Agriculture Reference
In-Depth Information
AMOUNT OF LIGHT
Most vegetable crops, including herbs, need 50 to 70 watts of light per
square foot of growing area. This is a general guideline. I have always
found that fruiting vegetables such as tomatoes, cucumbers, eggplants, and
peppers like a lot of light, so approach the higher figure for them. To calcu-
late the correct wattage of light needed for a specific area, simply multiply
the desired wattage of light by the area in square feet. Using our earlier
example from the “Plant Spacing” section, we had 8 tomato plants that
would eventually cover 32 square feet of floor space (8 plants × 4 square
feet/plant). We need at least 32 × 50 watts = 1600 watts. So you should
use a 1500-watt MH light. This 1500-watt MH will provide 47 watts per
square foot of growing area. To determine the number of units needed for a
specific wattage of bulb, divide the total growing area by the lamp wattage
times the desired watts per square foot. For example, if you want to use
600-watt lamps for the 32 square feet of growing area, the number of units
required is (32/600) × 50 = 3 (rounded up).
Lights need to be supported by chains. I have found that using jack
chains to support them allows you to change their position as the plants
grow. Start the plants immediately with the light. Keep the light about 3
feet above to get 510-foot candles at plant surface. Then lower the light
about a foot a week until it is about 1 foot above the plants as they grow
upward. Then keep moving the light up several times a week in increments
of 6 inches as the plants grow up 6 inches. When the light is close to the
plants you must take care not to burn them. Lights give off heat, so you
must dissipate the heat by using a fan that will move the air across the leaf
surface. Remember, as I discussed earlier, you want to keep your day tem-
peratures about 75°F (24°C). You can make more efficient use of the light
in the plant canopy by use of a reflector. Most of us just naturally take that
for granted. We do not want to have light going up above the plants, just for
it to focus on the plants themselves.
REFLECTORS AND WALL COVERING
Reflectors come in three basic forms: parabolic, horizontal, and conical.
Claims are made that parabolic reflectors give up to 18% more light than
the conical ones. They are supposed to focus more of the light on the plants
by directing the light below the horizontal plane. This also reduces glare to
your eyes. Conical reflectors give more side light so they are more useful
if you want some light to reach out from the plant canopy. Sunlight Supply
has parabolic reflectors in 32-inch and 48-inch diameters, and a cone
reflector in 42 inches (Figure 3.4). Some reflectors come with a fan to cool
