I visited
http://saxonbecnelandsons.com/ today and bought 30 trees. This is the end of the selling season and I'm out of rio grapefruit, navel oranges, and meyer lemon. They are growing 500,000 citrus trees in Orange, TX near the Louisiana border and just this year have started to sell trees. All their trees are 5 gallon and larger than other trees I have seen for sale in local nurseries.
They use Carrizo on all the trees. I mentioned that common knowledge around SE Texas is that trifoliate is the best rootstock for this area. His reply was "according to who?" and he proceeded to tell me why he thought carrizo was best. Much faster growing and makes bigger fruit=> bigger tree more cold hardy.
Hmm, I'm not convinced. Carrizo has hybrid vigor for sure and makes a large tree fast but I can't believe it is as cold hardy as trifoliate. I don't think it's best for cold hardy to have a vigorous rootstock.
I got to thinking why they prefer Carrizo. Big trees sell for more money and their customers the retail nurseries like Home Depot and don't ask/know about the rootstock.
I've found that another big grower outside of Houston
http://brazoscitrusnursery.com/ with 40,000 trees is not using trifoliate exclusively either.
However as far as I know
http://www.treesearchfarms.com/citrus.html and a local place Doremus wholesale nursery are growing with trifoliate rootsock/flying dragon.
I haven't seen trees for sale at local nurseries on trifoliate myself as the trees usually are on sour orange and sourced from the Texas Valley.
In my own experience not 1 person in 100 asks about the rootstock in my backyard nursery. Hmm, better for the grower but not as cold hardy for the retail buyer!!!