On the citrus I have that have sent up suckers from the roots, the thorns are much bigger than the grafted plant.. an inch plus long... Most of the citrus we get in northern California comes from 2 different growers.. I'm not sure what they graft on, or if a different rootstock is used in your part of the country. I can't see the graft line in your picture, but it's usually fairly obvious. If stuff is growing below the graft, it's a sucker, and you don't want it..
Four Winds Nursery has a good FAQ page on their site:
https://www.fourwindsgrowers.c...
