Why do greenhouse use diffused light?

meyermike_1micha(5)April 21, 2008

I was wondering..Is it better for a sun loving plant to get bright diffused light, such as through a window with frosted glass, or plastic to block out full sun? Why don't greenhouses use clear window for the sun to shine through? Am I suppose to grow my sun loving plants the same way? Right now I have many plants on my window sill, sitting in the windows with plastic sheets that cut the full sun down from hitting my plants. They are sun loving flowering plants. Can I uncover my windows and just have the sun shine on them? I am afraid to do so. I have never seen a greenhouse with just clear glass as a roof. They usually have panels that only let bright diffused light in...

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saucer(10 SF Bay Area)

I think the reason that they use frosted glass or shade cloth overhead is because mid-day direct sun can burn the plants. Most tropicals, even those that like bright light like shading from hot mid-day sun. Also, I think it keeps the greenhouse from getting too hot.

    Bookmark   April 21, 2008 at 7:08PM
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bihai(zone 9)

Its a common misconception that clear glazing materials used in greenhouse growing allow 100% light transmission. That's not true. Even clear glass does not emit 100% of the photonic capacity of the sun's rays. Different thicknesses of clear glass let differing amounts of light through. The thinner theglass, the more gets through. But its all still "filtered".

Polycarbonate, Lexan and Fiberglass all let less percentage of light through than glass. Polycarbonate can vary, from single wall (which IS basically Lexan) to double or even triple thickness. These materials are opaque.

Shade cloth is used in greenhouses for 2 reasons: to help regulate temperature by bouncing some of the suns rays back off of the greenhouse, and to provide shade when there isn't anything else providing it (like a tree canopy). While the clear glazing might not necessarily concentrate the light it transmits, it does concentrate the heat.

Its always much harder to control the temperature of a greenhouse in summer (ie COOL it) than it is to heat it in winter. So lots of folks rely on shade cloth to help with that.

Unless your glazing is 100% clear glass, it will eventually over time start to turn opaque anyway. Even if its treated with a UV Protectant, that protectant degrades over time and you end up with opaque glazing.

    Bookmark   April 21, 2008 at 8:10PM
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tapla (mid-MI z5b-6a)

Formerly, the most common types of glazing in greenhouse structures were either single strength (3/32") or double strength (1/8") window glass. Clear single strength offers an average of 91% visible light (that portion of the spectrum that is important to plants) transmission while double strength passes 90% of visible light. From this we see that thickness of the most popular types of glass used in greenhouses has little effect on light transmission. These products have fallen from favor, mainly because of insurability and safety issues. As a glazing contractor, I work with figures like this daily.

Polycarbonate and 'Lexan" are the same product. Lexan is simply GE's 'brand' of polycarbonate, just as Kleenex is a brand of tissue paper. Initially, polycarbonate and plexiglass both offer superior light transmission in comparison to the same thickness of glass, but eventual hazing (even with UV inhibitors onboard) due to UV effect, and soil (dust) accumulations due to an inherent static charge soon render them less effective than glass at allowing light to pass. None of the materials are opaque, as they would allow no light to pass if that were so.

There is a negligible difference in the amount of light passed through obscure (textured to diffuse light) glass as compared to clear. Though the light may appear softer, it is indeed as bright as that which passes through clear panels (within a % or 2).

The primary issue with placing indoor plants in full sun isn't sun load, but heat build-up. Passive solar gain w/o air movement can raise tissue temperatures to killing highs. The insulating glass in most home windows allows a maximum of around 71% light transmission, which is not too far off from 'dappled shade' outdoors, so you can see that it's heat build-up w/o air movement that takes the toll on your indoor plants.


    Bookmark   April 21, 2008 at 9:10PM
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bihai(zone 9)

well said. In Florida, almost no one uses glass due to the hurricane building codes. WHen we were considering attaching a greenhouse structure to our home that would have encompassed the swimming pool, we were of course concerned with the aesthetic appearance (ie we didn't want a utilitarian corrugated Lexan addition, LOL) so we explored with a few different contractors the possibility of using glass. For us in the Northern portion of the state, the hurricane danger used to be considered not as bad as that in the lower part of the state, but now the building codes have moved to become more standardized over the entire state and any new structure must have a wind resistance rating of at least 120 mph. Also, since it would have been attached to the home and used as a "habitation area" for people, specifications called for special tempered glass for safety codes.

It would have been so expensive we just couldn't do it.

I have single thickness corrugated Lexan on my greenhouse. When I built it, I was offered a choice of clear or opaque glazing, or a mixture of the two. It DOES come in OPAQUE. The opaque has the appearance of a frosted light bulb. It offers about a 60-65% light transmission. The company who built my greenhouse for me recommended the opaque to be used on my roof, because I have a 20 foot ceiling and it would be difficult for me to utilize shade cloth up there on a 1750 sq ft structure if I found that the clear offered too much light. They also recommended using the opaque on the Eastern exposure. They recommended it due to the intense sun and heat here in Florida in summer.

I still have the sample of it, if you have never seen the opaque Lexan up there in MI I can snap a photo of it for you. Maybe its a Florida thing?

But I didn't like the look of it so I chose the clear glazing for the entire greenhouse. I am glad I did because after 5 years I do have some hazing, and dirt buildup on the roof from leaf drop etc. I use a pressure cleaner to keep the walls clean (twice a year)

    Bookmark   April 22, 2008 at 7:44AM
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So, simply put, taking my plants out from the back of the plastic used for insulating windows that you can barely see through, will not harm my plants by getting the sun through clear glass?
It is heat and lack of airmovement around it that is to be more of a concern?I thought that the sun through clear glass alone would burn leaves or flower buds. I guess not to be concerned then and stick them in all the sun they like. Even outdoor sun once they are acclimated to it.

    Bookmark   April 22, 2008 at 12:38PM
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tapla (mid-MI z5b-6a)

I didn't say that. Chlorophyll is nature's sunscreen for plants, and some plants are genetically incapable of producing enough chlorophyll to prevent photo-oxidation (sunburn). These would be your direct sun-intolerant or shade-preferring plants. Just because a plant is tolerant of or prefers direct sun, is no reason to believe it will withstand the intense heat often associated with passive solar gain.

The blanket of unstirred air surrounding leaf surfaces is called the boundary layer. It helps insulate the leaf against water loss because it becomes nearly completely saturated with water vapor. The thickness of the boundary layer might only be a few thousandths of an inch, but depends on the degree of air movement, which blows away the boundary layer. Heat generated when photons are absorbed into the plant heats the leaf and this layer of air. If there is no air movement, a thicker, hotter, and more insulative layer results. Wind blows away this heated boundary layer and allows leaf tissues to give up heat to the cooler air, adding to this is the cooling effect of increased transpirational water loss.

Leaf surfaces will often be found to be cooler in full outdoor sun than in front of windows that only allow 70% light transmission because of the difference in or lack of air flow over leaf surfaces.


    Bookmark   April 22, 2008 at 3:24PM
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ok...To be more specific.Would a Gardenia Plant prefer direct sun outdoors or in a clear window facing south, or behind a sheild of plastic that would make diffused light blocking the direct sun? Or does it make a difference. Every grenhouse plant I see looks so healthy, and never in straight sun.
That is how they are seen growing in greenhouses..right? Or is it that I am wrong.
Are they growing them in diffused sun for better health, longer flowering, and fear of burnt plants, will mine be ok? Or will I pay a price sticking my sun loving plants in direct sun? Do I acclimate them from the diffuse sun they are use to growing in before I put them in full sun in my windows....? Or do you think they were use to outdoor sun, then shipped to a greenhouse,under inderect sun? I hope this is a simple question.
Are they, along with plants such as Plumerias better grown behind a plastic to diffuse direct sun, or in straight sun?

    Bookmark   April 22, 2008 at 4:28PM
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saucer(10 SF Bay Area)

To make a long story short, only full sun tolerant plants should be in full sun. "Bright filtered light" plants should be in an east or a little bit back from a south or west facing window. Even an hour or two of direct sun won't hurt, you just don't want to keep them in full sun most of the day or they will burn. Hope that helps Mike.

    Bookmark   April 23, 2008 at 5:59PM
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Hey..Thanks....Considering I havn't got much sleep for a few days, worrying about sick parents and lots of stress, I couldn't comprehend what everyone else said. The anwwers were not clear to me because of a tired brain..;-( I guess I should of just asked, will my sun loving plants grow better behind plastic in my sunny in my windows such as Gardenias,citrus and so on,the sun loving ones, as copmared to direct sun?
Thank you to all and a good night!!:-)

    Bookmark   April 23, 2008 at 6:06PM
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