I'm shopping for a greenhouse to protect my tropical fruit trees from winter frosts. While browsing, I noticed that greenhouses are predominantly green or clear. What are the advantages of each color? I don't quite see the advantage of a green greenhouse. Wouldn't any color in the windows block parts of the light spectrum?
Yes, the colour will block some light, but not too much. The only reason they sell these green plastic coverings on a frame is because they're thought to be more aesthetically pleasing than looking at clear plastic, and because the material they're made from is somewhat tougher than clear plastic.
I received an answer from an OGrow representative. OGrow is a manufacturer of greenhouses.
Me:
Can we get an official response on how to choose between the green and clear coverings? I'm mainly going to grow fruit trees and shrubs. Would fruit trees do better with the clear covering because bit provides full sun?
OGrow:
While we cannot determine which is better for the particular kind tree you are planting, we can advise you with some of the pros and cons regarding the Clear/PVC vs. PE greenhouse cover.
PVC: The plastic used on our clear covers generate extra heat and is better for cold climate area. Plants can be seen to add beauty to your garden!
PE: Customers and greenhouse experts prefer PE over PVC claiming that the PVC material contains a toxic load that can be damaging to sensitive plants. The PE material is made out of stronger elements, and protects from sun, for plants that do better with less exposure to the sun.
Good luck!
Wow! I had no idea that the clear covering could be poisonous. I guess that's why plastics smell bad when they're baking out in the sun. What fruit trees would benefit from shade though?
Several of the answers here are incorrect. Green plastic absorbs green light or reflects it which is why it appears green. Green is chosen because if you want to reduce the light intensity to the inside of the greenhouse, you use green because it's not used by the plants to any significant degree. The filtering is not perfect which is why you might still see some green on the other side of the plastic, or it could be that some white light is being reflected off another surface onto the plastic.
I have a little plastic greenhouse but the green is just green stripes running through clear plastic. However, at a distance, it looks green, and the same as in the image posted with the question.
While looking for an answer to this question myself, I came across this article which talks about actual studies that have been done regarding the colour of plastic greenhouse covers. In particular it states:
In studies, it has been found that using green plastic to cover a greenhouse results in plants which are slightly (but very slightly) shorter than plants grown in a greenhouse with a clear plastic covering. There isn’t all that much green in sunlight, it seems, since overall, green plastic lets in almost the same amount of light overall. It’s probably not preferable to clear plastic as a greenhouse covering, but if you find yourself in the unlikely situation of having only green plastic sheeting available for your greenhouse, it will do if need be.
Green filters, such as the green colored plastic, allows more green light than other wavelengths to pass through. For plants, the blue and red part of the spectrum are the most important. Since plant leaves are generally green, they reflect green light. I don't think the green color would be the best choice to grow plants under.
Red would probably be better. Some people like to use red colored plastic to cover the soil before planting tomatoes and claim that it helps improve yield.
Regarding the email you received... Plastic doesn't "generate" heat. It partially blocks light and heat and also helps retain it by not allowing cold air to enter and warmer air to leave.
All (most?) plastics leach some of their chemicals when in contact with water. Polyethelene may leach less or different substances I forget which but high density polyethelene (HDPE) I hear leaches less.
From what I hear those small greenhouses may not have UV stabilized plastic sheeting which means they will degrade faster under the sun's radiation. Maybe only lasting one season.
There are specific plastic films made to be used for outdoor greenhouses. I used one for my raised bed cover for the square foot gardening box I built last year. It's a UV stabilized low density polyethelene. I believe since it breaks down slower it won't leach as much of it's chemicals as fast.
Green filters, such as the green colored plastic, allows more green light than other wavelengths to pass through. For plants, the blue and red part of the spectrum are the most important. Since plant leaves are generally green, they reflect green light. I don't think the green color would be the best choice to grow plants under.
I'm pretty sure a green filter would block the green spectrum allowing the plants to absorb more of the others. The color you are seeing is the range of spectrum that ISN'T being absorbed. So if you think about it..if the material is green..it is reflecting the green spectrum..so less green light.
There are other questions to ask. Where are these Green covers coming from and which market were they designed for? Perhaps for example these green plastic greenhouses were made in China. Now since China is relatively close to Australia and has a strong trading relationship with them perhaps it's possible that they were designed for the Australian market. The sunlight in an Australian summer could be too powerful for many plants to survive. Perhaps this is why the concept of a green greenhouse has come about? Green obviously filters out much of the red and blue needed by plants, (more of the blue spectrum in early development and more of the red spectrum later on during flowering). Or perhaps Green just works out cheaper for the manufacturer. Generally green is more UV resistant so perhaps it is simply for longevity.
You are not likely to find a clear PVC cover (as in the OP second photo) that will survive sunlight for more than one season. The green one (as in the OP first photo) looks like a polyethylene mesh which is often treated to resist UV radiation, which means you should get two or more years out of it.
If treated, the green one will last at least twice as long as the other which should be a major purchasing consideration.
Chinese-made greenhouses use green plastic to communicate to uneducated consumers that they are greenhouses. It is mostly a marketing tool by people who don't know physics. Green filters do not block out green. On the contrary, they partially filter out the other colors, like red, which plants need. This is why plants emit green light, which they can't use. The Chinese suits making these atrocities should read my post and consider using red hues in their plastic coverings.
p.s. A more important question is the IR transmission vs. optical transmission coefficients, which if tweaked carefully can increase the winter air temperature of a greenhouse, another important variable.
Caveat: This answer assumes that the green plastic shown is pigmented green and thus transmits green light into the greenhouse. Certain metal coatings could reflect green light but transmit blue and red wavelengths (which would provide plants with an optimal environment but be rather expensive to engineer). To understand the illogic of green pigmented plastic, take a look at the absorption efficiency of plant pigments 
. The solid line, Clorophyll a, is the most important pigment for most of the terrestrial plants. The two peaks around 430 and 660 nm mean plants most efficiently convert blue and red light into energy for growth and respiration. PAR (photosynthetically active radiation) light meters are calibrated to measure blue and red light. Unfortunately, green-pigmented plastic is green because it absorbs about 50% of the PAR radiation. Green greenhouses may sell because they look nice, and still transmit about half of the PAR radiation, but a 50% shade cloth would produce far more cooling inside the greenhouse without reducing PAR one bit more. I've got a friend who just bought a green plastic greenhouse, and I am going to offer to help him replace it with a proper 6-mil UVA shielded greenhouse film. And see if we can get his tomatoes and peppers growing properly.
Aside from avoiding toxins, I think the primary benefit of using any color is to block light. The best light to block is probably UV rays, since it can stress plants. However, it may also increase nutritional value of the fruits and vegetables. So, it's give and take. Blocking infrared may help keep the greenhouse cooler, too, which would be helpful in the summer, in hot areas.
If you use green plastic, you might allow mostly only green light to go through (it depends on the color, which may merely look green, but actually allows other colors through in high amounts, too), which probably isn't going to do a lot for your plants, as others have said.
I don't recommend another color instead of green. Clear is best, in my opinion. It's more like outdoors and allows more light, which is generally a good thing. The only things I would consider blocking are UV rays and infrared, but you can still have a clear greenhouse with UV and infrared filters.
Most plastic probably blocks UV rays already. So, infrared making your greenhouse hot is probably mostly what you have to worry about.
If you can see color normally inside a green greenhouse, then it's letting other light besides green in, which is a good thing, in my opinion. I'm guessing most of them let lots of all colors, besides UV, in.
Given that the cover is visibly green this means that it is reflecting a large portion of green waves and absorbing (in this case allowing to pass through) the rest of the spectrum. If something looks green then it's not absorbing green so a green cover does not harm your plants and the truly best thing you could use would be grow bulbs which produce red and blue light at the intensity required by your particular plants. I hope this was helpful.
