The environment in our homes dictates which plants will grow vigorously and which will suffer. The most important environmental factor in growing plants indoors is adequate light.
It is recommended that a minimal level of illumination to grow Cactus indoors is around 15 watts per square foot (150 watts/ sq. meter). Fluorescent lighting should be placed 12 - 15 inches (28 - 35 cm) from the top of the plants. High Intensity Discharge Bulbs should be placed considerably further away (depending on wattage). Plants do much better if the day length is kept more or less constant, depending of course on the season. Be sure and use a timer set to 12 - 18 hours a day. Most plants grow best if the light, dark period matches that of their native habitats. When using artificial lights, be sure and use reflectors to catch and concentrate as much light as you can on the individual plants. For maximum growth, your plants should be rotated about every two weeks to assure even illumination. Cactus, like most plants are more sensitive to certain frequencies (colors) of light. This is usually towards the blue and red parts of the spectrum. For best results use a grow light type of tube for fluorescent lights, or for killer results, step up to a metal halide. These kind of lights produce more light in the colors that the plant can use. Metal Halide fixtures also produce a great deal of heat and some UV radiation, your Cactus will love it. These fixtures have proven to work well in an indoor environment as they have been used by "closet" growers successfully for years.
Light provides the energy source needed for plants to manufacture food. The amount of light is commonly measured in foot-candles (ft-c). The interior of a well-lighted home is often less than 100 ft-c, while outdoor light intensity on a clear sunny day may exceed 10,000 ft-c. Plants differ greatly in their light intensity requirements. Indoor plants are often classified by the amount of light necessary for growth:
- Low (minimum 100 ft-c, 75 to 200 preferred for good growth or ≈ 750 - 2.000 lux)
- Medium (minimum 100 to 150 ft-c, 200 to 500 preferred ≈ 2.000 - 5.000 lux)
- High (minimum 150 to 1000 ft-c, 500 to 1000 preferred ≈ 5.000 - 10.000 lux)
- Very high (minimum 1000 ft-c, 1000+ preferred ≈ more than 10.000 lux)
One footcandle ≈ 10.764 lux. The footcandle (or lumen per square foot) is a non- SI unit of illuminance, but it is still in fairly common use in the United States. About 100 ft-c for 12 hours per day are necessary simply to maintain plant quality for one year, and at least 200 ft-c for 12 hours per day are necessary for foliage plants to manifest any benefit from fertilization. For plants growing, light volume is very important. It's measure is lux, that represents divider between the light and ligtened area unit: lux=lm/m². If we use lux for measure unit, rather than ft-c, than we multiply with 10.764 (lux=10.764xft-c). For orientation, here are some examples of how much illuminance plants get:
- Full day-light 10.760 lux
- Morning light 10.760 lux
- Lightened room 3.500 lux
- North-side window 2.000 lux
Artificial lighting is widely used to supplement or replace natural light. Many indoor plants grow well under artificial light provided by fluorescent lamps or special incandescent lights. A large variety of fluorescent lamps are available. Philips makes a plant light they named "Agro-Lite", which is a minor variant of the wide spectrum Gro-Lux. The Philips tube resulted in 2 - 10% greater growth in a variety of terrestrial food crops when compared to other wide spectrum plant lights.
Generally, ordinary incandescent lamps are not recommended for plants, as plants placed under them tend to stretch or become "leggy." It is possible to make up for lack of sufficient light by increasing the time or duration that the plant is exposed to light. Sixteen hours of light and eight hours of darkness are satisfactory for most plants. Use an electric timer to ensure the correct cycle each day.
Since flowering on many tropical cacti (shade loving cacti) is begun in direct response to the length of day, plants grown under artificial light should have the daylength reduced in winter so that they are in darkness for more than 12 hours each day. Without these shortened days, such plants will not flower.