Can You Over-Lumen a Plant?

Comparing Light Units

• Although they seem to be the same, lumens and footcandles are not the same measurement of light. Lumens measure the output of light from a source, such as a bulb or the sun. Footcandles refer to the amount of light received from the light source. Watts refer to the amount of energy --- namely electricity --- consumed in order for a bulb to produce light. The amount of watts used by a light bulb affects its lumen output, and the distance of a plant from the bulb affects the footcandle rating.

Light Intensity

• Lumens can directly affect how plant leaves grow and undergo photosynthesis. A more intense light source has more potential to harm leaves, or supply more light energy for photosynthesis. Not all plants produce leaves with similar thicknesses or concentrations of pigments to absorb light energy. Light is usually accompanied by heat, and if intense light also increases temperature in leaf tissues, more water is needed to sustain the leaves. Overly intense light will harm leaves.

Plant Thresholds

• Plants native to habitats that bask in sunlight or are from regions where sunlight intensity is stronger do not sustain damage from intense light as quickly as plants native to shady forests or higher latitudes. While a cactus or oak tree will fare just fine in intense, all-day sunshine, a nearby fern, hosta or moss will scald or desiccate from the overly intense light energy. The angle of the sun also affects the health of plant foliage. If plants cannot maintain leaves long enough to conduct photosynthesis, the plant weakens and eventually dies, since it cannot make its own food, even if water and nutrients are available.

Light Emitted and Received

• In the case of sunlight and its relationship to plants, lumen and footcandle measurements suggest completely different implications. The sun's lumen output is constant. However, during the different seasons, the amount of light received from the constant sun varies. The lower-angled light of winter is less intense and damaging to foliage compared to the more direct light that occurs in summer. Plants experience higher footcandles in summer compared to winter. Footcandles refers to the light energy actually received. Likewise, on cloudy days, fewer footcandles occur on plants compared to a clear, sunny sky.