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Grow Lights Position & Size

Growing plants indoors under grow lights is a very different ballgame to outdoor growing.

Plant shape for indoor grow lightsIndoors, distance from the light source is crucial. Too close and your plants suffer from localised low humidity and heat stress. Too far and the amount of incident energy hitting your plants quickly diminishes to almost nothing.

The most productive indoor growers train and manipulate their plants away from the highly inefficient “Christmas tree” shape and instead aim for low, wide candelabra-like canopies with multiple flowering and fruiting sites at the same vertical height (see article Topping & LST). This way a larger proportion of your plant is basking in that “sweet zone” enjoying optimum light levels within a temperature range that’s ideal for photosynthesis.

air-cooled-lightsPosition the lamp as close as possible to the top of plants without causing photo-respiration or burning of foliage will best utilize lamp output (see Table 1 for “minimum gap” guidelines). Air cooled lights should be used because they enable lamps to be positioned much closer to the foliage. These are particularly beneficial for 1,000W lamps (Fig 3). Ensure lamps are hung so that their height can be easily adjusted as the plants grow.

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.

A Simple Grow Light Heat Test: Hold the back of your hand level with the highest part of your plant directly underneath the grow light. Leave it there for a few minutes. It should not feel overly warm or as if it’s heating up. Alternatively use an infrared thermometer and measure the temperature of the leaf surface directly. If it measures over 28 °C (82 °F) your lights are too close.

Sizing grow lamps

hid-lamp-sizeThe lamps wattage (W) is essential for ensuring plants receive adequate light. The size of the growing area will partly determine the required wattage – see Fig 1 and Table 1, “Area of coverage”.

The height of a plant must also be considered when determining lamp wattage. Light intensity diminishes rapidly as distance from the lamp increases. Table 1 shows the drop in intensity from 1ft to 3ft. As an example, imagine your plants covered an area 0.9 x 0.9 m. You may be tempted to use a 400W HPS instead of a 600W HPS – the 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 for that foliage. This intensity may be insufficient for the plant species being grown. By choosing a 600W HPS, the intensity would be roughly double – assuming the same size gap between the lamp and the foliage.

For more see page 19 of the FloraMax Hydroponics Manual.

© Andrew M Taylor

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