How high to hang grow lights

If you are just starting out with growing, it can be difficult to estimate the correct hanging height for grow lights. How high you should hang the fixtures will depend not only on their output, but also on the age of the plants and other factors. In this article, we explain how to determine the hanging height of different types of lights according to the growth stage of the plants.

In indoor growing, light is one of the main factors affecting plant development. While the length of the light period determines whether plants will flower or invest energy in vegetative growth, light intensity is crucial for the amount of energy they obtain through photosynthesis. If plants receive just the right amount of light, they will grow quickly and develop according to their genetic potential. Too intense light will burn the leaves, and if there is too little light, they will have elongated stems and grow slowly.

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The best way to measure the intensity of grow lights is by using photosynthetic photon flux density (PPFD), which expresses the amount of active photons falling on a given surface per second (μmol/m²/s). The older the plants are, the more light they are able to use effectively for photosynthesis.

• PPFD for seedlings: 200 to 400 μmol/m²/s
• PPFD for plants in the vegetative growth stage: 400 to 600 μmol/m²/s
• PPFD for the flowering stage: 600 to 1000 μmol/m²/s
• With CO₂ supplementation: up to 1,000 µmol/s/m² during the growth period and up to 1,500 μmol/m²/s during the flowering period.

Grow LED lights are designed so that at a certain distance from the plants they cover the largest area at the ideal PPFD for growth and flowering. You will usually find this value in the manual. It is not recommended to illuminate a larger area than corresponds to the light’s output. You will end up with weak plants and slow growth. If you buy a light with a dimmer, you can easily change the intensity by turning the dial. If you have a fixture without dimming, hang the light high above the plants during the first days and weeks of the plants’ life and gradually move it closer.

Approximate hanging height of LED modules for different growth stages according to power input (W)

HPS and HID discharge lamps

If you are lighting plants with high-pressure sodium discharge lamps, follow these parameters:

• 250 W HPS will effectively illuminate 0.3 m2 of growing area and you will achieve the optimal PPFD for growth and flowering at a distance of 30 cm from the tops of the plants
• 400 W HPS will effectively illuminate 0.5 - 1 m2 of growing area and you will achieve the optimal PPFD for growth and flowering at a distance of 30-40 cm from the tops of the plants
• 600 W HPS will effectively illuminate 1 – 1.4 m2 of growing area and you will achieve the optimal PPFD for growth and flowering at a distance of 40-50 cm from the tops of the plants
• 1000 W HPS will effectively illuminate 1 – 1.5 m2 of growing area and you will achieve the optimal PPFD for growth and flowering at a distance of 70 cm from the tops of the plants

Although it may sound complicated, after a few growing cycles you will easily recognise whether the plants are receiving just the right amount of light, too little or too much. The first signs of light burn are burnt tips or yellow faded spots on the leaves, usually around the tops closest to the fixture. If you notice such damage on the plants, reduce the light intensity. If you notice that the plants are weak, stretching towards the light or developing leaves without chlorophyll, give them more light. If you want to measure light intensity really accurately, equip yourself with a PAR meter.

You can find more useful tips and guides for indoor and outdoor growers on the Higarden blog.