We have windows in our house that blocks UV rays. Is that bad for house plants sit near the window?

Not at all. UV rays are not helpful to plants, as far as I know. Greenhouse panels are specifically coated with UV blocking filters so your windows would be just like the effect produced by greenhouses.

dave said:Not at all. UV rays are not helpful to plants, as far as I know. Greenhouse panels are specifically coated with UV blocking filters so your windows would be just like the effect produced by greenhouses.

I had always heard the opposite...
* puts on his homework hat*

I do know that glass "eats" light! As does greenhouse film and polycarbonate panels.
Somewhere is the range if 8%-20% of light will be lost to EACH layer, if your windows are modern double pane, then that's 2 layers...

Also dust and the like on windows will eat a small, but ever increasing amount of light.

For many houseplants, this is not a bad thing, for some, it is!

Light requirements vary species by species and even vary within species themselves. Lumens measure light intensity wereas light spectrum measures wavelength.

There has been a lot of research done that identifies what plants need what sort of light (wavelengths) and intensity to germinate, grow, and flower. Generally speaking plants utilize two sorts of light and for discussion I will call them "red" and "blue". The wavelenghs between the peak red and blue spectrum are more what our eyes see. This "visable" wavelength is something like 430-660 nanometers. At the extremes are UV and infrared, and neither of these are beneficial to plants, and if an overabundance, can cause plant mutation and even death. UV is less than 400 nm and infrared greater than 700 nm. Excess UV mutates cells. Excess infrared burns plants. (For orchid growers and many other tropical plant growers, this infrared is what "sunburns" our plant's leaves).

The so-called "visable" light, the range that our eyes can actually see, are not really beneficial to plant growth/bloom. That means that the vast majority of incandescent lighting won't help you much.

Most plants need a good measure of both red and blue spectrums, and these spectrums are tight. The useful blue is 400-450 and the useful red is 650-700 nm.

For spring and summer bloomers, during the fall our light will go more from the red spectum to the blue spectrum. I'm talking northern hemisphere. As the sun gets lower in our southern sky, the blue spectrum increases. The blue triggers the plants to change "gears" from growing (vegetative) to blooming (budding). For fall/winter bloomers, just the opposite occurs. During the spring/summer, as the sun gets more overhead, there is more of the red spectrum and this triggers the plants to bloom in the fall/winter.

Lumens (light intesity) is a whole other topic.

Thanks, Dr. Dawg! I often wondered about that. And I didn't know that the useful parts of the spectrum were so narrow. Before, all I had seen were absorption spectra, which I thought had a fairly narrow "valley" around green wavelengths, not a big 200 nm gap.

That's good to know.

So it may not matter much to gardeners, but conventional window glass does absorb some, or a lot of, UV.

I forget whether it mostly absorbs short-wave or long-wave UV. We had sterile rooms in school, flooded with UV when not in use. When we worried about that UV passing through glass windows and frying our eyes, we were told "don't worry, glass absorbs UV". (Perhaps he meant that the more dangerous, shorter wavelength UV-B and UV-C are mostly absorbed.)

When optics in scientific instruments have to transmit all the UV, they have to use expensive quartz (fused silica, I think) instead of cheaper silicate glass. Of course, they also have to pump all the air out of the light path, to prevent absorption of "vacuum UV" and "extreme UV".

And yet, some UV, of some wavelengths, must pass through glass, because there are more expensive products available for blocking 98% or 99+% of UV, for example to protect million-dollar artwork from any fading.

So if some unusual plant species DID need UV (for example long-wave, low-energy UV-A), window glass probably would block a lot of it, but not ALL.

Excellent explanation, DrDawg.
I had to learn the light spectrums/wavelengths some years back when I was choosing which kind of grow light system I wanted to buy. (Depending on what you want your plants to do, bloom or grow vegetation only is a factor in which kind of light I needed, and which offer mostly red or mostly blue.) Your explanation is something I'll *star for future reference.

JoycalynAnnSkinner, WELCOME to ATP! Hope to see you around the site.

Shoe

Thanks, Horseshoe - I love that name!

I have posted off and on ATP and DG for several years but my main interest is orchids and my passion orchids. Thus my involvement in lighting and greenhouses.

Kens answer is right on, conventionally thinking, there's some thinking now that usable spectrum being only red/blue may not be the case...

It certainly stands to reason that in the billions of years that plants have relied on the sun, that they may derive something, from the unusable spectrum.
There's some interesting results coming out of the MMJ field that are showing real results from UV lights.

I think of it like nutrients, N-P-K is like red/blue, the bulk of plants needs are meet here, but there's micro and even macro nutrients, on top of that there's been recent discoveries that plants use more elements than we ever thought, on the micro/macro scale.
Who's to say what we will come to learn, know, and accept in the future.

Question every "fact" you doubt is true - continue to challenge "the usual way of doing things". When we quit learning, we quit living.

While on the subject... I have double pane throughout the entire house, cheapo ones the builders used to save a buck.
While the conservatory on the back is name brand 4 seasons windows, double pane, and nice too!
It's very bright out there, much more so than Inside, yet the response from the true sun lovers is clear!
They would all rather be inside, getting shined through the cheapos.

Interesting. It is important to know where the windows face and then what blocks sunlight (trees). I love an easterly exposure best, followed by southerly, then westerly, and (dead-last) northerly. During the late spring and summer months you have to be really careful of the sun coming in from the west. This mid-late afternoon sun will cook plants! There is typically not enough light to grow/flower plants well when the light comes in from the north.

That is interesting Sway. You might want to look up the model of the new windows installed and try to find out what if any coating(s) was applied. Also whether an inert gas was used in the void, even though unlikely.