It was because of Weedhound that I had to go buy a laser thermometer, and no I didnā??t get a good deal on it. It all started with me lowering the light. I have always thought that if the plant doesnā??t show signs of heat stress, then itā??s not too hot. Well I have always been wrong according to Weedhound. Hereā??s what she has to say on the subject:

ā?¦the REAL key is this.....(shhhhh) when the leaf surface temps go over 80 degrees (usually at about 85 degrees room temp or so depending on ventilation, how close your light is etc..) the plants growth will slow down tremendously. Photosynthesis drops a HUGE amount (in fact almost to 0 if I remember correctly) when leaf surface temp exceeds 80

I lowered my light to about 16ā? from the plants after lights out one night. I kept watching the plants the next day to see if it looked like they were getting heat stressed, but I didnā??t notice signs of any stress. The day after that I was looking at my plants and I realized that I hadnā??t really noticed much growth over the past two days. Usually I can notice growth in half of a day, and I was expecting faster growth with the light closer, not slower growth. Not too much later I was reading that post of Weedhounds, and it suddenly dawned on me that I may have the very problem that she is talking about. Iā??ve never heard of it before, but I learn new things all the time that are so basic and simple, that I should have known them a long time ago.

Now Weedhound, I love and respect you, and I deeply want to steal your dogs, but I usually donā??t take anyoneā??s word at face value. Itā??s not that I donā??t trust you, far from it. Itā??s just that I have noticed that even people that I have greatly respected have sometimes been mistaken. I spent a little time trying to find something to corroborate your hypothesis, but I didnā??t have any luck. (Weedhound if youā??re reading this, any chance of a link or something?) Iā??ll probably look more in the future, but the important thing is that I believe in what you are preaching, so much that I got to thinking about my dilemma concerning my plantsā?? apparent lack of proper growth. I got to wondering how I could find out what the real temperature is of individual leaves is, and the only plausible thing I could come up with is to get a laser thermometer. I wanted one now and decided not to try to get the best deal possible. I found out that they are very hard to come by in these parts, but fortunately an auto parts shop in a nearby hick town had one. Turns out that even though the air temp around the plants was supposedly 85, the leaves were showing anywhere from 80 to 91 when using the laser thermometer. Just as many, if not more of the readings were above 85 than were below. I thought that this is not good according to Weedhound, even though I have heard of growing in temperatures over 100. It definitely would explain my apparent lack of growth for a couple days immediately following the lowering of the light. So I kept raising my light until all my leaf readings were under 80. Itā??s now almost 3 feet above the plants.

Tomorrow I am going to put a variable speed switch on one of my inline fans and run it through that louvered opening you can see in the 2nd picture, and another one you canā??t see about a foot above it. Perhaps then I will be able to lower my light.

Summary: Assuming Weedhound is correct, and she probably is, then the only real way to find out what distance your light needs to be from your plants, is to use a laser thermometer and measure the leaf temperature. Keep raising your light until all leaves read less than 80.