I recently posted a response in a thread about a hobbyist’s frustration with burning off a vine on a new Hoya growing under lights. There is some good information within the post so I thought I would post it here as well.
Welcome to the fun of growing Hoyas under lights! Vines getting singed off on fluorescent tubes happens all to frequently to me, and it makes me just as mad, especially if it is a new plant and has just started growing well.
Your question is a good one, and I don’t have a scientific answer, but I can tell you that the law of diminishing returns applies. The higher that you move those tubes, the less results you will see. The plant’s growth will be weaker and fewer blooms will appear. I have too many plants in that situation currently. There is not enough punch in the fluorescent tubes to do much good for the majority of the plant.
I have started experimenting with a couple of the newer LED fixtures with amazing early results. They are expensive, but so much more powerful and they don’t get hot because they run with internal fans. You can keep your hand on it all day and it is barely warm so it won’t burn off a vine tip. All of that being said, 90% of my plants still grow under T-5s and they do work very well for most Hoyas. Singed growing tips and vines are just going to be part of the hobby. Moving the lights too high to prevent it will just not get you the results.
I will soon have some information on the specifics of the LED lights that I am currently using – Stay tuned!