Hello,

I have a potted dwarf tango mandarin tree in my room by a south-facing window (right now it barely gets any light from the window). I am using the SANSI 36 W Full Spectrum LED Grow Light and the tree is 36 inches tall. Currently, I am hanging my grow light about 17 inches above the tree. I built an adjustable light fixture, so I can raise it or lower it.

My question: Is this a safe distance or do you recommend it closer or higher, especially since it does not get any light from the south-facing window?

Thank you!

Window light would be negligible at best. In general, a 36 watt (true LED consumption) is appropriate for one square foot of growing area at about two feet. Obviously that can't happen for any but one level of the foliage. Bu inverse square law, if the light is two feet above the top foliage, the foliage two feet lower will bet on one-quarter of the light. Even much less than that, since the upper foliage will shade it out. In your first picture, you can see the great difference in light intensity.

We deal with this sort of thing all the time in hydroponic where so often, the only light is artificial. In large growing environments, higher power lights are mounted much higher, so that the percentage difference from top to bottom of the plants is of no concern. But it is a common problem in smaller environment, like grow tents. Lower foliage can suffer greatly.

You can't make up for it by mounting the light close to the top. That can damage the top foliage. If I had the problem, I think I would move the tree to the side or away from the wall, so that I could direct the light from the side so that most of the length of the tree was always within the range of 1.5 to 3 feet from the light, and I would rotate the tree frequently to give all of it a shot at light. If that light arm has a lot of extension left, it would be nice if the bulb could be moved away from the wall to look back at the tree from about a foot lower. It would still need to be rotated.

But you get the idea, and you may be able to work out an even better way to light most of the tree evenly and with enough light.

And I don't know if you know this, so I'll say it anyway, since I don't see any obvious timer. Artificial light for plants should follow their preferred natural day/night cycle. Plants use the light in building energy supply during the day and use the energy during dark hours. They need both.

Citrus needs a minimum of six hours of sunlight or equivalent. Ten hours is probably a good average. The hours of light affect the character of the tree. At six hours of direct sun, they tend to be shorted and bushier. With eight to ten hours, they grow taller and broader. A simple timer is all you need.

The tree will let you know when you get it right. If it doesn't have enough light, new leaves will be small, because it doesn't have the energy to make them large. And it may get unnaturally taller and leggy as it reaches for the weak light. Rotating it will also help keep it symmetrical by not having it favor one light direction. There are nice plant caddies with wheels to make it easy.

Thank you for the response. I can move the light about a foot further away from the wall. From my understanding, it is more important to light the entire plant than to have the light above the plant. So my plan is to move the light about a foot lower and a foot away from the wall. I have the pot on wheels so I can spin it. Should I rotate it like 33% every day or throughout the day... that might be a dumb question but I want the plant to be happy

Thank you again for the response! I'll post a picture of my updated set up once I adjust it!

Here is the new set up. Do you think the tree will benefit from the lamp being lowered?

I think your tree would do better either outside or with 3 or 4 more of those spotlights. Yes, the plant would benefit from the lamp being lower.

Yes. I think lower. I've changed my thinking a bit after looking again at your photos. Note the apparent very wide beam width. The rough rule I mentioned presumes a much tighter beam, at least two feet up, more concentrated on the growing area. In effect, a great deal of your light is not getting to the plant. I think you're getting even less effect from that light than I was at first thinking. So, I think you can move it significantly closer.

The maker claims a beam angle of 60 degrees. That means that for the light to cover just a bit more than about 20 inches, which looks to me about the length of leafed portion of the tree from the point of view of the lamp, the distance would be about 17 inches.

So, I think that means you can get the light closer than I first thought without danger to the foliage. And it means that, although we're kind of guessing about light distribution, you can cover the entire foliage from a position somewhat in front of it and about 17 inches distant at the closest point. So, I'd look for a position that would make a 60-degree beam address the entire plant and not much more. I've got a feeling you're going to end up about that17 inches from the plant at the closest point, maybe less, and that there won't be so great a difference between the closest point and the farthest to be a significant problem of insufficient light anywhere.

Just in case, keep an eye on the leaves closest to the light so you catch any browning or yellowing from light burn early. But I think the light intensity is little enough that it's unlikely to burn.