I am currently growing about a dozen citrus trees on the 8b / 9a line in Louisiana, most of which are some variety of Satsuma. Three of the trees are over 20 years old, the rest have been planted over the last 5 years.
One of my 20+ year old trees is in poor condition, it is planted in a more exposed location than the others, was well as in a spot where the soil stays a bit wetter. I cover my citrus trees and provide them with heat lamps during unusually cold winter weather, and during one of these freeze events a couple of years ago the cover blew off of this one tree during a night where the low temperatures reached about 15F. This resulted in considerable die back, it lost all its leaves and did not start having any signs of life until the following August with eventual loss of about 1/3 of the tree (the entire northern side). Since then fruit production on this tree has been light (I would estimate under 100 Satsumas this last season) and growth has been sparse and scraggly, perhaps due to freeze damaged wood.
This leads to consider cutting back at least one of the main trunks and possibly grafting in stems of scion wood from my Miho Satsuma tree which is reportedly one of the most cold hardy varieties.
Any advice you can give on the subject is most welcome, as I have little experience grafting, and no experience grafting citrus. Also my limited reading suggests that top working is best done on trees under 20 years of age, but I have found no details on this age limit.
thanks