Two months ago, Erica posted comparison photos showing seedlings started in a south-facing window vs seedlings started under full-spectrum grow lights.
The results were surprising to many readers (including me!) and at least a few readers invested in grow lights after seeing the difference light makes.
There are a few reasons why windowsill started seedlings can rarely compete with those grown under good quality light, and it’s worth understanding the science of why.
Even the sunniest window doesn’t provide the hours of sun that we think it does. A seedling sitting a foot back from a three foot wide, south-facing window receives direct illumination over (at best) a 110 degree arc of the sun’s passage – or about 7 1/3 hours. The same seedling under artificial light can enjoy 12-16 hours of uninterrupted illumination.
Quite a difference, but the duration of illumination isn’t the only thing to think about.
Let’s Get Sciencey About Light
Humans see these different wavelengths as colors: longer waves as red, on through orange, yellow, green, blue and violet with the shortest of visible wavelength.
Longward of the visible color spectrum is the infrared, which transmits heat and is the source of those nifty thermal imaging videos shot from helicopters. Shortward is the ultraviolet which fades your furniture and gives you suntans and skin cancer.
(Incidentally, if you have trouble remembering the color spectrum, I’d recommend this catchy tune from They Might Be Giants off the excellent Here Comes Science children’s album. Gather the kids around and watch together.)
Plants require light that falls in a wavelength range called the Photosynthetically Active Radiation or PAR. This stretches from about 400 nanometers to 700 nanometers. The edges of this range – 400 to 450 nanometers and 680 to 700 nanometers are particularly important.
Note the double peaks of the chart below showing where a plant’s photosynthesis is most active across the spectral range.
You do not see the world in the same way a plant does
Superficially, the photosynthetically active radiation range lines up pretty well with the human visual range of, typically, 390-750 nanometers, and our region of peak sensitivity of around 550 nanometers (greenish). Incidentally, if you want to really have fun with this, consider one primary role of vision in a foraging proto-human: identifying yummy plants.
Our eyes have their peak sensitivity in the green range, precisely the range reflected by photosynthetic organisms. This gives the human eye its greatest light sensitivity just at the color where plants are most visible. Fans of evolution can postulate how natural selection would favor the development of peak vision in this range. Fans of intelligent design can marvel at the intentionally interconnected nature of everything.
Our eyes and the needs of our green-leaved friends are not as compatible as it might seem – and us humans love to tweak our environment. We like to stay cool, we like our sofas and carpets to last a long time, and we like pleasing colors that don’t force us to go around squinting.
To accomplish this, we deliberately select what light will reach the insides of our homes by applying various coatings to our windows. Modern windows are designed to reflect infrared radiation to keep things cool, reflect ultraviolet radiation to stop fabrics, books, art, and paint from fading, and present a pretty and not-too-bright view of outside. To do this they filter (absorb or reflect) some of the incoming radiation.
A simple single-pane window, such as you might find in an older home or a greenhouse, will transmit 80-85% of the light in the region that a plant cares about. A blinged-up, coated, multi-pane window might transmit as little as 25-50% of the light in the key 680-700nm region.
Various coating materials are applied to, selectively, block out some of the UV and blue light – and while the aim here is really at stopping the 300-380 nanometer range most treatments seem to extend into the 400’s. From a consumer standpoint that’s just fine, since we tend to see blue light as harsh and unpleasant but to a plant with high-light needs blocking light in that spectrum is akin to a semi-starvation diet.
Meanwhile, the desire to keep things comfortable inside leads to treatments that block longer wavelength radiation including, of course, that critical 680-700 nanometer region.
And that’s a big part of understanding what is going on with those sad window-grown seedlings. Not only are they suffering from a lack of time under illumination, but – as perceived by their plant-ish standards – it’s also poor quality illumination,
So don’t go and build a greenhouse out of spare double-pane household windows, okay? Instead, go to that salvage yard and pull some leaky single-panes. How about some of those great 1950’s picture windows…perfect!
What about other materials?
The polycarbonate materials used in most greenhouses (including ours) are pretty good at transmitting the PAR region – even though they sometimes have a “milky” look to our eyes, the right light is still making it through. They are about 80-90% transmissive all through the visual and PAR range.
The plastic sheeting that we use for a lot of our winter time cloching transmits about 70% of light – and though I haven’t found precise spectral transmission curves for this stuff, I found a lot of discussions on boards dedicated to growing…ahem…cannabis. Most plastics are pretty evenly transmissive in the visual and photosynthetic wavelengths. It is only when they are treated or coated that stuff starts to get blocked out.
I dug into finding the spectral transmission properties of Reemay – and unfortunately got little beyond bland statements that it transmits 75-85% of light. Nothing on what spectral content gets transmitted, unfortunately, except for some research done that indicates it is about 20% transmissive shortward of 380 nanometers.
But practical experience tells me that Reemay is just fine as a cover material. It doesn’t seem to hamper the growth of plants unduly, probably because it is mostly open space. Plenty of light (of all spectral domains) can flow through the interstices between the little teeny strands of polyester. It shades a bit, but primarily not by filtering light but by blocking it.
There’s actually a lot more to this topic – plants live in a world of light and derive a lot of seasonal queues from the spectral content of the light, the length of daylight vs. night, etc. That’s why some tricks (like red plastic mulch) can help induce behavioral changes (like fruiting and fruit ripening) in some plants.3