Light is used by plants for photosynthesis, in some cases to signal flowering. Plants that use light as a signal for flowering can sometimes be manipulated using light to induce flowering out of season. Photosynthesis converts energy, water and carbon dioxide to carbohydrates (used for plant growth) and oxygen.

Light energy is collected by chloroplasts in leaves, which contain chlorophyll pigments. These pigments appear green because they absorb red and violet-blue light, and reflect green light, giving most plant leaves a green appearance. The chlorophyll pigment itself is made of carbon, nitrogen and magnesium.

Plants without enough access to nitrogen or magnesium suffer from diminished photosynthesis. The energy from the collected light is used to convert water and phosphorous containing chemicals to create nicotinamide adenine dinucleotide phosphate (NADPH), adenosine triphosphate (ATP) and oxygen gas (O2). The oxygen gas is vented off though stomates (holes in the leaf structure). NADPH and ATP are used by the reactions that follow as an energy source.

In carbon fixation, carbon dioxide from the air is incorporated with ribulose bisphosphate(RuBP) in a process known as the Calvin cycle. Using ATP and NADPH, the resulting compounds are then reduced to form carbohydrates. These carbohydrates are then used for plant growth.

Light, temperature and the amount of available carbon dioxide are all factors in the photosynthetic reaction. Since photosynthesis is so important to plant growth, it behoves the gardener to make sure these three resources are available and within the range of plant tolerance.

Since not only water, but also carbon dioxide is a requirement for photosynthesis, it must be available. Luckily, there is a fair amount (400 ppm and growing) of carbon dioxide in the atmosphere already, so simply making sure fresh air is available to the plant will meet the minimum requirements for growth.

With indoor gardens, additional CO2 (if used) is usually generated by chemical reaction, combustion, biological reaction or slowly released from pressurized tanks.

Carbon dioxide pads use a chemical reaction that is activated with moisture. Small gas fires can be used to create CO2 by burning them. Fungi (especially oyster mushrooms) often produce CO2 as part of their life cycle.

Another alternative is pressurized CO2 tanks, which can be used with a regulator to slowly release their contents into a garden area. As a side note, adding carbohydrates directly to a nutrient solution does not increase the amount of sugars available to the plant for growth. Carbohydrates added to the nutrient solution are used to encourage fungal and bacterial growth in and around the root system.

In some plants, light is used not only as an energy source for growth, but the duration of the dark periods can signal when a plant should flower.

Photoperiodism in plants refers to the length of the dark period being the determining factor in triggering flowering. In an unfortunate miscalculation by earlier botonists, the terms "short day" and "long day" were chosen to describe plants that are photodependant for flowering.

A better way to refer to them would be "long night" and "short night" plants, since it is actually the length of the dark periods, and not the length of the light periods that trigger flowering.

The presence or absence of light is detected by a type of photoreceptor known as a phytochrome (and in some cases a photoreceptor known as cryptochrome). Red light-absorbing phytochromes (Pr) will turn into far red light absorbing phytochromes (Pfr) in the presence of red light.

To reverse the reaction, Pfr will convert back into Pr over time in darkness (or under far red light). The ratio of Pr to Pfr will indicate how long the dark periods are. If much of the Pr has been converted to Pfr then the plant has been in light for a long time.

If a lot of the Pfr has converted back into Pr, then the plant has been in uninterrupted darkness for a long time. Since the reaction to start converting Pr into Pfr takes only a short while, only a brief interruption of the dark period is needed to raise Pfr levels high enough to prevent the flower trigger in long night plants.

Short day/long night plants such as chrysanthemums will flower in fall or spring (and winter), whereas long day/short night plants such as California poppies will flower in summer. Some plants like tomatoes are "day neutral" and use a different mechanism to trigger flowering (such as chronological age).

In outdoor gardens, spring, fall and winter all have longer nights than the short nights of summer. Short day/long night plants bulk up during the growth stage of summer until the longer nights of fall trigger flowering. If these plants are set outside too early in the spring, when the nights are long, they take a cue from the long nights to start flowering.

The four seasons are important for plants that use the length of darkness (night) to trigger flowering. Winter Solstice is around Dec. 21, and marks the day of the year that has the longest night (take note that the dates given are reversed south of the equator, where midsummer occurs in December and midwinter is in June).

While in many areas the cold of winter prevents outdoor gardening, it is an excellent time for indoor gardening, as cooler outdoor temperatures usually make indoor lighting heat issues easier to cope with.

The Spring Equinox is around March 20, when day-night approaches 12-12. Depending on the date of last frost, some areas can use photoperiodism-triggered plants for an early spring harvest. The Summer Solstice is around June 22 and has the longest day and shortest night. Artificially lit gardens often use light-to-dark periods of 18 hours on and six hours off to simulate this lighting condition and prevent flowering in long night plants.

The Autumn Equinox is around Sept. 22, when the longer nights and shorter days again approach 12-12. This is the traditional fall harvest season. As the weather cools, keep an eye out for bud rot and other fungal diseases, which can thrive in the moist, cool fall.

To replicate seasonal lighting indoors for long night plants, many gardeners use zero to six hours of darkness to simulate the short nights of summer, encouraging short night plants to grow and develop without flowering.

When the plants have developed sufficiently, the dark periods are increased to 12 hours or so, giving them long nights and triggering flowering. The reverse is done for short night plants, where they put on size during long nights and bloom under short ones.

With even a basic understanding of photosynthesis and photoperiodism, lights, cold frames and greenhouses can be used to manipulate the light that the plant is exposed to. By covering the plants at dusk and/or dawn with an opaque covering, nights can be extended. To shorten the dark periods, electric lights can be added to interrupt long nights.

Many photoperiod-dependent plants can be grown out of sync with their normal growing season using these two techniques, allowing for harvest times to be manipulated while still taking advantage of outdoor lighting as the primary growth light source.

(The lights used to interrupt dark periods are usually not bright enough to encourage much growth but are high enough to convert the Pr back to Pfr and prevent flowering).