All plants have a pigment - phytochrome - that essentially measures the dark period. This generally tells a plant when to leaf out, bloom, drop its leaves, etc. With plants that are very photoperiod sensitive, such as pointsettas, which only bloom at the time of year with the longest nights (corresponding coincidentally to Christmas but also to the rainy season in its native land), interrupting the dark period for even a very short time (opening the closet in which the plant is stored at night, turning the lights on in a greenhouse for a second or two) can disrupt the blooming cycle. I can't think of any conifer that would be disturbed by lights at night. Bt maybe I am missing something...

Probably a CYA statement they put on the label because someone on the light crew said they heard something about lights affecting trees at night. < roll eyes >

From what I've learned about growing plants under lights, it takes some pretty strong lights to affect plants.

And although it doesn't matter, trees don't exist above the arctic circle. lol

Yes, it's moot. I only have them on for an hour or so or whenever there is company. I was just struck by the statement because I'd never heard such a thing before.

I appreciate everyone's comments. Just one thing to correct: trees certainly do exist above the Arctic circle, although the exact tree line varies based on local factors. A specific latitude in one portion of the Earth may be barren tundra, while in another area (at the same latitude) there may be forest.

I'll agree with you on that.

The tree line is jagged and doesn't follow the A.C. exactly. A truer statement would be 'Trees do not exist very far above of the Arctic circle'. But you get my point. ;o)

Picture of Tree line:

actually I recently read article about light pollution , they mentioned it's effect on plants ...

but can't remember any details ,,,,, lots of help

ron

Read somewhere that most plant growth actually occurs at night. Also note the original post used the term "optimal growth". Assume that means growth can still occur, just not as fast.

Enjoyed the "Artic Circle" picture!

The wife & I made the bus trip to the A.C. from Fairbanks several years ago. The topography varied along the way, from low ground mucky 'perma frost' areas, where we were told that spruce trees there were 100 years old and only about 10 ft tall. Not much growth I guess when only the top foot or two of soil thaws during the summer. The other more elevated areas consisted more of gravel and rocks that wasn't froze year round like the bog areas.

Even before we got to the A.C. there were rocky/gravelly 'high' areas with no trees, just a few scrubby shrub like plants and lichens on the rocks. Below, me and the wife at a stop area, guessing 50 mi (?) before reaching the A.C. (notice in the background (left) you can see forever).

In the lower, boggy areas, we learned about Fireweed (Chamaenerion angustifolium).

And CottonGrass (Eriophorum angustifolium):

You could dig with your hand into the loose sod and find it was very cold (froze?) only a few inches in.

Finally the picture op at the end of journey (wife pointing to where we are). :-)