How high to hang grow lights

If you are just starting with cultivation, estimating the correct height to hang grow lights can be tricky. How high to hang fixtures will depend not only on their power but also on the age of the plants and other factors. In this article we will explain how to determine the hanging height of different types of lights according to the plants' growth stage.

In indoor growing, light is one of the main factors that influence plant development. While day length determines whether plants will flower or invest energy into vegetative growth, light intensity is crucial for the amount of energy they obtain through photosynthesis. If plants receive 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 light is insufficient, stems will stretch and growth will be slow.

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

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

Grow LED lights are designed so that at a certain distance from the plants they cover the largest area at an 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 the light’s capacity. You will end up with weak plants and slow growth. If you buy a light with a dimmer, you can simply change the intensity by turning the knob. If you have a fixture without a dimming function, hang the light high above the plants during the first days and weeks of their life and gradually lower it.

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

HPS and HID lamps

If you illuminate plants with high-pressure sodium lamps, follow these parameters:

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

Although it may sound complicated, after a few growth cycles you will easily recognize whether plants are receiving too little, just the right amount, or too much light. The first signs of light burn are burned tips or yellow faded spots on the leaves, usually near the tops closest to the fixture. If you notice such damage on the plants, reduce the light intensity. If you notice that plants are weak, stretching toward the light, or have leaves lacking chlorophyll, increase the light. If you want to measure light intensity precisely, equip yourself with a PAR meter.

Further useful tips and guides for indoor and outdoor growers can be found on the Higarden blog.