Not all houseplants need artificial light, but if yours demonstrate a need for supplemental light in the winter or are in a low-light position there are a few things to keep in mind.
Artificial light is not as strong as natural sunlight. The amount of artificial light needed will depend upon a plant's natural light needs and the amount of light it is getting without an artificial supplement. The type and strength of the artificial light you choose will also impact the number of hours that will be necessary.
For most plants getting some natural light, 12 to 14 hours of artificial light should do but plants can need over 16 hours of supplemental light if there is little natural light. These are estimates and you will need to think about how high your particular plants' light needs are and what is available from natural sources. And remember that all plants need some hours of darkness to remain healthy.
Most flowering houseplants are long-day plants that bloom when the sunlit hours outnumber the hours of darkness. Short-day plants that typically bloom in winter (Christmas cactus, African violet and poinsettia are examples) are exceptions that need short-day photoperiods provided by artificial light to flourish. Give them only about 10 hours of artificial light per day until buds form.
Specialized horticultural lights have become increasingly available and are most houseplant owners' choice for high intensity light with relatively little plant-scorching heat. They are expensive compared with some other options, but reliable and very long-lasting. These LED (Light-emitting diode) bulbs can be purchased from many different vendors but you will get the most guidance from a horticultural supplier or plant nursery. Standard LED lights are not designed for plant growth; look for full-spectrum grow bulbs specifically designed for horticulture.
There are other options. Amongst traditional types of artificial light sources, fluorescent high intensity (T5) bulbs offer high output efficiency (low watts are needed to produce high light value) and relative economy. They give off low heat so they can be positioned near plants and are generally easy to set up in flexible configurations. They offer enough light for sun-loving plants.
Standard Fluorescent bulbs (T12) on the other hand, are weaker in intensity and a good option only if your light needs are modest, for instance when starting seeds or supplementing naturally available light. Compact fluorescent bulbs (CFL's) can fit in a traditional light fixture and are best for very limited light needs.
Incandescent bulbs do not provide the specific spectrum or intensity of light suitable to plant growth and are inefficient in conversion of electricity to light energy.
A second type of specialized horticultural light, HID (High Intensity Discharge) bulbs, are mostly used in commercial growing environments. Two types of HID bulbs, each requiring a different fixture, are frequently used together. HID high pressure sodium is low spectrum and best for flowering. HID metal halide is high spectrum, supporting leaf growth. They are powerful and energy efficient, but give off too much heat and take up too much space for most home environments.
Keep in mind that the efficiency of some bulbs in delivering light decreases as they age, and you may need to adjust exposure to compensate. You should also clean your grow lights twice a month to maintain light efficiency.
Most houseplants need both warm and cool light. The colors of the light spectrum are described in degrees Kelvin (k) with the higher the number the cooler the light. Around 6500k is cool blue light and is the best for foliage. Around 3000k is warm red light, needed for plants to flower and fruit. Even if insignificant in visual impact, flower or seed production may be an important part of your plant's annual cycle that you need to support.
Bulbs described as cool-white do not have much red light and are not enough for a houseplant on their own unless they are supplementing generous sunlight. White or daylight fluorescent bulbs are also not good for indoor plants. Full-spectrum LED or fluorescent grow bulbs designed for plants have enough red light to balance the blue.
Artificial lighting loses impact dramatically as you move it away from your plants. You will need to balance the heat that the light source emits with the plant's need for light when placing your artificial light system. You can experiment! If a plant needs a little less than your grow lights are giving it, you can move the lights a bit further away or reduce the time you have them on.
Keeping the lights far enough away to avoid burning your plant depends upon the type of light you are using. In general, T5 Fluorescent bulbs are low heat and can go 3 to 12 inches from the plant, LEDs should go 12 to 24 inches and HID 24 to 60 inches. Keep a close eye on plants in the initial period after placing lights to detect any signs of heat stress or damage.
The leaves of your entire plant needs to receive light. For a large plant, that may mean positioning multiple lights to reach upper and lower leaves. For smaller plants, remember to rotate the plants with respect to the artificial and natural light sources regularly. Plants with largely horizontal leaves (many trees, for instance), need to receive light primarily from above for photosynthesis to occur efficiently. Artificial light aimed upwards toward the leaves of your plant will not be very effective. Most of the chloroplasts in the leaves are arranged just below the top skin of the leaf and will not receive light efficiently from below through extra layers of leaf. There are some chloroplasts near the bottom leaf surface, but far fewer. The leaf also has the stoma, through which CO2 and water vapor are exchanged, on the more protected underside of the leaf. If additional heat from the lighting is received on the undersurface, it can cause the stoma to close, as they would in the time of a heat wave. That will also disrupt the process of photosynthesis and the availability of energy to your plant.
This is a common question to which there is no exact answer and it is one of the most difficult aspects of artificial light. We are usually asked about watts when a houseplant owner wants to purchase LED grow bulbs online, which is a confusing prospect. Watts, unfortunately, measure the energy required to produce light rather than the light produced, so the output of two bulbs with the same wattage can vary considerably based on efficiency. Plants also vary enormously in their needs and the natural light available to them.
A nursery or a horticultural supply company will be able to guide you best with choosing a high-quality system of lights that will benefit the plants you are caring for effectively. If you would like to purchase bulbs and fixtures online or from non-specialized retailers, be prepared to experiment. The light product you purchase may not be reliable, efficient or thoughtfully designed for plants. Monitor your plants closely.
Ensure that the number of lights you buy will provide light to the entire plant or group of plants without being too distant to be helpful. For green plants, about 30 watts of horticultural LED light per square foot of plant area is a good place to start. If the plants you own have medium light intensity needs, they may need as much as 10x the delivered light as a plant with low light intensity needs. This cannot be translated directly into watts. Observation, adjustment and light repositioning should be expected with artificial growing lights.