Growing plants indoors under grow lights is very different to outdoor growing. Pay particular attention to the lamp size and type, plant height and density, and the distance between the lamp and foliage.
Fig 3.4 Light meters are used to check if all foliage is receiving enough light and if lamps have degraded.
Lamp Size & Type
The lamps size (wattage) and type (spectrum) is essential for ensuring plants receive adequate light. When using HID or fluorescent lamps, lux* meters are useful for checking if all foliage is receiving the correct amount of light . Ensure to check both high and low:
• Clones /seedlings: 5,000–7,000 lux. Use fluorescent. • Vegetative growth: 15,000–50,000 lux. Use MH lamp. • Flowering: 45,000–70,000 lux. Use HPS lamp.
*PAR meters are more accurate and can also be used to test LED lights, however they are very expensive.
Can be caused by foliage being too far from the lamp i.e. lamp is too high, or plants are too tall, or area of coverage is inadequate (see below). May also be due to shading from over-planting, inadequate plant shaping/ training, incorrect lamp size/type, or due to a faulty lamp or incompatible ballast. Insufficient light will produce sparse foliage, spindly branches and poor flowering.
Fig 3.3 From Table 3 we can estimate that 2 x 600W HPS lamps would be suitable for lighting an area 2.4m x 1.2m (4 x 8ft).
Too Much Light?
Can be caused by the lamp being too close to foliage or incorrect lamp size. Symptoms will be evident on upper most foliage and may include stunted growth, and folding, bleaching or burning of leaves.
Area of Coverage
Fig 7.2 The heat from HID lamps is a big problem, especially in summer. ‘Ducting’ heat away through ‘air cooled’ lights enables lights to be positioned closer to foliage.
The lamps wattage will largely determine the size of the ‘effective’ growing area – see Fig 3.3 and Table 3, “Area of coverage”. However, to unleash the lamp’s full potential, it is critical to also choose a lamp shade that provides maximum reflection efficiency and focuses the light evenly (without hot spots) and with the correct amount of spread i.e. not too wide or narrow. The use of reflective material on walls and other large surfaces helps ensure that light is not wasted through absorption.
Fig 8.5 Strategic pruning (topping) and low stress training (LST) combined with horizontal netting will create a plant that uses light most efficiently.
Plant Height
Light intensity diminishes rapidly as distance from the lamp increases. Therefore, along with choosing the correct wattage, it is important to ensure the majority of foliage is positioned within lamp’s “sweet-zone”. This is best achieved by training plants into a low, wide and flat canopy (Fig 8.5) – see section on Topping and LST (Fig 8.1).
Lamp Height
To best utilize lamp output, position the lamp as close as possible to the top of plants without causing photo-respiration or burning of foliage (see Table 3 for “minimum gap” guidelines). Air cooled lights should be used because they enable lamps to be positioned closer to the foliage. These are particularly beneficial for 1,000W lamps (Fig 7.2). Ensure lamps are hung so that their height can be easily adjusted as the plants grow.
Plant Density/Shading
Shading becomes an issue when plants are positioned too close to one another. It is generally more productive to plant fewer plants, rather than more.
