Mizz -- yes, you have a grafted tree. The non-thorny side is likely the kumquat scion, which was grafted to some other variety as an understock, and the thorny branches are very likely coming from that understock. They should be trimmed off where they emerge, leaving no nubs.

hmm... this might be an issue as half the tree at this point is the thorny part. How do I find out what kind of tree the understock is? And if I choose not to cut it off, will the kumquat half die?

I've taken photos of both sides, but I'm not sure how to upload them to this post...

It's my understanding that grapefruit is often used as understock for kumquat, in some parts of the world. More likely it's trifoliate orange or another sour orange. Once you let the root stock get the better of the grafted species, it can be difficult to recover it. Only you can decide if you want to try or not.

Thanks for the help, guys! I guess I'll just have to do some research and find out what the understock is, and whether or not it's desireable. I love my little kumquats, but I'll only be left with half a tree, which wouldn't be too cute, if I cut off the root stock. ::sigh::

But, thanks again, everyone!

Okay, so after doing a bunch of research, it looks like half my tree is Kumquat, and half is Kaffir Lime. So I guess my last question is: can I possibly keep both sides of my tree happy, so I can have both fruits, or do I have to choose one or the other?

Yes, you could. Just don't let one side overwhelm the other, through the use of judicious pruning. It's surprising that KL would be used as a rootstock, but maybe. Does it really have that humongous petiole wing? Or might it be something smaller, like grapefruit?

The petiole wings are smaller than most of the Kaffir Lime pics that I found, but then I came across a few KL pictures that seemed very similar. And my plant has large thorns, as well, which the lime plant seems to. I would agree with you, though, in that I think the leaves probably more closely resemble that of a grapefruit plant, but I haven't found anything that states that grapefruit plants have the same thorns that mine does.

I would estimate that my plant is probably about 5 years old.

What do you think?
Thanks for all your help!

Yes, grapefruit seedlings often have big, sharp thorns. Most seedling citrus do. And grapefruit leaves do have a sizable petiole wing, although much smaller than that of KL.

Thank you! You've been a big help. I think I'll keep both sides growing and see if I can't get a fruit salad out of the tree by the end of it.

Thanks again!

This was an extremely helpful thread as I am dealing with the exact same issue with a kumquat that was given to me about eight months ago. I can't seem to find a match for the leaves of the "alien" plant that has sprouted up. Can anyone tell me what type of plant these leaves belong to?

Thank you!

Well that's not a kumquat ray its somekind of trifold rootstock as to what kind I have no clue. It looks very healthy but has the strangest leaf I have ever Sean.I attached a photo of a kumquat seedling to show what the leafs look like.
Trace

Ray, that is trifoliate rootstock. I would cut that off at the trunk to let your kumquat revive itself, if there is any of the original cultivar scion left. If not, you've just got rootstock growing,

Patty S.