Quality of sunlight through glass?

cynna_leaf(8b)July 2, 2010

I've got a pretty big vegetable garden on my west-facing condo balcony, and July/August are generally 30+ degrees (86+ in Fahrenheit) - prime growing time.

We've just been notified that they'll be doing extensive work on the building, and we'll have to have everything off our balcony for an unknown amount of time, which by the sound of it could be up to 4 weeks, starting soon.

My question is this: is the quality of sunlight *through* glass anywhere near what direct sunlight's is? Obviously plants in greenhouses do pretty well, but I know my plants do way better outside in the sun than in the window, unless that's just my perception.

What I'm growing are: peppers, eggplants, potatoes, carrots, basil, parsley, oregano, beans, and chives. I also have peas and lettuce out there which were to be screened to not have super-direct sun when it gets real hot.

My plan is to set up shelving inside the window so the plants will have residual sun from morning, and direct sunshine from about 2pm on. They will get strong heat in the window (curtain will be 'inside' the room around them), but we'll have the a/c inside so we can control the amount of heat. I don't yet know if there will be any of that green 'netting' covering the side of the building, but there's none set up so fingers crossed.

Thoughts on if this is worth trying? Or will the plants not get what they'll need? I feel crushed right now after all the hard work that's gone into growing all these guys from seed for the last 4 months, and am hoping to not have to give them all away. :(

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My thought is it would be a lot better if you have a friend with yard space that would let you use it for the time needed. Al

    Bookmark   July 3, 2010 at 8:49AM
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col_sprg_maters(Front Range 5)

I agree with Calistoga. If it was only 5 or so days, you could find a way in your home, but not for a month.

Also ask the HOA/landlord for suggestions. It may be a "dry hole", but you never know.


    Bookmark   July 3, 2010 at 11:26AM
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tapla (mid-MI z5b-6a)

I own a glazing contracting business (a glass company), so if you can give me the specifics of the type of glass (tinted, low emissivity coating, insulated, heat mirror?) I can give you some specifics, but the main issue lies in the fact that you have only window openings for light to pass through. Outdoors, you have much more light hitting the plant from reflective sources. Another issue is heat build-up. Indoors in direct light, the boundary layer surrounding leaves tends to trap heat in the leaf. You need air movement to disturb the boundary layer and help disperse heat, or foliage on many plants in direct sun, even full-sun veggies, can burn foliage. Air conditioning helps only a little because it doesn't provide enough air movement to disturb the boundary layer sufficiently, and the temperature increase on leaf surfaces isn't affected much by ambient temperatures as it's closely related to relative (solar) gain as light is turned to heat energy at the leaf surface INSIDE the boundary layer. Short version -> get fans.

BTW -I agree with Al, too. I only offered my comments in case you don't have that option.


    Bookmark   July 3, 2010 at 12:26PM
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Thanks for the responses. Unfortunately I don't have the option of moving everything to a yard. A girlfriend lives nearby and has a small balcony, I could possibly drop 1-3 plants there if needed but she's got a million plants already. The size of most of my containers also makes that fairly prohibative though.

I do have a great rotating tower fan I've planned on using to circulate air around them, so that's covered (but thanks for explaining why it's important).

As for the glass, this is a 30 year old building, they're double glazed and seem 'average'. Here's the info printed on the corners of the patio door glass:
Guardian - Tillsonburg, Ont
Ansi Z974-1975
Safety Tempered
16 CFR 1201 11
5.0 mm U 3/16
CGSB 12-GP-1 (These numbers will mean more to you, I think)

The west/balcony windows are 8' high and 14' wide, the north windows next to it are 5' high and 16' wide, so there's definitely a lot of light that comes into the room. I'd planned to keep the curtains on the west somewhat closed behind the plants, mostly because it does get crazy hot in here in the afternoon. 17 stories up, nothing to shadow us. I'll be getting creative with making sure the plants have enough sun while not melting my furniture!

I know my seedlings do great in the windows, and due to a cold June ('Junuary' this year) the eggplants haven't even gone outside yet and they're growing great in the window area, same with the basil that's flourishing, though I have plonked them outside a few times in the sunshine.

Al, would putting something bright white behind the guys help for reflecting light? I'll also be turning them often so they 'tan' evenly. It's light here until about 10pm, then dusky until 11ish, so they will be getting a good 8 hours of direct sun, hopefully you can advise on the quality of that through this glass; I might be able to pop them outside a few hours some days too, depending on what goes on outside, really we have no idea.

My hope is the balcony only has to stay cleared for a week, but since this is a building envelope issue if there's a problem on a balcony it'll definitely be 4 weeks. Hopefully mine is less, but even then we've not been given a timeline for what they have to do. Workmen will be plonking onto my balcony willy-nilly, we'll be fined if there's anything there while they do this - nothing like keeping your giant barbecue in your living room for a month. :( Whole thing is frustrating, but I mostly want my plants to be happy.

    Bookmark   July 3, 2010 at 4:22PM
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tapla (mid-MI z5b-6a)

The label doesn't say anything about the glass, other than it's tempered and meets the ANSI 1975 safety glazing code requirements. Of the visible portion of the spectrum that strikes the glass, just under 70% of it will pass. When you factor in the fact that the highest % of your room is walls, your plants will probably only be getting about 30-40% of the light they would have received in outdoor sun.

A white background would be helpful in reflecting a portion of light back toward the plants that would normally have been absorbed on other surfaces as passive solar (heat) gain. I think you'll be ok if it's only a week or two, but you might see some shedding of foliage on some plants (normal response to a significant decrease in photo-exposure). Sorry you're having to deal with such an inconvenience - we're all pulling for you, CL


    Bookmark   July 3, 2010 at 6:15PM
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