LIGHT HEIGHT

My garden is lit by a 600-watt lamp that is at its top height, 165 cm. Two of the three plants are 110 cm, so the plant tops are only 55 cm from the lamp. I am worried that the tops will burn if they get too close to the lamp.

My garden is lit by a 600-watt lamp that is at its top height, 165 cm. Two of the three plants are 110 cm, so the plant tops are only 55 cm from the lamp. I am worried that the tops will burn if they get too close to the lamp.

What is a safe distance to keep the plants from the light? The lamp I am using gets quite hot.
Dave

When gardens are supplemented with CO₂, plant metabolism, photosynthesis and growth increase as the leaf temperature climbs to 30 degrees. At higher temperatures there is often some stress that results in looser buds. Without supplemental CO₂, plant metabolism and photosynthesis is limited by CO₂ availability so the plants function better at a lower temperature, around 24 degrees.

The amount of heat that a lamp directs towards the canopy varies greatly. Lamps emit much of their heat as infrared light. When this light hits a surface such as a plant leaf, it is absorbed and its energy is converted to heat.
In addition the lamp heats the air around it and this reaches the plant as well. Bleached out leaves, often called light burn, result from too high a temperature, not from too much light. The conditions should more properly be called heat burn.

Air-cooled reflectors capture most of the infrared light and heat the bulb produces, so much less gets to plant leaf. Water-cooled lamps capture almost all of it. Air circulation is another factor, because it removes heated air from the area. For this reason there is no set distance between lamp and the top of the canopy.

The best way to determine the proper distance is to check the leaf surface temperature with a surface temperature thermometer. This inexpensive device uses an infrared beam for measurement. Leaf temperature has a close correlation with air circulation and air temperature since the leaf, being very thin, radiates its heat to the surrounding air. Using the surface thermometer you can determine the right lamp height and canopy air temperature so that proper leaf temperature is maintained. For instance, if the leaf temperature is too high and the space has good air circulation, you would lower the ambient temperature to cool the leaf.

You mentioned that your plants are over a meter tall. If the plants' canopy cover 2/3 of the garden area, it is time to force them to flower. If much of the canopy is bare, cut the plants back a bit. This encourages them to branch out and spread over the garden while eliminating the height problem.