Light

• The visible light of the solar radiation is a source of energy for plants.
• The light energy thus utilized is trapped in the carbohydrate.
• If the light intensity is diminished, photosynthesis slows down and hence the growth.
• If higher than optimal light intensities are provided, growth again slows down because of the injury to the chloroplasts.
• Green house crops are subjected to light intensities varying from 129.6klux on clear summer days to 3.2 Klux on cloudy winter days.
• Photosynthesis does not increase at light intensities higher than 32.2klux.
• Rose and carnation plants will grow well under summer light intensities.
• In general, for most other crops foliage is deeper green if the greenhouse is shaded to the extent of about 40% from mid spring (May) to mid fall (August and September).
• Visible and white light has wavelength of 400 to 700nm.
• Far red light (700 to 750nm) affects plants, besides causing photosynthesis.
• Infrared rays of longer wavelengths are not involved in the plant process.
• In the blue and red bands, the photosynthesis activity is higher,
• Blue light (shorter wavelength) alone is supplied to plants, the growth is retarded, and the plant becomes hard and dark in color.
• Red light (longer wavelength): growth is soft and internodes are long, resulting in tall plants.
• Visible light of all wavelengths is readily utilized in photosynthesis.

Greenhouse orientation (direction of the ridge)

a. East-West

• More light interception
• More permanent shadows
• More snow blown off roof by wind ( <40 ⁰ N or S)

b. North-South

• Less light interception
• Less permanent shadows
• Better natural ventilation ( < 40 ⁰N or S).

Incidence Angle of Light

• If light strikes roof at 90°, then have maximum light transmission
• If light strikes roof not at 90°, then less light transmitted
• Light transmission is affected by glazing materials and the maintenance of them

a. Glazing Material (% light transmission)

• Glass (low iron) (93%)
• Exolite (double acrylic) (92%)
• Lexan (double polycarbonate) (78%)

b. Cleaning glazing material

• Several times a year (usually rainfall will do this)
• Remove shading compound by mid-October

Remove objects that shade

a. Adequate plant spacing

• Reduces shade avoidance response
• Don’t overdo with the numbers of hanging baskets

b. Objects close to greenhouse(trees, buildings, etc)

• Distance away = 2 x Height of object