This young Cara Cara navel orange tree looks fine, right? That’s what I thought until I crouched down and noticed this little devil:
That branch growing from the base of the trunk — that’s the little devil. It’s not a branch of Cara Cara navel orange; it’s rootstock.
Almost every citrus tree you can buy from a nursery (the main exception being Meyer lemon) is actually two trees in one. There is a rootstock on the bottom, and there is a scion on top. The scion is the part that gives you the fruit you desire.
Usually you can spot the union where the rootstock and scion were grafted together because the bark has a change in shape there, sometimes an extreme bump but sometimes only a subtle line. Can you see the graft union on my tree above? It’s just below where the branches start. It looks a bit like a “V”.
Beware. If the rootstock on a citrus tree sends up a branch, often referred to as a sucker, it will be vigorous and it will eventually take over the whole tree. Within a couple years, you’ll find that all of your tree’s fruit taste sour, and that’s because it’s not really your tree’s fruit — it’s not the scion’s fruit, that is. It’s rootstock fruit.
So keep an eye out, and cut any rootstock suckers off immediately.