Insufficient light will produce sparse foliage, spindly branches and poor flowering.  This can occur because lamp size (and type) is inadequate; lighting duration is inadequate; foliage is too far away from the lamp; or foliage is being shaded by other plants.

Lamp size:  A lamp’s size is measured in watts (W). The size of the growing area will partly determine the required wattage - see Table 10.1 & Fig 10.3.

Plant height:  A plant’s height must be considered when determining lamp size because light intensity diminishes rapidly as distance from the lamp increases (Table 10.1 shows the drop in intensity from 1ft to 3ft).  For example, imagine your plants covered an area 0.9 x 0.9m.  You may be tempted to use a 400W lamp instead of a 600W HPS (i.e. power costs are 50% less, and it generates less heat).  However, if a significant proportion of the foliage is 3ft from the lamp, the intensity is only ~5,555 lm/sq.ft.  This intensity may be insufficient for the species being grown.  By choosing a 600W HPS, the intensity would be roughly double (assuming equal gap between lamp and foliage).
 

Plant density/ Shading:  Shading becomes an issue when plants are positioned too close to one another.  Hence it is generally more productive to plant less than more.

Lamp height:  To best utilize lamp output, position lamp as close as possible to the top of plants without causing photo-respiration or burning of foliage (see Table 10.1 for “minimum gap” guidelines).  Air cooled shades should be used because they enable lamps to be positioned much closer to foliage - particularly beneficial for 1,000W lamps (Fig 10.4).

Lamp shades:  These help maximize the amount of light directed towards the plants (Fig 10.5).  Shades need to be hung so that their height can be easily adjusted as plants grow.  For safety, ensure mounts are securely fastened to the ceiling.

Light cycles:  Many plant processes respond to daylight length.  For example, certain species require ~18 hours of light per day during the seedling and vegetative phase.   However, flowering is then induced by lowering the lighting duration to ~12 hours per day. 

Be aware that growth can suffer if lighting intervals are irregular.  Therefore, employ a timer to help ensure consistency.  Note also, vital processes occur during the night (lights off) period therefore avoid interrupting it by turning lights on.

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