It is very close. You can even put curved foil behind the tree to magnify the sun to get 2x power or so. Hanging a white sheet behind the tree makes the entire north side of the plant bright. I have found this works best.

A word of warning, mirrors around plants break often. foiles around plants are hard to work with and plants get broken. Sheats often fall and ensnare plants. Go ahead and give it a try

Thanks. Do you use aluminum foil or a linen sheet? I was going to look for an outdoor mirror.

I use a white window shade and I pull it down and out enough to not touch the tree. My trees are very small as they are all from seed. A sheet would work better for larger trees. I remove the sheet at night so that the warmth of the room reaches the tree. I tried a mirror but found that the reflected light has a very small focus on the tree.

I have a long mirror I use to provide light to the north side of my cherry tree when the sun travels to low and my house shades the south side. That worked great for an extra month of sun, but i had to constantly move the mirror Go with cloth over foil and forget the mirror. They often break when you water the plants.

But be aware that on cloudy days the mirror can make things worse! In cloudy conditions diffuse light from the sky comes from all directions. So your tree, already in a dim area due to the building, is now even darker because the mirror blocks light from that side.
I have a greenhouse with lots of potted citrus inside on the north side. I thought about covering the glass on that side with reflective foil. But on sunny days, the foil would be in the shade of the trees and reflect little light. On cloudy days it would block diffuse light entering from the north. Probably not worth doing!

citrange I have been debating that with my west and north wall of my greenhouse. I have trees to the west and my house is the north wall. I use bright white paint on my house. This really brightens things up and helps out greatly. The west wall is forest background and a bright white insulated hard wall will work better in my case, sun or no sun.

Jack_Tripper I should of read your question a little more carefully. Because I am a northerner I assumed your tree was in a window and the north diffused light would be nil and thus a white sheet would work best.

Being the tree is outdoors a mirror would work best. Buy and indoor mirror and paint the back side with an exterior paint to make it an out door mirror and set it up when Its
sunny.

Very inventive everyone, and yet this idea is not new...

Did you know that Rjukan in Norway where my friend lives actually use this concept?

Because this town is at the base of surrounding mountains, they get no sun at all, all winter...So they installed these ginormic mirrors on the mountain tops to reflect sunlight down to the town thus giving them sunlight.

Imagine how much you would appreciate this living there trying to grow a citrus tree? Take a peek..YOu'll love this article.

Good thinking ....

Mike

Here is a link that might be useful: Huge Mirrors for sunlight...