To get the best possible harvest, force spring plants to flower in June. If plants are forced June 1, they will be ready in mid- to late-July. If they are forced July 1, they will ripen in mid August.


Ultra-violet B light is a spectrum of light that is invisible to us but is visible to insects and some other organisms. In humans it causes suntan and sunburn and is implicated in the formation of eye cataracts. It is the light emitted by tanning bulbs.

UVB light also affects marijuana potency. The potency of high quality marijuana increases in direct ratio to the amount of UVB light it receives. This is very significant. In California, where the medical dispensaries operate in an unrestricted market; many dispensaries reject fall harvested outdoor material as inferior. They have found it lacks the potency of indoor crops and is a harsh smoke. However, when they were presented with marijuana grown outdoors but forced to ripen August 10, they accepted it as if it were indoor because of its high potency and lack of harshness. I think the harshness results from cool nights.

Indoors, under fluorescent and HPS lamps, gardens receive little UV-B light. Metal halides emit a bit more. However, there are ways of supplying your garden with UV-B light. Tanning lamps work, that is, lamps that tan people, because of the UV-B light they emit. Using tanning lamps will increase the THC content of the crop. Reptiles and lizards require the spectrum to stay healthy. So the spectrum usually comprises about 10 percent of their output. If you want to try tanning lamps they are available on the Internet. Use between 5-10 percent of your total wattage to these lamps. For a 1000-watt garden use 100 watts of special lighting.

Adding UV-B light to your garden will enhance your marijuana naturally, without “special formulas” and chemicals.


As plants evolved for hundreds of millions of years they never actually had to deal with separation of light spectrums or unusual lighting regimes. When they received light it came from the sun in a mixture of spectrums and they could pick and choose which to use. It was only with the advent first of gas and then electric lighting that plants encountered unusual regimens and splintered spectrums.

Plants measure day length using the red light spectrum. While they use other spectrums for other purposes, they are not sensitive to them as far as flowering is concerned. They are almost totally insensitive to green light and for this reason reflect it back to us while absorbing most other spectrums.

Plants’ insensitivity to green light can be used to a gardener’s advantage. You know that turning the light on in the middle of the dark cycle disturbs the plants’ flowering paradigm. The light, HPS, fluorescent and MH lamps all emit red light. Green fluorescent and LED lights contain no red light and will not disturb the dark period. You can go in the garden under adequate light to work, as long as it is green.

Plants use blue light for certain regulatory processes and also for photosynthesis. Chlorophyll absorbs both blue and red light and uses the light’s energy to power the complex process in which water and atmospheric carbon dioxide are converted to sugar and oxygen gas. Blue light does not affect the regulation of flowering.

When blue light is turned on during the dark period, plants photosynthesize but their flowering isn’t affected. This results in more growth as the plants produce more sugars. Before LED lights it was difficult to create a pure blue light. Instead, most of the time other spectrums were filtered out, which can be an inconvenient process. Try using between 20 and 40 watts of mixed blue light per 1000 watts of regular light. I have done only initial experimentation with this so test this in a limited way first. I suspect that the additional light is an efficient way of increasing total yield

Aside from red and blue light, plants also use orange light for photosynthesis. I haven’t experimented with them yet, but orange LEDs might also help increase yield and probably can be lit continuously, just like the blues. More on this as the news breaks—or at least, as it fractures.


Ask Ed
Ask Ed
Ask Ed
The plants were placed outside early in the season and would have started flowering under natural light, which included a dark period of slightly less than 11 hours. To keep the plants growing vegetatively and prevent flowering, the plants were “sprayed” with light in the middle of each dark period. The brief lighting reset the plants’ chemical countdown of the critical dark period. The frame was made from PVC pipe. The gardener told me that if he were to do it again at least part of the framing would be made from steel and aluminum tubing to make it more stable.
An automated HPS light made by PAR Lighting for regulating flowering. This lamp, held over the garden using wires, oscillates back and forth, covering an area of about 2000 square feet. A timer was used to turn the lamp on for 10 minutes every 2 hours of the dark period.
The plants, lined up neatly in rows were covered each day using white/ black plastic to limit the lighted period to only 12 hours. At this point the sun was rising at about 6 AM. The plants were covered at 6 PM and then the curtain was removed at 9:30 PM, after dusk. This limited the humidity and dew on the buds. The white part of the plastic, placed on the outside, reflected light to keep the interior cool. The opaque black interior absorbed any light that might penetrate.
This lightweight portable carport was used as a greenhouse. The translucent top dispersed the light so there was less shading. The open sides kept air circulating during hot periods. With the threat of inclement weather clear plastic was attached using Velcro to keep the plants dry.
These blue LED lights can be used to supplement HPS lamps. Plants regulate flowering using red light, so these pure blue lights do not affect it. The do use blue light for photosynthesis and the spectrum also promotes, stocky stem growth.
Tanning lights, as shown in this tanning bed, emit high levels of UVB light. This spectrum increases THC content of high potency plants. This light is unhealthy to be around and promotes the formation of eye cataracts. Make sure to wear protective clothing and UVA/UVB wrap around sunglasses when working around these lamps. It’s best to shut it these lights off when you are in the garden.
This headband light with green LEDs can be used to work around the garden during the dark period. Plants are insensitive to green light so it won’t affect plant flowering.

[ The Book | New | Store | About Ed | Contact | Resources | Home ]

© 2010 Quick Trading Company