Light can be measured as irradiance or photosynthetic photon flux. Irradiance is a relative measure of radiant energy per unit area and is useful for estimating the energy absorbed by a crop. It is used in greenhouse models to control vents or schedule irrigation.

Photosynthetic photon flux is the photosynthetically active radiation measured in the wavelengths most important for photosynthesis - 400 to 700 nanometers.

Light source Photosynthetic Photon Flux
umol × m-2 × sec-1
watts × m-2

Solar radiation

Full sunlight 2000 450
Heavy overcast 60 15

Artificial light

Metal halide lamps 400 watts at 2 meters 19 4
Fluorescent lamps 40 watts at 0.5 meters


Radiant energy from normal solar radiation can be high for many propagation activities because of the heat associated with sunlight.

It is common to shade emerging seedlings, unrooted cuttings, and initial grafts to reduce the heat load.

However, once these propagules are established photosynthetically active radiation becomes important for growth.

Photo of a covered greenhouse where overhead sunlight is reduced to protect plants.

In cases where solar radiation is inadequate for optimal growth, supplemental lighting from high intensity discharge lamps are commonly used.

Photo of circular high intensity discharge lamps in a greenhouse.

Photo of rectangular high intensity discharge lamps in a greenhouse.