Cover a lens with blue cellophane or a blue filter, and you turn the light blue. This can create a dramatic effect in a theatre and an equally dramatic effect on plants. The colour of the light that reaches a growing plant determines how it will grow. Shine green light on a plant, and nothing happens because the plant reflects green light, which is why it appears green to our eyes. But subject a plant to red or blue light, and it will have entirely different reactions.
Sunlight is full spectrum and contains all the colours so that plants receive balanced stimulation to grow, flower and produce fruit. Plants use sunlight for photosynthesis, converting carbon dioxide into sugars and oxygen. The green chlorophyll in plants allows them to absorb red spectrum and blue spectrum light to fuel photosynthesis. But red and blue spectrum light triggers specific growth activities in plants as well.
Blue light activates the production of chlorophyll in leaves and stems and causes the plant to leaf out and grow a thicker, stockier stem. Isolating a plant under a blue light source would keep all the plant's energy available for this growth because none of it would be diverted to producing flowers and fruit. Blue cellophane filters out the rest of the spectrum and admits only blue light. So, growing a plant under blue light alters how it develops. You end up with a greener, leafier and possibly shorter plant than one exposed to full spectrum light like sunlight. Blue light won't limit a plant's height, but it promotes growth habits that may result in more volume and less height.
In a greenhouse or a seedling bed, fluorescent light is used to give seedlings a vigorous start. Fluorescent light is mostly blue spectrum light that encourages leafy growth. Add red to that light to balance it and the plant will begin to flower and eventually produce seeds or fruit. But filter out the blue light and provide only red spectrum light and a plant will go leggy and spindly. Red light controls maturation in plants and when it is not balanced by blue light, the plant will race to mature but won't develop healthy leaves and stem. You could get a taller plant than normal at the expense of overall vigour. If the plant is spindly enough, it may not even be able to support itself.
It's possible to manipulate plant growth with coloured gels or coloured cellophane. You could use blue cellophane to encourage seedlings to greater vigour and more leaves early in the growth cycle. Then remove the blue filter to add red light to the mix at the point in the growth process when you want it to flower. Incandescent lamps are orange-red spectrum lights but they put out so much heat that they are not a good source of red light for plants. In the absence of sunlight, full-spectrum grow lights can be used to complete the plant cycle but exposing the plants to sunlight will give you the best results, sturdy green plants with healthy flowering and fruiting.
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- Colorado State University Extension; Plant Growth Factors: Light; David Whiting
- University of Arizona Cooperative Extension: Environmental Factors That Affect Plant Growth
- Kids Gardening; Luminous Lessons: Investigating Light; Eve Pranis
- Government of Alberta: Agriculture and Rural Development: House Plant: Artificial Light