I remember that I read it is possible to reach 3 inches in diameter trunk in 18 months but I can find the post again in the forum.Search doesn't help here.
It seems impossible to me now but my mind tells me that I have read it.

Do I remember right and who wrote it, can you remember?

I do not remember reading it here.

link

this link mentions getting rootstock pencil size in 9 months

Hi Skeeter, I remember I read it here but I am just not sure about the time and thickness.My mind tells me that it was written by Ned or Malcolm_Manners, but not sure

Thanks for the link bodavid

In the link that bodavid posted, there is a post by Dr. Manners stating that rootstock seedlings in Florida are grown at 100F. I find this interesting, in all of the literature I have read states at temperatures above 95F citrus growth stops. - Millet

Growing in 100 deg F could be possible if there is enough humidity, available water source and enough light. Some metabolic enzymes though could break down, but it depends upon, if the tree has been acclimatized well before reaching that temperature and has enough other proteins to protect or adjust their metabolic enzymes.

I do know that they grow citrus in Florida under cover, greenhouses and sometimes type of fabric tents. Of course they are required to do this now, but in the commerical operations I have seen they were doing this before the trees had to be screen. The heat under the plastic cover was hot and humid. I am sure it often got to, or exceeded, 100 degrees during the day. Also it helps by keeping the trees warmer at night. I figure they do this to push growth, especially in winter. For the nurseryman rapid growth equals greater profit. I have seen greenhouses used in this manner outside of Florida too.

Ned