Mirror to reflect light?

[Mudman]

I have a large mirror which would fit great in the back of the cold frame I am building. Does this make any sense?

[dipchip2000]

Mudman

I lined my cold frame with insulation board with the shiny side in. The plants survived two nights down to 16degrees with no damage at all. They grew very well and I think the shiny board helped during daylight hours by reflecting the sunlight. I think a mirror would possibly do the same thing however I would still line the box with insulation of some sort for those extra cold temps.I put my plants in the box at the end of January which is typically the coldest part of the winter here in Oklahoma. The coldframe worked excellent and will use it every year from now on.
ron

[Doug9345]

White paint will also work. I've seedlings in a couple of south south east windows and the curtains have a cream colored backing on them. It makes a huge difference as I keep then closed to keep the cat from scattering the cups.

[Skaggydog]

Big M growers say flat white paint. If mirrors worked better they would use them.

[JamesL]

Mirror is a bad idea. Well, it really is a good idea, but the result won't be. You will run the risk of burning the plants and creating too much heat in your cold frame.
White works much better. If you don't want to use paint, Tyvek building wrap is very effective, providing good reflectivity.

[Mudman]

Originally Posted by JamesL

Mirror is a bad idea. Well, it really is a good idea, but the result won't be. You will run the risk of burning the plants and creating too much heat in your cold frame.
White works much better. If you don't want to use paint, Tyvek building wrap is very effective, providing good reflectivity.

Ok, now I am not a very scientific person, but I wouldn't be adding any light coming into the box, only reflect the stuff coming in, so how could I burn or create too much heat?

I have used the white paint before, but one of the reasons I thought the mirror would be nice is that I am making the front wall high in order to get bigger plants into the box. I am probably creating a dark spot in front and thought this might compensate for it.

[janezee]

I'm very interested in this, mudman.

Recently, I drove by a demolition site, and they had some bathroom cabinet
doors that had been covered in mirror. I brought them home, thinking that I
would mount them on the back wall of my cold house, above the water tank,
to bring some more light into the space, and hopefully help heat it in the cooler
weather. I'd take them down in the summer, or cover them.

I'd go for it, but what do I know?

j

[Doug9345]

The problem with a mirror is that where ever you reflect the sun will be quite hot. The advantage of a white wall is that it will help even the light out from one place to another.

[Mudman]

Originally Posted by janezee

I'd go for it, but what do I know?
j

I'm in the same predicament.

[ScottinAtlanta]

I have heard too that white paint is safer, but I put up sheet of 25 by 4 foot mylar on the fence about 4 feet from the garden - I think it will help a great deal in raising light energy by a few % in my garden. I am not worried about burning the garden as, unlike a mirror, the surface is not smooth (due to imperfections in the fence), and the reflection is more diffuse.

[JamesL]

Mudman,
Possible sunburn, and perhaps to much heat retention in the cold frame.
Think about the difference in the amount of sun you get when you go fishing versus sitting in the yard. The sun is the same, right? Well, why is my face fried? (it has happened to me more than once!)
It's the reflectivity. The sun reflects strongly off the water so you are getting it coming down and also reflected back up.
It might be the case that you won't have a problem using the mirror depending on angle of the sun, time of day it it hits it, etc.
If you had the ability to reflect the weaker morning light only as opposed to the stronger noon to afternoon light you probably wouldn't have an issue.
Or if you could use the mirror to reflect just to the dark area of your cold frame.

Only way to know for sure is to try it!!! Or maybe try to test it (if you do let us know what happens....)
I can't come up with a really good way to do this though. A thermometer would be useful, but not for testing sunburn....

Here is some more info if I haven't bored you yet....
Color and reflection-
Black - Absorbs visible light and holds it long enough to convert it to heat.
White - releases visible light almost as soon as it is absorbed, staying cooler and reflecting more.
Most materials absorb some colors better than others, which is what makes them the colors that we see.
Plants appear green and tomatoes appear red as that is the color that is the most reflected back to us.

Smoothness - A rough surface reflects in all directions. A very smooth surface reflects in a very ordered fashion, sending most light out in a single direction, i.e. a mirror.
Good mirrors are silvered because of color and smoothness.
Silver is probably the most "white" of all metals. It reflects almost every color.

Heat - mirrors and other reflective objects don't actually reflect heat as the suns rays are not themselves "hot". They interact with whatever object they are hitting, causing heat.

Here are some links about sun reflectivity off of energy efficient windows causing serious issues.
http://www.greenbuildingadvisor.com/...flections-caus

http://www.dailytech.com/Green+on+Gr...ticle23866.htm

[Mudman]

Ok James. You definitely are not boring me, I appreciate the comments. Your water reflection argument is enough for me to abandon the idea for the spring, but I think I will definitely add the mirror for late fall into the winter.
Here is another thing that bothers me. White definitely reflect the most light, but that would mean that you are not getting the optimum heat retention that you could get if you painted it black. So there seems to be a trade off. Which is more important, heat or light? Where I am, I might say heat.

[JamesL]

Mike,
I feel like the naysayer and discourager and I don't like that role.
Re-reading this thread, Skaggydog commented that the Big M growers don't use them. I went and read some of the Big M boards and that's right, they don't. The Big M people are always cutting edge, way out in front of everyone else. But they are also growing indoors for the most part and typically more concerned with heat dissipation so they don't show up so much to the infrared helicopter sensors.
Janzee also commented about using a mirror to bring light in to help generate heat in cooler weather.
That might be the key - cooler weather.
It occurs to me that the very reason not to use a mirror most places (too much reflectivity) might be the same reason to use it in a northern climate.
You know I hadn't really thought much about location, but your being up north, the light is going to be weaker until summer gets going, no? Maybe you could build your frame and have the mirror as an insert. Start out the season with it as a test drive, and be able to lift it out as your season gets going if it is causing too much heat retention or other issues.

Don't know if this is a valid idea or not, but it might be worth additional consideration.

[John3]

Mudman I am no expert on the subject but after reading the posts it sounds like to me there is a line between a cold frame and of turning a cold frame into a solar oven or solar cooker (and they do work in the winter cold)

[Keiththibodeaux]

OK, I may be really off here, but didn't I read somewhere about red mulch being effective on tomatoes. What about reflecting the light using red plastic as a filter.