Addendum to Greenhouse Coverings
September 5, 2020

Ultraviolet light basics

About a billion years ago ancient plant ancestors called algae formed or came to this planet and started to multiply. Their energy making activities created oxygen gas as part of the photosynthetic process. This gas became part of the protective layer called the atmosphere that blocked out most of the life destroying ultraviolet rays hitting the earth. It also allowed the proliferation of lifeforms that depended on oxygen to breathe and survive. The dominance of plants continues to this day. Plants and algae create, maintain, and protect the conditions to make life viable on our planet.

Most of us cannot see the light waves that are not in the visible spectrum. What we can see with our eyes is a tiny portion of what is happening. We see red orange yellow green blue and violet. Plus the mixtures of colors like pink magenta and turquoise. Scientists investigating the properties of light found that there was something happening at the extreme ends of the rainbow of light. Above the purple violet was something else – they called this ultra violet. Ultra meaning more than/extremely/excessive, like Ultra man! Beyond the red part of the rainbow they found another kind of wave – they called this infrared. Infra meaning below.

As the scientists made more observations, they discovered a wider range of waves in this electromagnetic spectrum that is the energy reaching the earth from the sun. They identified waves of energy with super short wavelengths like gamma rays and x rays. Waves that were so short and tiny and full of energy that they could easily penetrate bodies of living organisms. Scientists found waves of energy with long wavelengths, like microwaves and radio waves.

Over time, we have harnessed and made use of the physical properties of these waves: x rays at the doctors to see past the muscles but see the dense bones; microwaves in the kitchen to excite the water molecules and heat up your supper; microwaves for radar to detect motion and measure movement and velocity; radio waves for radios, mobile phones, wireless networks; infrared waves for night vision scopes and heat seeking missiles.

Of the ultraviolet rays of light hitting the earth from the sun, much of it is absorbed by the atmosphere composed of nitrogen and oxygen gas. This is good, because otherwise most of life on land would be dead. The UV waves that do make it through can break chemical bonds and damage cells in humans. This is why we ask you gardeners to wear sun protection. Long sleeved shirts and a nice wide brimmed sun hat, not a baseball cap. Otherwise the tips of the ears and back of the neck are uncovered and often susceptible to skin cancer melanomas. Folks with darker skin can produce more natural sunblock which is melanin, they do not burn so easily. Fairer folks ought to wear sun block and reapply accordingly. Otherwise the sunblock wears off with time, and the sun’s UV rays start baking the skin cell/sunblock residue matrix. But UV light is not all bad. Vitamin D is made when your skin is exposed to the sunlight and that little bit of UV; Vitamin D helps you absorb calcium, magnesium, and phosphate. It also helps your immune system and protects you against disease. So some sun is good for you.

In the garden we also harness UV light for its beneficial qualities. In a pond system we will use a UV light alongside the filter to kill the algal spores in the water. This keeps the pond water nice and clear and not green. UV light is also used to disinfect and treat water at the wastewater treatment plant before discharging it or before it is channeled to the recycled water stream. In entomology class we collected scorpions at night by shining a hand held black light while wandering around the desert. Scorpions fluoresce and glow in response to the UV light, and are easy to catch this way.

So back to coverings. Glass has the excellent light transmission. It absorbs and blocks all of the shorter wavelength UV rays (UV-B) but lets in most of the longer wavelength UV rays (UV-A). The plants in the glass greenhouse get all the light they need to proceed with photosynthesis. Glass, composed of sand, limestone, and sodium carbonate, is great stuff!

In a poly film covering, however, the plastic covering is composed of long chains of carbon and hydrogen; it degrades and breaks down in the presence of ultraviolet light. Before long the plastic is discolored, cracking and falling apart. Therefore, a chemical compound that absorbs the UV radiation is added to the plastic in the process of manufacture to prolong its life. These are similar compounds as found in sunblock and cosmetics. This way, your plastic greenhouse holds up over time, and you can grow happy plants.