My experiencing of overwintering Citrus under growth lights
I recently bought a gold nugget mandarin from four winds this summer. It's looking very good with many small fruits, I picked off all but 4. Late this summer, there are very many new growth. But the new growth grew very slowly, the new leaves getting bigger in almost one month time.
I moved it in my basement under a 1000w growth light, alone with many other tropical plants. But it's taking the prime location, lots of light. The temperature is in 78F when lights on and 68F when lights off. Humidity is low ranging from 40% to 50%. After about three days, most of the leaves except the new growth are drooping, like underwatered, but there is plenty of mositure in the soil. Plus, the lower side of the leaves started turning yellow in veins and gradually dropped. I immediated move it into my solarium, which will hot (85F) in the day if sunny, and cold in the night (60F), but humidity is around 40% in the day and 70% in the night. It recovered nicely, the drooping stopped, leaves stay hydrated even during hottest part of the day. There is no single leaves lost since.
On the other hand, my myer lemon, which has been with me the 3rd year, is doing splendid under the same light. No single leaves lost, exploded with fat fragrant blossoms. Leaves stay very hydrated.
Also, my improved myer lemon, sitting in a not so optimal location at the edge of the growth light, receiving far less light, sent out a several foot long new shoot with dark green leaves. But leaves are super sized, like my palm.
Now I do think that citrus will fair much better overwinter indoors if they already experienced the overwintering before. This also proved that they will hate the warm and dry conditions by shedding leaves or drooping their leaves if the lights is not optimal. Not even under a powerful growth light.