Light distance is a critical factor in indoor plant cultivation, and improper light distance can cause physical damage to plants. When a light is too close to a plant, the light intensity and the heat from the light can cause light bleaching or thermal stress. Light bleaching occur when the light is too intense, and thermal stress occurs when the heat from the light is too high.
Many growers notice light bleaching when they see white or bleached tips on the leaves, and many growers notice thermal stress when the leaves begin to curl upward. Conversely, if a person positions a light too far away from a plant, the plant will not receive enough light energy, and the plant will begin to stretch towards the light. This stretching results in long, spindly, and weak stem, and these weak stems can cause the plant to collapse later in the growth cycle.
Right Light Distance for Indoor Plants
A person must adjust the light distance as the plant moves through different stage of growth because plants has different light requirement during different stages of growth. A seedling is a small and fragile plant, so a seedling require a light that is positioned far away from the seedling to avoid damaging the new growth. As a plant moves into the vegetative stage, the plant develops a higher tolerance for light, so you can move the light closer to the plant to provide more energy for the vegetative structure.
When a plant reaches the flowering stage, the plant require maximum light intensity to develop flowers, so you should position the light closer to the plant canopy during the flowering stage. Because the light requirement change, the light distance must also change. Every light fixture has different properties, and different light fixtures require different light distances.
High-pressure sodium (HPS) lights produce a large amount of heat, so you must maintain a larger distance between the HPS light and the plant to prevent heat damage. Moddern LED lights are more efficient than HPS lights, and modern LED lights often allow you to maintain a closer light distance. Furthermore, different types of LED lights have different light distributions, so a wide-spreading LED quantum board requires a different light distance then a highly intense LED COB chip.
You cannot use the same light distance for every light fixture because the physics of how light fixtures disperse energy and heat are different for every light fixture. A person can measure light intensity using a PAR meter to ensure the light distance is correct. Measuring the distance with a ruler is a method to find the light distance, but a ruler does not measure the actual amount of light energy hitting the leaves.
A PAR meter measures photosynthetic photon flux density (PPFD), and measuring PPFD is the only way to know the exact amount of light energy the plant is receiving. If the PPFD is too high at a certain light distance, you must raise the light higher or dim the light. If the PPFD is too low at a certain light distance, you must lower the light or increase the light intensity.
Temperature and ventilation are also important factors when you manage light distance. High-intensity lights generate heat, and high-intensity lights can create hot spots if the ventilation are insufficient. Even if the light distance is correct, stagnant air can cause heat to pool at the plant canopy, and this pooled heat can cause the same damage that a light that is too close would cause.
You must use an exhaust fan and a ventilation system to move the air so that the heat doesnt damage the plant. Successful growers do not set the light distance once and leave it alone, but instead, successful growers makes frequent adjustments to the light distance. You should use adjustable ratchet hangers so that you can make small adjustments to the light distance as the plants grow larger.
You must observe the leaves of the plant to see if the plant is showing signs of stress. If the leaves have long, pale stems, the plant isnt receiving enough light, and you must lower the light. If the leaves have crispy, brown edges, the plant is receiving too much light or too much heat, and you must raise the light.
By observing the leaves and adjusting the light distance, you can maintain the correct environment for the plants.