People used to think that plants were vegetables; that they have no way of reacting quickly to their environment and were more like couch potatoes with very slow reactions. There were always signs in clear sight that this isn’t necessarily so. Sunflowers move so they always face the sun. The Venus flytrap closes on its victims the second its prey touches it. A fraction of a second after lights are turned on, plants start photosynthesizing. Mimosa pudica, also called “the sensitive plant,” quickly collapses its leaves upon being touched. However, when it is touched repeatedly by the same stimulus, it becomes habituated and stops reacting. After not stimulating the plant for weeks, it still “remembered” the stimulus and didn’t react. It had “learned.”
Rhodopsin is a pigment that is very sensitive to light. A version of Rhodopsin is found in bacteria, and it is used in our sophisticated sight system. It’s also found in plants and helps cannabis regulate its flowering by distinguishing light from dark periods.
Plants also share stress responses with animals regarding UV light. In animals, dark skin has high melanin content to protect against UV light. Light skin develops more intense stress reactions
and respond to the light by producing melanin, causing tanning, or more severely, sunburn, which actually results in destruction of layers of skin and other destructive reactions.
Plants growing under natural sunlight develop resistance to these harmful UV rays in several ways. They grow longer protective cells (palisade cells) to disperse the light to minimize its intensity, and they produce higher levels of pigments, flavonoids and terpenes as sun shields. In various experiments and anecdotal reports THC production increased by 10 percent. Terpene levels also increase significantly. There are positive effects in other plants, too. For instance, tomatoes grow thicker skins and contain more flavonoids.
When plants grown indoors are placed outdoors in late spring or summer, they sometimes get sunburned. Their leaves droop or dry out, and they suffer tissue damage. Whether or not they survive and thrive, they are set back. For this reason plants should be gently, gradually introduced to direct sun, perhaps first placed in the shade or by using shade cloth to protect against the sun’s intensity.
You might think that plants in greenhouses are getting full sun. However, most plastics and glass are opaque to UV light. One exception is acrylic sheet, often known by its brand name, “PLEXIGLAS.”
Indoors, fluorescents and HPS lamps produce no UV light. Metal halide lamps often produce small but significant amounts of UV, but the plate glass required for safely enclosing the lamp in the reflector is opaque to it.
Some LED manufacturers include the spectrum in their mixes, but emitters in these spectrums are still costly.
The lamps need only be used during the last 10 to15 days of flowering, for six hours a day. For instance, plants growing outdoors receive the highest amount of UV light in the summer when they are in the vegetative stage. I haven’t seen the results of any experimentation on this. This is an area where there is a lot of room for experimentation.
Photo 1: This room is illuminated using tanning lamps. Usually they are used five hours a day in conjunction with HPS lamps. In this photo the HPS lamps have been turned off for illustrative
Photo 2: Reptile lights can be used to supply UV light.
Photo 3: A tanning lamp with reflector.
Photo 4: A view of the room showing the six-foot tanning lamps.
Photo 5: Garden of the month – Garden of the Month® Coral Cove Greenhouse, Jamaica.